4.2: Aseptic Technique Basic Concepts
Standard Versus Transmission-Based Precautions
Standard Precautions
Standard precautions are used when caring for all patients to prevent health care associated infections. According to the Centers for Disease Control and Prevention (CDC), standard precautions are “the minimum infection prevention practices that apply to all patient care, regardless of suspected or confirmed infection status of the patient, in any setting where health care is delivered.” [1] They are based on the principle that all blood, body fluids (except sweat), nonintact skin, and mucous membranes may contain transmissible infectious agents. These standards reduce the risk of exposure for the health care worker and protect the patient from potential transmission of infectious organisms.
Current standard precautions according to the CDC (2019) include the following:
- Appropriate hand hygiene
- Use of personal protective equipment (e.g., gloves, gowns, masks, eyewear) whenever infectious material exposure may occur
- Appropriate patient placement and care using transmission-based precautions when indicated
- Respiratory hygiene/cough etiquette
- Proper handling and cleaning of environment, equipment, and devices
- Safe handling of laundry
- Sharps safety (i.e., engineering and work practice controls)
- Aseptic technique for invasive nursing procedures such as parenteral medication administration [2]
Each of these standard precautions is described in more detail in the following subsections.
Transmission-Based Precautions
In addition to standard precautions, transmission-based precautions are used for patients with documented or suspected infection, or colonization, of highly-transmissible or epidemiologically-important pathogens. Epidemiologically-important pathogens include, but are not limited to, Coronavirus disease (COVID-19), C. difficile (C-diff), Methicillin-resistant Staphylococcus aureus (MRSA), Vancomycin-resistant enterococci (VRE), Respiratory Syncytial Virus (RSV), measles, and tuberculosis (TB). For patients with these types of pathogens, standard precautions are used along with specific transmission-based precautions.
There are three categories of transmission-based precautions: contact precautions , droplet precautions , and airborne precautions . Transmission-based precautions are used when the route(s) of transmission is (are) not completely interrupted using standard precautions alone. Some diseases, such as tuberculosis, have multiple routes of transmission so more than one transmission-based precautions category must be implemented. See Table \(\PageIndex{1}\) outlining the categories of transmission precautions with associated PPE and other precautions. When possible, patients with transmission-based precautions should be placed in a single occupancy room with dedicated patient care equipment (e.g., blood pressure cuffs, stethoscope, thermometer). Transport of the patient and unnecessary movement outside the patient room should be limited. However, when transmission-based precautions are implemented, it is also important for the nurse to make efforts to counteract possible adverse effects of these precautions on patients, such as anxiety, depression, perceptions of stigma, and reduced contact with clinical staff.
| Precaution | Implementation | PPE and Other Precautions |
|---|---|---|
| Contact | Known or suspected infections with increased risk for contact transmission (e.g., draining wounds, fecal incontinence) or with epidemiologically important organisms, such as C-diff, MRSA, VRE, or RSV |
Note: Use only soap and water for hand hygiene in patients with C. difficile infection. |
| Droplet | Known or suspected infection with pathogens transmitted by large respiratory droplets generated by coughing, sneezing, or talking, such as influenza, coronavirus, or pertussis |
|
| Airborne | Known or suspected infection with pathogens transmitted by small respiratory droplets, such as measles |
Fit-tested N-95 respirator or PAPR
|
View a list of transmission-based precautions used for specific medical conditions at the CDC Guideline for Isolation Precautions.
Patient Transport
Several principles are used to guide transport of patients requiring transmission-based precautions. In the inpatient and residential settings, these principles include the following:
- Limiting transport for essential purposes only, such as diagnostic and therapeutic procedures that cannot be performed in the patient’s room
- Using appropriate barriers on the patient consistent with the route and risk of transmission (e.g., mask, gown, covering the affected areas when infectious skin lesions or drainage is present)
- Notify other health care personnel involved in the care of the patient of the transmission-based precautions. For example, when transporting the patient to radiology, inform the radiology technician of the precautions. [4]
Appropriate Hand Hygiene
Hand hygiene is the single most important practice to reduce the transmission of infectious agents in health care settings and is an essential element of standard precautions. [5] Routine handwashing during appropriate moments is a simple and effective way to prevent infection. However, it is estimated that health care professionals, on average, properly clean their hands less than 50% of the time it is indicated. [6] The Joint Commission, the organization that sets evidence-based standards of care for hospitals, recently updated its hand hygiene standards in 2018 to promote enforcement. If a Joint Commission surveyor witnesses an individual failing to properly clean their hands when it is indicated, a deficiency will be cited requiring improvement by the agency. This deficiency could potentially jeopardize a hospital’s accreditation status and their ability to receive payment for patient services. Therefore, it is essential for all health care workers to ensure they are using proper hand hygiene at the appropriate times. [7]
There are several evidence-based guidelines for performing appropriate hand hygiene. These guidelines include frequency of performing hand hygiene according to the care circumstances, solutions used, and technique performed. The Healthcare Infection Control Practices Advisory Committee (HICPAC) recommends health care personnel perform hand hygiene at specific times when providing care to patients. These moments are often referred to as the “ Five Moments for Hand Hygiene .” [8] See Figures \(\PageIndex{1}\) [9] and \(\PageIndex{2}\) [10] for an illustration and application of the five moments of hand hygiene. The five moments of hand hygiene are as follows:
- Immediately before touching a patient
- Before performing an aseptic task or handling invasive devices
- Before moving from a soiled body site to a clean body site on a patient
- After touching a patient or their immediate environment
- After contact with blood, body fluids, or contaminated surfaces (with or without glove use)
- Immediately after glove removal
When performing hand hygiene, washing with soap and water, or an approved alcohol-based hand rub solution that contains at least 60% alcohol, may be used. Unless hands are visibly soiled, an alcohol-based hand rub is preferred over soap and water in most clinical situations due to evidence of improved compliance. Hand rubs are also preferred because they are generally less irritating to health care worker’s hands. However, it is important to recognize that alcohol-based rubs do not eliminate some types of germs, such as Clostridium difficile (C-diff).
When using the alcohol-based handrub method, the CDC recommends the following steps. See Figure \(\PageIndex{3}\) [11] for a handrub poster created by the World Health Organization.
- Apply product to the palm of one hand in an amount that will cover all surfaces.
- Rub hands together, covering all the surfaces of the hands, fingers, and wrists until the hands are dry. Surfaces include the palms and fingers, between the fingers, the backs of the hands and fingers, the fingertips, and the thumbs.
- The process should take about 20 seconds, and the solution should be dry. [12]
When washing with soap and water, the CDC recommends using the following steps. See Figure \(\PageIndex{4}\) [13] for an image of a handwashing poster created by the World Health Organization.
- Wet hands with warm or cold running water and apply facility-approved soap.
- Lather hands by rubbing them together with the soap. Use the same technique as the handrub process to clean the palms and fingers, between the fingers, the backs of the hands and fingers, the fingertips, and the thumbs.
- Scrub thoroughly for at least 20 seconds.
- Rinse hands well under clean, running water.
- Dry the hands using a clean towel or disposable toweling.
By performing hand hygiene at the proper moments and using appropriate techniques, you will ensure your hands are safe and you are not transmitting infectious organisms to yourself or others.
Hand Hygiene for Healthcare Workers [15]
CDC Handwashing for Children in School [16]
For more information about hand hygiene recommendations, use the following links:
WHO Guidelines on Hand Hygiene
WHO Hand Hygiene Brochure
CDC Hand Hygiene in Healthcare Settings
Joint Commission on Hand Hygiene
Personal Protective Equipment (PPE)
Personal Protective Equipment (PPE) includes gloves, gowns, face shields, goggles, and masks used to prevent the spread of infection to and from patients and health care providers. Depending on the anticipated exposure, PPE may include the use of gloves, a fluid-resistant gown, goggles or a face shield, and a mask or respirator. When used for a patient with transmission-based precautions, PPE supplies are typically stored in an isolation cart next to the patient’s room, and a card is posted on the door alerting staff and visitors to precautions needed before entering the room.
Gloves
Gloves protect both patients and health care personnel from exposure to infectious material that may be carried on the hands. Gloves are used to prevent contamination of health care personnel hands during activities such as the following:
- anticipating direct contact with blood or body fluids, mucous membranes, nonintact skin, and other potentially infectious material
- having direct contact with patients who are colonized or infected with pathogens transmitted by the contact route, such as Vancomycin-resistant enterococci (VRE), Methicillin-resistant Staphylococcus aureus (MRSA), and Respiratory Syncytial Virus (RSV)
- handling or touching visibly or potentially contaminated patient care equipment and environmental surfaces [17]
Nonsterile disposable medical gloves for routine patient care are made of a variety of materials, such as latex, vinyl, and nitrile. Many people are allergic to latex, so be sure to check for latex allergies for the patient and other health care professionals. See Figure \(\PageIndex{5}\) [18] for an image of nonsterile medical gloves in various sizes in a health care setting. At times, gloves may need to be changed when providing care to a single patient to prevent cross-contamination of body sites. It is also necessary to change gloves if the patient interaction requires touching portable computer keyboards or other mobile equipment that is transported from room to room. Discarding gloves between patients is necessary to prevent transmission of infectious material. Gloves must not be washed for subsequent reuse because microorganisms cannot be reliably removed from glove surfaces and continued glove integrity cannot be ensured. [19]
When gloves are worn in combination with other PPE, they are put on last. Gloves that fit snugly around the wrist should be used in combination with isolation gowns because they will cover the gown cuff and provide a more reliable continuous barrier for the arms, wrists, and hands.
Gloves should be removed properly to prevent contamination. See Figure \(\PageIndex{6}\) [20] for an illustration of properly removing gloves. Hand hygiene should be performed following glove removal to ensure the hands will not carry potentially infectious material that might have penetrated through unrecognized tears or contaminated the hands during glove removal. One method for properly removing gloves includes the following steps:
- Grasp the outside of one glove near the wrist. Do not touch your skin.
- Peel the glove away from your body, pulling it inside out.
- Hold the removed glove in your gloved hand.
- Put your fingers inside the glove at the top of your wrist and peel off the second glove.
- Turn the second glove inside out while pulling it away from your body, leaving the first glove inside the second.
- Dispose of the gloves safely. Do not reuse.
- Perform hand hygiene immediately after removing the gloves. [21]
Gowns
Isolation gowns are used to protect the health care worker’s arms and exposed body areas and to prevent contamination of their clothing with blood, body fluids, and other potentially infectious material. Isolation gowns may be disposable or washable/reusable. See Figure \(\PageIndex{7}\) [23] for an image of a nurse wearing an isolation gown along with goggles and a respirator. When using standard precautions, an isolation gown is worn only if contact with blood or body fluid is anticipated. However, when contact transmission-based precautions are in place, donning of both gown and gloves upon room entry is indicated to prevent unintentional contact of clothing with contaminated environmental surfaces.
Gowns are usually the first piece of PPE to be donned. Isolation gowns should be removed before leaving the patient room to prevent possible contamination of the environment outside the patient’s room. Isolation gowns should be removed in a manner that prevents contamination of clothing or skin. The outer, “contaminated,” side of the gown is turned inward and rolled into a bundle, and then it is discarded into a designated container to contain contamination. See more information about putting on and removing PPE in the subsection below. [24]
 Masks are used to protect these sites from entry of large infectious droplets. See Figure \(\PageIndex{8}\) [25] for an image of nurse wearing a surgical mask. Masks have three primary purposes in health care settings:
- Used by health care personnel to protect them from contact with infectious material from patients (e.g., respiratory secretions and sprays of blood or body fluids), consistent with standard precautions and droplet transmission precautions
- Used by health care personnel when engaged in procedures requiring sterile technique to protect patients from exposure to infectious agents potentially carried in a health care worker’s mouth or nose
- Placed on coughing patients to limit potential dissemination of infectious respiratory secretions from the patient to others in public areas (i.e., respiratory hygiene) [26]
Masks may be used in combination with goggles or a face shield to provide more complete protection for the face. Masks should not be confused with respirators used during airborne transmission-based precautions to prevent inhalation of small, aerosolized infectious droplets. [27]
It is important to properly wear and remove masks to avoid contamination. See Figure \(\PageIndex{9}\) [28] for CDC face mask recommendations for health care personnel.
Goggles/Face Shields
Eye protection chosen for specific work situations (e.g., goggles or face shields) depends upon the circumstances of exposure, other PPE used, and personal vision needs. Personal eyeglasses are not considered adequate eye protection. See Figure \(\PageIndex{10}\) [29] for an image of a health care professional wearing a face shield along with a N95 respirator.
Respirators and PAPRs
Respiratory protection used during airborne transmission precautions requires the use of special equipment. Traditionally, a fitted respirator mask with N95 or higher filtration has been worn by health care professionals to prevent inhalation of small airborne infectious particles. A user-seal check (formerly called a “fit check”) should be performed by the wearer of a respirator each time a respirator is donned to minimize air leakage around the facepiece.
A newer piece of equipment used for respiratory protection is the powered air-purifying respirator (PAPR). A PAPR is an air-purifying respirator that uses a blower to force air through filter cartridges or canisters into the breathing zone of the wearer. This process creates an air flow inside either a tight-fitting facepiece or loose-fitting hood or helmet, providing a higher level of protection against aerosolized pathogens, such as COVID-19, than a N95 respirator. See Figure \(\PageIndex{11}\) [30] for an example of PAPR in use.
The CDC currently recommends N95 or higher level respirators for personnel exposed to patients with suspected or confirmed tuberculosis and other airborne diseases, especially during aerosol-generating procedures such as respiratory-tract suctioning. [31] It is important to apply, wear, and remove respirators appropriately to avoid contamination. See Figure \(\PageIndex{12}\) [32] for CDC recommendations when wearing disposable respirators.
How to Put On ( Don ) PPE Gear
Follow agency policy for donning PPE according to transmission-based precautions. More than one donning method for putting on PPE may be acceptable. The CDC recommends the following steps for donning PPE: [33]
- Identify and gather the proper PPE to don. Ensure the gown size is correct.
- Perform hand hygiene using hand sanitizer.
- Put on the isolation gown. Tie all of the ties on the gown. Assistance may be needed by other health care personnel to tie back ties.
- Based on specific transmission-based precautions and agency policy, put on a mask or N95 respirator. The top strap should be placed on the crown (top) of the head, and the bottom strap should be at the base of the neck. If the mask has loops, hook them appropriately around your ears. Masks and respirators should extend under the chin, and both your mouth and nose should be protected. Perform a user-seal check each time you put on a respirator. If the respirator has a nosepiece, it should be fitted to the nose with both hands, but it should not be bent or tented. Masks typically require the nosepiece to be pinched to fit around the nose, but do not pinch the nosepiece of a respirator with one hand. Do not wear a respirator or mask under your chin or store it in the pocket of your scrubs between patients.
- Put on a face shield or goggles when indicated. When wearing an N95 respirator with eye protection, select eye protection that does not affect the fit or seal of the respirator and one that does not affect the position of the respirator. Goggles provide excellent protection for the eyes, but fogging is common. Face shields provide full-face coverage.
- Put on gloves. Gloves should cover the cuff (wrist) of the gown.
- You may now enter the patient’s room.
How to Put On a Respirator [34]
Donning and Doffing PPE [35]
How to Take Off ( Doff ) PPE Gear
More than one doffing method for removing PPE may be acceptable. Train using your agency’s procedure, and practice until you have successfully mastered the steps to avoid contamination of yourself and others. There are established cases of nurses dying from disease transmitted during incorrect removal of PPE. Below are sample steps of doffing established by the CDC: [36]
- Remove the gloves. Ensure glove removal does not cause additional contamination of the hands. Gloves can be removed using more than one technique (e.g., glove-in-glove or bird beak).
- Remove the gown. Untie all ties (or unsnap all buttons). Some gown ties can be broken rather than untied; do so in a gentle manner and avoid a forceful movement. Reach up to the front of your shoulders and carefully pull the gown down and away from your body. Rolling the gown down is also an acceptable approach. Dispose of the gown in a trash receptacle. If it is a washable gown, place it in the specified laundry bin for PPE in the room.
- Health care personnel may now exit the patient room.
- Perform hand hygiene.
- Remove the face shield or goggles. Carefully remove the face shield or goggles by grabbing the strap and pulling upwards and away from head.
- Remove and discard the respirator or face mask. Do not touch the front of the respirator or face mask. Remove the bottom strap by touching only the strap and bringing it carefully over the head.  For masks, carefully untie (or unhook ties from the ears) and pull the mask away from your face without touching the front.
- Perform hand hygiene after removing the respirator/mask. If your workplace is practicing reuse, perform hand hygiene before putting it on again.
Putting on PPE [37]
Removing PPE [38]
Respiratory Hygiene
Respiratory hygiene is targeted at patients, accompanying family members and friends, and health care workers with undiagnosed transmissible respiratory infections. It applies to any person with signs of illness, including cough, congestion, rhinorrhea, or increased production of respiratory secretions when entering a health care facility. See Figure \(\PageIndex{13}\) [39] for an example of a “Cover Your Cough” poster used in public areas to promote respiratory hygiene. The elements of respiratory hygiene include the following:
- Education of health care facility staff, patients, and visitors
- Posted signs, in language(s) appropriate to the population served, with instructions to patients and accompanying family members or friends
- Source control measures for a coughing person (e.g., covering the mouth/nose with a tissue when coughing and prompt disposal of used tissues, or applying surgical masks on the coughing person to contain secretions)
- Hand hygiene after contact with one’s respiratory secretions
- Spatial separation, ideally greater than 3 feet, of persons with respiratory infections in common waiting areas when possible. [40]
Health care personnel are advised to wear a mask and use frequent hand hygiene when examining and caring for patients with signs and symptoms of a respiratory infection. Health care personnel who have a respiratory infection are advised to avoid direct patient contact, especially with high-risk patients. If this is not possible, then a mask should be worn while providing patient care. [41]
Environmental Measures
Routine cleaning and disinfecting surfaces in patient-care areas are part of standard precautions. The cleaning and disinfecting of all patient-care areas are important for frequently touched surfaces, especially those closest to the patient that are most likely to be contaminated (e.g., bedrails, bedside tables, commodes, doorknobs, sinks, surfaces, and equipment in close proximity to the patient).
Medical equipment and instruments/devices must also be cleaned to prevent patient-to-patient transmission of infectious agents. For example, stethoscopes should be cleaned before and after use for all patients. Patients who have transmission-based precautions should have dedicated medical equipment that remains in their room (e.g., stethoscope, blood pressure cuff, thermometer). When dedicated equipment is not possible, such as a unit-wide bedside blood glucose monitor, disinfection after each patient’s use should be performed according to agency policy. [42]
Disposal of Contaminated Waste
Medical waste requires careful disposal according to agency policy. The Occupational Safety and Health Administration (OSHA) has established measures for discarding regulated medical waste items to protect the workers who generate medical waste, as well as those who manage the waste from point of generation to disposal. Contaminated waste is placed in a leak-resistant biohazard bag, securely closed, and placed in a labeled, leakproof, puncture-resistant container in a storage area. Sharps containers are used to dispose of sharp items such as discarded tubes with small amounts of blood, scalpel blades, needles, and syringes. [43]
Sharps Safety
Injuries due to needles and other sharps have been associated with transmission of blood-borne pathogens (BBP), including hepatitis B, hepatitis C, and HIV to health care personnel. The prevention of sharps injuries is an essential element of standard precautions and includes measures to handle needles and other sharp devices in a manner that will prevent injury to the user and to others who may encounter the device during or after a procedure. The Bloodborne Pathogens Standard is a regulation that prescribes safeguards to protect workers against health hazards related to blood-borne pathogens. It includes work practice controls, hepatitis B vaccinations, hazard communication and training, plans for when an employee is exposed to a BBP, and record keeping.
When performing procedures that include needles or other sharps, dispose of these items immediately in FDA-cleared sharps disposal containers. Additionally, to prevent needlestick injuries, needles and other contaminated sharps should not be recapped. See Figure \(\PageIndex{14}\) [44] for an image of a sharps disposal container. FDA-cleared sharps disposal containers are made from rigid plastic and come marked with a line that indicates when the container should be considered full, which means it’s time to dispose of the container. When a sharps disposal container is about three-quarters full, follow agency policy for proper disposal of the container.
If you are stuck by a needle or other sharps or are exposed to blood or other potentially infectious materials in your eyes, nose, mouth, or on broken skin, immediately flood the exposed area with water and clean any wound with soap and water. Report the incident immediately to your instructor or employer and seek immediate medical attention according to agency and school policy.
Textiles and Laundry
Soiled textiles, including bedding, towels, and patient or resident clothing may be contaminated with pathogenic microorganisms. However, the risk of disease transmission is negligible if they are handled, transported, and laundered in a safe manner. Follow agency policy for handling soiled laundry using standard precautions. Key principles for handling soiled laundry are as follows:
- Do not shake items or handle them in any way that may aerosolize infectious agents.
- Avoid contact of one’s body and personal clothing with the soiled items being handled.
- Place soiled items in a laundry bag or designated bin in the patient’s room before transporting to a laundry area. When laundry chutes are used, they must be maintained to minimize dispersion of aerosols from contaminated items. [45]
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- “ cycphceng.pdf ” by www.health.state.mn.us/index.html is in the Public Domain ↵
- Siegel, J. D., Rhinehart, E., Jackson, M., Chiarello, L., & Healthcare Infection Control Practices Advisory Committee. (2019, July 22). 2007 guideline for isolation precautions: Preventing transmission of infectious agents in healthcare settings. https://www.cdc.gov/infectioncontrol/guidelines/isolation/index.html ↵
- Siegel, J. D., Rhinehart, E., Jackson, M., Chiarello, L., & Healthcare Infection Control Practices Advisory Committee. (2019, July 22). 2007 guideline for isolation precautions: Preventing transmission of infectious agents in healthcare settings. https://www.cdc.gov/infectioncontrol/guidelines/isolation/index.html ↵
- Siegel, J. D., Rhinehart, E., Jackson, M., Chiarello, L., & Healthcare Infection Control Practices Advisory Committee. (2019, July 22). 2007 guideline for isolation precautions: Preventing transmission of infectious agents in healthcare settings. https://www.cdc.gov/infectioncontrol/guidelines/isolation/index.html ↵
- Centers for Disease Control and Prevention. (2015, November 5). Background I. Regulated medical waste. https://www.cdc.gov/infectioncontrol/guidelines/environmental/background/medical-waste.html#i3 ↵
- “Sharps-Containers.jpg” by Federal Drug Administration (FDA) is in the Public Domain . Access for free at https://www.fda.gov/medical-devices/safely-using-sharps-needles-and-syringes-home-work-and-travel/sharps-disposal-containers ↵
- Siegel, J. D., Rhinehart, E., Jackson, M., Chiarello, L., & Healthcare Infection Control Practices Advisory Committee. (2019, July 22). 2007 guideline for isolation precautions: Preventing transmission of infectious agents in healthcare settings. https://www.cdc.gov/infectioncontrol/guidelines/isolation/index.html ↵


5.5: Hearing
Learning Objectives
- Describe the basic anatomy and function of the auditory system
- Explain how we encode and perceive pitch
- Discuss how we localize sound
Our auditory system converts pressure waves into meaningful sounds. This translates into our ability to hear the sounds of nature, to appreciate the beauty of music, and to communicate with one another through spoken language. This section will provide an overview of the basic anatomy and function of the auditory system. It will include a discussion of how the sensory stimulus is translated into neural impulses, where in the brain that information is processed, how we perceive pitch, and how we know where sound is coming from.
Anatomy of the Auditory System
The ear can be separated into multiple sections. The outer ear includes the pinna , which is the visible part of the ear that protrudes from our heads, the auditory canal, and the tympanic membrane , or eardrum. The middle ear contains three tiny bones known as the ossicles , which are named the malleus (or hammer), incus (or anvil), and the stapes (or stirrup). The inner ear contains the semi-circular canals, which are involved in balance and movement (the vestibular sense), and the cochlea. The cochlea is a fluid-filled, snail-shaped structure that contains the sensory receptor cells (hair cells) of the auditory system.
Sound waves travel along the auditory canal and strike the tympanic membrane, causing it to vibrate. This vibration results in movement of the three ossicles. As the ossicles move, the stapes presses into a thin membrane of the cochlea known as the oval window. As the stapes presses into the oval window, the fluid inside the cochlea begins to move, which in turn stimulates hair cells , which are auditory receptor cells of the inner ear embedded in the basilar membrane. The basilar membrane is a thin strip of tissue within the cochlea.
The activation of hair cells is a mechanical process: the stimulation of the hair cell ultimately leads to activation of the cell. As hair cells become activated, they generate neural impulses that travel along the auditory nerve to the brain. Auditory information is shuttled to the inferior colliculus, the medial geniculate nucleus of the thalamus, and finally to the auditory cortex in the temporal lobe of the brain for processing. Like the visual system, there is also evidence suggesting that information about auditory recognition and localization is processed in parallel streams (Rauschecker & Tian, 2000; Renier et al., 2009).
Pitch Perception
Different frequencies of sound waves are associated with differences in our perception of the pitch of those sounds. Low-frequency sounds are lower pitched, and high-frequency sounds are higher pitched. How does the auditory system differentiate among various pitches?
Several theories have been proposed to account for pitch perception. We’ll discuss two of them here: temporal theory and place theory. The temporal theory of pitch perception asserts that frequency is coded by the activity level of a sensory neuron. This would mean that a given hair cell would fire action potentials related to the frequency of the sound wave. While this is a very intuitive explanation, we detect such a broad range of frequencies (20–20,000 Hz) that the frequency of action potentials fired by hair cells cannot account for the entire range. Because of properties related to sodium channels on the neuronal membrane that are involved in action potentials, there is a point at which a cell cannot fire any faster (Shamma, 2001).
The place theory of pitch perception suggests that different portions of the basilar membrane are sensitive to sounds of different frequencies. More specifically, the base of the basilar membrane responds best to high frequencies and the tip of the basilar membrane responds best to low frequencies. Therefore, hair cells that are in the base portion would be labeled as high-pitch receptors, while those in the tip of basilar membrane would be labeled as low-pitch receptors (Shamma, 2001).
In reality, both theories explain different aspects of pitch perception. At frequencies up to about \(4000\) Hz, it is clear that both the rate of action potentials and place contribute to our perception of pitch. However, much higher frequency sounds can only be encoded using place cues (Shamma, 2001).
Sound Localization
The ability to locate sound in our environments is an important part of hearing . Localizing sound could be considered similar to the way that we perceive depth in our visual fields. Like the monocular and binocular cues that provided information about depth, the auditory system uses both monaural (one-eared) and binaural (two-eared) cues to localize sound.
Each pinna interacts with incoming sound waves differently, depending on the sound’s source relative to our bodies. This interaction provides a monaural cue that is helpful in locating sounds that occur above or below and in front or behind us. The sound waves received by your two ears from sounds that come from directly above, below, in front, or behind you would be identical; therefore, monaural cues are essential (Grothe, Pecka, & McAlpine, 2010).
Binaural cues, on the other hand, provide information on the location of a sound along a horizontal axis by relying on differences in patterns of vibration of the eardrum between our two ears. If a sound comes from an off-center location, it creates two types of binaural cues: interaural level differences and interaural timing differences. Interaural level difference refers to the fact that a sound coming from the right side of your body is more intense at your right ear than at your left ear because of the attenuation of the sound wave as it passes through your head. Interaural timing difference refers to the small difference in the time at which a given sound wave arrives at each ear. Certain brain areas monitor these differences to construct where along a horizontal axis a sound originates (Grothe et al., 2010).
Hearing Loss
Deafness is the partial or complete inability to hear. Some people are born deaf, which is known as congenital deafness . Many others begin to suffer from conductive hearing loss because of age, genetic predisposition, or environmental effects, including exposure to extreme noise (noise-induced hearing loss, as shown in the figure below), certain illnesses (such as measles or mumps), or damage due to toxins (such as those found in certain solvents and metals).
Given the mechanical nature by which the sound wave stimulus is transmitted from the eardrum through the ossicles to the oval window of the cochlea, some degree of hearing loss is inevitable. With conductive hearing loss, hearing problems are associated with a failure in the vibration of the eardrum and/or movement of the ossicles. These problems are often dealt with through devices like hearing aids that amplify incoming sound waves to make vibration of the eardrum and movement of the ossicles more likely to occur.
When the hearing problem is associated with a failure to transmit neural signals from the cochlea to the brain, it is called sensorineural hearing loss . One disease that results in sensorineural hearing loss is Ménière's disease . Although not well understood, Ménière's disease results in a degeneration of inner ear structures that can lead to hearing loss, tinnitus (constant ringing or buzzing), vertigo (a sense of spinning), and an increase in pressure within the inner ear (Semaan & Megerian, 2011). This kind of loss cannot be treated with hearing aids, but some individuals might be candidates for a cochlear implant as a treatment option. Cochlear implants are electronic devices that consist of a microphone, a speech processor, and an electrode array. The device receives incoming sound information and directly stimulates the auditory nerve to transmit information to the brain.
 This is called deaf culture . In the United States, deaf individuals often communicate using American Sign Language (ASL); ASL has no verbal component and is based entirely on visual signs and gestures. The primary mode of communication is signing. One of the values of deaf culture is to continue traditions like using sign language rather than teaching deaf children to try to speak, read lips, or have cochlear implant surgery.
When a child is diagnosed as deaf, parents have difficult decisions to make. Should the child be enrolled in mainstream schools and taught to verbalize and read lips? Or should the child be sent to a school for deaf children to learn ASL and have significant exposure to deaf culture? Do you think there might be differences in the way that parents approach these decisions depending on whether or not they are also deaf?
Summary
Sound waves are funneled into the auditory canal and cause vibrations of the eardrum; these vibrations move the ossicles. As the ossicles move, the stapes presses against the oval window of the cochlea, which causes fluid inside the cochlea to move. As a result, hair cells embedded in the basilar membrane become enlarged, which sends neural impulses to the brain via the auditory nerve.
Pitch perception and sound localization are important aspects of hearing. Our ability to perceive pitch relies on both the firing rate of the hair cells in the basilar membrane as well as their location within the membrane. In terms of sound localization, both monaural and binaural cues are used to locate where sounds originate in our environment.
Individuals can be born deaf, or they can develop deafness as a result of age, genetic predisposition, and/or environmental causes. Hearing loss that results from a failure of the vibration of the eardrum or the resultant movement of the ossicles is called conductive hearing loss.
Glossary
- basilar membrane
- thin strip of tissue within the cochlea that contains the hair cells which serve as the sensory receptors for the auditory system
- binaural cue
- two-eared cue to localize sound
- cochlea
- fluid-filled, snail-shaped structure that contains the sensory receptor cells of the auditory system
- cochlear implant
- electronic device that consists of a microphone, a speech processor, and an electrode array to directly stimulate the auditory nerve to transmit information to the brain
- conductive hearing loss
- failure in the vibration of the eardrum and/or movement of the ossicles
- congenital deafness
- deafness from birth
- deafness
- partial or complete inability to hear
- hair cell
- auditory receptor cell of the inner ear
- incus
- middle ear ossicle; also known as the anvil
- interaural level difference
- sound coming from one side of the body is more intense at the closest ear because of the attenuation of the sound wave as it passes through the head
- interaural timing difference
- small difference in the time at which a given sound wave arrives at each ear
- malleus
- middle ear ossicle; also known as the hammer
- Ménière's disease
- results in a degeneration of inner ear structures that can lead to hearing loss, tinnitus, vertigo, and an increase in pressure within the inner ear
- monaural cue
- one-eared cue to localize sound
- pinna
- visible part of the ear that protrudes from the head
- place theory of pitch perception
- different portions of the basilar membrane are sensitive to sounds of different frequencies
- sensorineural hearing loss
- failure to transmit neural signals from the cochlea to the brain
- stapes
- middle ear ossicle; also known as the stirrup
- temporal theory of pitch perception
- sound’s frequency is coded by the activity level of a sensory neuron
- tympanic membrane
- eardrum
- vertigo
- spinning sensation
Contributors and Attributions
-
Rose M. Spielman with many significant contributors. The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the creative commons license and may not be reproduced without the prior and express written consent of Rice University. For questions regarding this license, please contact<EMAIL_ADDRESS>.Textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0 license. Download for free at<EMAIL_ADDRESS>.


5.12: Conclusion
“Captive Greece has conquered her rude conqueror,” the Roman poet Horace famously wrote in the late first century BCE. This comment about the deep influence of Greek culture on the Roman world, even after the Roman conquest of Greece was complete, continued to be the case well after the days of Horace. Ultimately, the impact of the Hellenization of the Mediterranean and the Middle East, which started with Alexander’s conquests, lasted far beyond the Hellenistic kingdoms, as the Greek language continued to be the language of the Eastern Roman Empire and, subsequently, the Byzantine Empire up until the conquest of that territory by the Ottomans in 1453 CE. In some respects, this spread of the Greeks and their civilization ultimately changed what it meant to be Greek–or, rather, it created a more universal Greek identity, which largely replaced the polis- specific view of citizenship and identity that existed before Philip’s conquest of Greece. And yet, certain cultural constants persisted."
The first of these was Homer, whose epics continued to be as great an inspiration to the Greeks of the Roman world as they were to their Archaic Age counterparts. For instance, the Homeric values were likely the reason for the minimal advances in military technology in the Greek world, as honor was more important than military success at all cost. The second cultural constant was the work of the philosophers Plato and Aristotle, in whose shadows all subsequent philosophers of the Greco-Roman world labored. Even as the Greek-speaking portions of the Roman Empire turned to Christianity, they could not abandon their philosophical roots, resulting, for instance, in the Gnostic heresies. Horace’s cheeky comment thus proved to be true far longer than he could have expected.


5.2: What is Central Tendency
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Learning Objectives
- Identify situations in which knowing the center of a distribution would be valuable
- Give three different ways the center of a distribution can be defined
- Describe how the balance is different for symmetric distributions than it is for asymmetric distributions
What is "central tendency," and why do we want to know the central tendency of a group of scores? Let us first try to answer these questions intuitively. Then we will proceed to a more formal discussion.
 Today your instructor is walking around the room, handing back the quizzes.  Written in bold black ink on the front is "\(3/5\)." How do you react? Are you happy with your score of \(3\) or disappointed? How do you decide? You might calculate your percentage correct, realize it is \(60\%\), and be appalled. But it is more likely that when deciding how to react to your performance, you will want additional information. What additional information would you like?
If you are like most students, you will immediately ask your neighbors, "Whad'ja get?" and then ask the instructor, "How did the class do?" In other words, the additional information you want is how your quiz score compares to other students' scores. You therefore understand the importance of comparing your score to the class distribution of scores. Should your score of \(3\) turn out to be among the higher scores, then you'll be pleased after all. On the other hand, if \(3\) is among the lower scores in the class, you won't be quite so happy.
This idea of comparing individual scores to a distribution of scores is fundamental to statistics. So let's explore it further, using the same example (the pop quiz you took with your four classmates). Three possible outcomes are shown in Table \(\PageIndex{1}\). They are labeled "\(\text{Dataset A}\)," "\(\text{Dataset B}\)," and "\(\text{Dataset C}\)." Which of the three datasets would make you happiest? In other words, in comparing your score with your fellow students' scores, in which dataset would your score of \(3\) be the most impressive?
In \(\text{Dataset A}\), everyone's score is \(3\). This puts your score at the exact center of the distribution. You can draw satisfaction from the fact that you did as well as everyone else. But of course it cuts both ways: everyone else did just as well as you.
| Student | Dataset A | Dataset B | Dataset C |
|---|---|---|---|
| You | 3 | 3 | 3 |
| John's | 3 | 4 | 2 |
| Maria's | 3 | 4 | 2 |
| Shareecia's | 3 | 4 | 2 |
| Luther's | 3 | 5 | 1 |
Now consider the possibility that the scores are described as in \(\text{Dataset B}\). This is a depressing outcome even though your score is no different than the one in \(\text{Dataset A}\). The problem is that the other four students had higher grades, putting yours below the center of the distribution.
Finally, let's look at \(\text{Dataset C}\). This is more like it! All of your classmates score lower than you so your score is above the center of the distribution.
Now let's change the example in order to develop more insight into the center of a distribution. Figure \(\PageIndex{1}\) shows the results of an experiment on memory for chess positions. Subjects were shown a chess position and then asked to reconstruct it on an empty chess board. The number of pieces correctly placed was recorded. This was repeated for two more chess positions. The scores represent the total number of chess pieces correctly placed for the three chess positions. The maximum possible score was \(89\).
Two groups are compared. On the left are people who don't play chess. On the right are people who play a great deal (tournament players). It is clear that the location of the center of the distribution for the non-players is much lower than the center of the distribution for the tournament players.
We're sure you get the idea now about the center of a distribution. It is time to move beyond intuition. We need a formal definition of the center of a distribution. In fact, we'll offer you three definitions! This is not just generosity on our part. There turn out to be (at least) three different ways of thinking about the center of a distribution, all of them useful in various contexts. In the remainder of this section we attempt to communicate the idea behind each concept. In the succeeding sections we will give statistical measures for these concepts of central tendency.
Definitions of Center
Now we explain the three different ways of defining the center of a distribution. All three are called measures of central tendency.
Balance Scale
One definition of central tendency is the point at which the distribution is in balance. Figure \(\PageIndex{2}\) shows the distribution of the five numbers \(2, 3, 4, 9, 16\) placed upon a balance scale. If each number weighs one pound, and is placed at its position along the number line, then it would be possible to balance them by placing a fulcrum at \(6.8\).
For another example, consider the distribution shown in Figure \(\PageIndex{3}\). It is balanced by placing the fulcrum in the geometric middle.
Figure \(\PageIndex{4}\) illustrates that the same distribution can't be balanced by placing the fulcrum to the left of center.
Figure \(\PageIndex{5}\) shows an asymmetric distribution. To balance it, we cannot put the fulcrum halfway between the lowest and highest values (as we did in Figure \(\PageIndex{3}\)). Placing the fulcrum at the "half way" point would cause it to tip towards the left.
The balance point defines one sense of a distribution's center. The simulation in the next section "Balance Scale Simulation" shows how to find the point at which the distribution balances.
Smallest Absolute Deviation
Another way to define the center of a distribution is based on the concept of the sum of the absolute deviations (differences). Consider the distribution made up of the five numbers \(2, 3, 4, 9, 16\). Let's see how far the distribution is from \(10\) (picking a number arbitrarily). Table \(\PageIndex{2}\) shows the sum of the absolute deviations of these numbers from the number \(10\).
| Values | Absolute Deviations from 10 |
|---|---|
| 2 | 8 |
| 3 | 7 |
| 4 | 6 |
| 9 | 1 |
| 16 | 6 |
| Sum | 28 |
The first row of the table shows that the absolute value of the difference between \(2\) and \(10\) is \(8\); the second row shows that the absolute difference between \(3\) and \(10\) is \(7\), and similarly for the other rows. When we add up the five absolute deviations, we get \(28\). So, the sum of the absolute deviations from \(10\) is \(28\). Likewise, the sum of the absolute deviations from \(5\) equals \(3 + 2 + 1 + 4 + 11 = 21\). So, the sum of the absolute deviations from \(5\) is smaller than the sum of the absolute deviations from \(10\). In this sense, \(5\) is closer, overall, to the other numbers than is \(10\).
We are now in a position to define a second measure of central tendency, this time in terms of absolute deviations. Specifically, according to our second definition, the center of a distribution is the number for which the sum of the absolute deviations is smallest. As we just saw, the sum of the absolute deviations from \(10\) is \(28\) and the sum of the absolute deviations from \(5\) is \(21\). Is there a value for which the sum of the absolute deviations is even smaller than \(21\)? Yes. For these data, there is a value for which the sum of absolute deviations is only \(20\). See if you can find it. A general method for finding the center of a distribution in the sense of absolute deviations is provided in the simulation "Absolute Differences Simulation."
Smallest Squared Deviation
We shall discuss one more way to define the center of a distribution. It is based on the concept of the sum of squared deviations (differences). Again, consider the distribution of the five numbers \(2, 3, 4, 9, 16\). Table \(\PageIndex{3}\) shows the sum of the squared deviations of these numbers from the number \(10\).
| Values | Squared Deviations from 10 |
|---|---|
| 2 | 64 |
| 3 | 49 |
| 4 | 36 |
| 9 | 1 |
| 16 | 36 |
| Sum | 186 |
The first row in the table shows that the squared value of the difference between \(2\) and \(10\) is \(64\); the second row shows that the squared difference between \(3\) and \(10\) is \(49\), and so forth. When we add up all these squared deviations, we get \(186\). Changing the target from \(10\) to \(5\), we calculate the sum of the squared deviations from \(5\) as \(9 + 4 + 1 + 16 + 121 = 151\). So, the sum of the squared deviations from \(5\) is smaller than the sum of the squared deviations from \(10\). Is there a value for which the sum of the squared deviations is even smaller than \(151\)? Yes, it is possible to reach \(134.8\). Can you find the target number for which the sum of squared deviations is \(134.8\)?
The target that minimizes the sum of squared deviations provides another useful definition of central tendency (the last one to be discussed in this section). It can be challenging to find the value that minimizes this sum. You will see how you do it in the upcoming section "Squared Differences Simulation."
Contributor
-
Online Statistics Education: A Multimedia Course of Study ( http://onlinestatbook.com/ ). Project Leader: David M. Lane, Rice University.
- David M. Lane and Heidi Ziemer


17.11.12: Works Consulted and Further Reading
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Adshead, S.A.M., Central Asia in World History . New York: Palgrave, 1993.
Allsen Thomas T., Culture and Conquest in Mongol Eurasia . Cambridge: Cambridge University Press, 2001.
Christian, David. Inner Eurasia from Prehistory to the Mongol Empire. Oxford: Blackwell, 1998.
Findley, Carter Vaughn, The Turks in World History. Oxford: Oxford University Press, 2005.
Halperin, Charles J., Russia and the Golden Horde: The Mongol Impact on Medieval Russian History . Bloomington, IN: Indiana University Press, 1987.
Khan, Paul. ed., The Secret History of the Mongols: The Origin of Chingis Khan. Boston: Cheng and Tsui, 1985.
Morgan, David, The Mongols. Oxford: Wiley-Blackwell, 1996.
Soucek, Svat, A History of Inner Asia . Cambridge: Cambridge University Press, 2000.


6.5: Solution Concentrations and Dilutions
- Define concentration and use the terms concentrated and dilute to describe the relative concentration of a solution.
- Calculate (w/v) percent concentration.
- Determine concentration in molarity.
- Complete dilution calculations.
Solution Concentration
There are several ways to express the amount of solute present in a solution. The concentration of a solution is a measure of the amount of solute that has been dissolved in a given amount of solvent or solution . A concentrated solution is one that has a relatively large amount of dissolved solute. A dilute solution is one that has a relatively small amount of dissolved solute . However, these terms are relative, and we need to be able to express concentration in a more exact, quantitative manner. Still, concentrated and dilute are useful as terms to compare one solution to another (see figure below). Also, be aware that the terms "concentrate" and "dilute" can be used as verbs. If you were to heat a solution, causing the solvent to evaporate, you would be concentrating it, because the ratio of solute to solvent would be increasing. If you were to add more water to an aqueous solution, you would be diluting it because the ratio of solute to solvent would be decreasing.
Weight/Volume Percent
One way to describe the concentration of a solution is by the percent of the solute in a solution. Percent means "parts per hundred". It means the number of parts of solute present every 100 parts of solution. The most commonly used percent measurement is weight/volume percent. The weight/volume percent unit %(w/v), is commonly used for intravenous (IV) fluids. It is calculated by dividing the grams of the solute by the milliliters of the solution and expressing the result as a percent.
For example, if a solution is prepared from \(10. g \: \ce{NaCl}\) in enough water to make a \(150 \: \text{mL}\) solution, the weight/volume percent is
\[\begin{align} \text{weight/volume percent =} & \frac{\text{grams solute}}{\text{volume solution}} \times 100\% \\ &= \frac{10. \: \text{g} \: \ce{NaCl}}{150 \: \text{mL solution}} \times 100\% \\ &= 6.7\% (w/v) \end{align}\]
Molarity
A concentration unit based on moles of solute is molarity. The molarity \(\left( \text{M} \right)\) of a solution is the number of moles of solute dissolved in one liter of solution . To calculate the molarity of a solution, you divide the moles of solute by the volume of the solution expressed in liters.
\[\text{Molarity} \: \left( \text{M} \right) = \frac{\text{moles of solute}}{\text{liters of solution}} = \frac{\text{mol}}{\text{L}}\]
Note that the volume is in liters of solution and not liters of solvent. When a molarity is reported, the unit is the symbol \(\text{M}\), which is read as "molar". For example, a solution labeled as \(1.5 \: \text{M} \: \ce{NH_3}\) is a "1.5 molar solution of ammonia".
A solution is prepared by dissolving \(42.23 \: \text{g}\) of \(\ce{NH_4Cl}\) into enough water to make \(500.0 \: \text{mL}\) of solution. Calculate its molarity.
Solution
Step 1: List the known quantities and plan the problem.
Known
- Mass of \(\ce{NH_4Cl} = 42.23 \: \text{g}\)
- 1 mole of \(\ce{NH_4Cl} = 53.50 \: \text{g}\)
- Volume of solution \(= 500.0 \: \text{mL} = 0.5000 \: \text{L}\)
Unknown
- Molarity \(= ? \: \text{M}\)
The mass of the ammonium chloride is first converted to moles. Then, the molarity is calculated by dividing by liters. Note that the given volume has been converted to liters.
Step 2: Solve.
\[42.23 \: \text{g} \: \ce{NH_4Cl} \times \frac{1 \: \text{mol} \: \ce{NH_4Cl}}{53.50 \: \text{g} \: \ce{NH_4Cl}} = 0.7893 \: \text{mol} \: \ce{NH_4Cl}\]
\[\frac{0.7893 \: \text{mol} \: \ce{NH_4Cl}}{0.5000 \: \text{L}} = 1.579 \: \text{M}\]
Step 3: Think about your result .
The molarity is \(1.579 \: \text{M}\), meaning that a liter of the solution would contain 1.579 moles of \(\ce{NH_4Cl}\). Having four significant figures is appropriate.
Dilutions
When additional water is added to an aqueous solution, the concentration of that solution decreases. Dilution is adding more solvent (water) to a solution. Dilution is important in health care because some drugs must be diluted to proper concentration before being administered. The number of moles of the solute does not change, but the total volume of the solution increases.
An useful equation to use when doing dilutions is
\[C_1 \times V_1 = C_2 \times V_2\]
The concentration can be in any other unit as long as \(C_1\) and \(C_2\) are in the same unit.
Suppose that you have \(100. \: \text{mL}\) of a \(2.0 \: \text{M}\) solution of \(\ce{HCl}\). You dilute the solution by adding enough water to make the solution volume \(500. \: \text{mL}\). The new molarity can easily be calculated by using the above equation and solving for \(C_2\).
\[C_2 = \frac{C_1 \times V_1}{V_2} = \frac{2.0 \: \text{M} \times 100. \: \text{mL}}{500. \: \text{mL}} = 0.40 \: \text{M} \: \ce{HCl}\]
The solution has been diluted by a factor of five, since the new volume is five times as great as the original volume. Consequently, the molarity is one-fifth of its original value. Another common dilution problem involves deciding how much a highly concentrated solution is required to make a desired quantity of solution with a lower concentration. The highly concentrated solution is typically referred to as the stock solution.
Nitric acid \(\left( \ce{HNO_3} \right)\) is a powerful and corrosive acid. When ordered from a chemical supply company, its molarity is \(16 \: \text{M}\). How much of the stock solution of nitric acid needs to be used to make \(8.00 \: \text{L}\) of a \(0.50 \: \text{M}\) solution?
Solution
Step 1: List the known quantities and plan the problem.
Known
- Stock \(\ce{HNO_3} \: \left( C_1 \right) = 16 \: \text{M}\)
- \(V_2 = 8.00 \: \text{L}\)
- \(C_2 = 0.50 \: \text{M}\)
Unknown
- Volume of stock \(\ce{HNO_3} \: \left( V_1 \right) = ? \: \text{L}\)
The unknown in the equation is \(V_1\), the necessary volume of the concentrated stock solution.
Step 2: Solve.
\[V_1 = \frac{C_2 \times V_2}{V_1} = \frac{0.50 \: \text{M} \times 8.00 \: \text{L}}{16 \: \text{M}} = 0.25 \: \text{L} = 250 \: \text{mL}\]
Step 3: Think about your result.
\(250 \: \text{mL}\) of the stock \(\ce{HNO_3}\) solution needs to be diluted with water to a final volume of \(8.00 \: \text{L}\). The dilution from \(16 \: \text{M}\) to \(0.5 \: \text{M}\) is a factor of 32.
Contributors and Attributions
- Deboleena Roy, Ph.D. (Deparment of Science and Engineering @ American River College)
-
Allison Soult , Ph.D. (Department of Chemistry, University of Kentucky)


5.5: Writing Formulas for Ionic Compounds
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Learning Objectives
- Write the correct formula for an ionic compound.
- Recognize polyatomic ions in chemical formulas.
Ionic compounds do not exist as molecules. In the solid state, ionic compounds are in crystal lattice containing many ions each of the cation and anion. An ionic formula, like \(\ce{NaCl}\), is an empirical formula. This formula merely indicates that sodium chloride is made of an equal number of sodium and chloride ions. Sodium sulfide, another ionic compound, has the formula \(\ce{Na_2S}\). This formula indicates that this compound is made up of twice as many sodium ions as sulfide ions. This section will teach you how to find the correct ratio of ions, so that you can write a correct formula.
If you know the name of a binary ionic compound, you can write its chemical formula . Start by writing the metal ion with its charge, followed by the nonmetal ion with its charge. Because the overall compound must be electrically neutral, decide how many of each ion is needed in order for the positive and negative charges to cancel each other out.
Example \(\PageIndex{1}\): Aluminum Nitride and Lithium Oxide
Write the formulas for aluminum nitride and lithium oxide.
Solution
| Write the formula for aluminum nitride | Write the formula for lithium oxide | |
|---|---|---|
| 1. Write the symbol and charge of the cation (metal) first and the anion (nonmetal) second. | \(\ce{Al^{3+}} \: \: \: \: \: \ce{N^{3-}}\) | \(\ce{Li^+} \: \: \: \: \: \ce{O^{2-}}\) |
| 2. Use a multiplier to make the total charge of the cations and anions equal to each other. |
total charge of cations = total charge of anions 1(3+) = 1(3-) +3 = -3 |
total charge of cations = total charge of anions 2(1+) = 1(2-) +2 = -2 |
| 3. Use the multipliers as subscript for each ion. | \(\ce{Al_1N_1}\) | \(\ce{Li_2O_1}\) |
| 4. Write the final formula. Leave out all charges and all subscripts that are 1. | \(\ce{AlN}\) | \(\ce{Li_2O}\) |
An alternative way to writing a correct formula for an ionic compound is to use the crisscross method . In this method, the numerical value of each of the ion charges is crossed over to become the subscript of the other ion. Signs of the charges are dropped.
Example \(\PageIndex{2}\): The Crisscross Method for Lead (IV) Oxide
Write the formula for lead (IV) oxide.
Solution
Exercise \(\PageIndex{2}\)
Write the chemical formula for an ionic compound composed of each pair of ions.
- the calcium ion and the oxygen ion
- the 2+ copper ion and the sulfur ion
- the 1+ copper ion and the sulfur ion
- Answer a:
- CaO
- Answer b:
- CuS
- Answer c:
- Cu 2 S
Be aware that ionic compounds are empirical formulas and so must be written as the lowest ratio of the ions.
Example \(\PageIndex{3}\): Sulfur Compound
Write the formula for sodium combined with sulfur.
Solution
Exercise \(\PageIndex{3}\)
Write the formula for each ionic compound.
- sodium bromide
- lithium chloride
- magnesium oxide
- Answer a:
- NaBr
- Answer b:
- LiCl
- Answer c:
- MgO
Polyatomic Ions
Some ions consist of groups of atoms bonded together and have an overall electric charge. Because these ions contain more than one atom, they are called polyatomic ions. Polyatomic ions have characteristic formulas, names, and charges that should be memorized. For example, NO 3 − is the nitrate ion; it has one nitrogen atom and three oxygen atoms and an overall 1− charge. Table \(\PageIndex{1}\) lists the most common polyatomic ions.
| Name | Formula |
|---|---|
| ammonium ion | NH 4 + |
| acetate ion | C 2 H 3 O 2 − (also written CH 3 CO 2 − ) |
| carbonate ion | CO 3 2 − |
| chromate ion | CrO 4 2 − |
| dichromate ion | Cr 2 O 7 2 − |
| hydrogen carbonate ion (bicarbonate ion) | HCO 3 − |
| cyanide ion | CN − |
| hydroxide ion | OH − |
| nitrate ion | NO 3 − |
| nitrite ion | NO 2 − |
| permanganate ion | MnO 4 − |
| phosphate ion | PO 4 3 − |
| hydrogen phosphate ion | HPO 4 2 − |
| dihydrogen phosphate ion | H 2 PO 4 − |
| sulfate ion | SO 4 2 − |
| hydrogen sulfate ion (bisulfate ion) | HSO 4 − |
| sulfite ion | SO 3 2 − |
The rule for constructing formulas for ionic compounds containing polyatomic ions is the same as for formulas containing monatomic (single-atom) ions: the positive and negative charges must balance. If more than one of a particular polyatomic ion is needed to balance the charge, the entire formula for the polyatomic ion must be enclosed in parentheses, and the numerical subscript is placed outside the parentheses. This is to show that the subscript applies to the entire polyatomic ion. An example is Ba(NO 3 ) 2 .
Writing Formulas for Ionic Compounds Containing Polyatomic Ions
Writing a formula for ionic compounds containing polyatomic ions also involves the same steps as for a binary ionic compound. Write the symbol and charge of the cation followed by the symbol and charge of the anion.
Example \(\PageIndex{4}\): Calcium Nitrate
Write the formula for calcium nitrate.
Solution
| Crisscross Method | Write the formula for calcium nitrate |
|---|---|
| 1. Write the symbol and charge of the cation (metal) first and the anion (nonmetal) second. | \(\ce{Ca^{2+}} \: \: \: \: \: \ce{NO_3^-}\) |
| 2. Transpose only the number of the positive charge to become the subscript of the anion and the number only of the negative charge to become the subscript of the cation. | |
| 3. Reduce to the lowest ratio. | \(\ce{Ca_1(NO_3)_2}\) |
| 4. Write the final formula. Leave out all subscripts that are 1. If there is only 1 of the polyatomic ion, leave off parentheses. | \(\ce{Ca(NO_3)_2}\) |
Example \(\PageIndex{5}\)
Write the chemical formula for an ionic compound composed of the potassium ion and the sulfate ion.
Solution
| Explanation | Answer |
|---|---|
| Potassium ions have a charge of 1+, while sulfate ions have a charge of 2−. We will need two potassium ions to balance the charge on the sulfate ion, so the proper chemical formula is \(\ce{K2SO4}\). | \(\ce{K_2SO_4}\) |
Exercise \(\PageIndex{5}\)
Write the chemical formula for an ionic compound composed of each pair of ions.
- the magnesium ion and the carbonate ion
- the aluminum ion and the acetate ion
- Answer a:
- \(\ce{MgCO3}\)
- Answer b:
- \(\ce{Al(CH3COO)3}\)
Recognizing Ionic Compounds
There are two ways to recognize ionic compounds.
Method 1
Compounds between metal and nonmetal elements are usually ionic. For example, \(\ce{CaBr2}\) contains a metallic element (calcium, a group 2 [or 2A] metal) and a nonmetallic element (bromine, a group 17 [or 7A] nonmetal). Therefore, it is most likely an ionic compound (in fact, it is ionic). In contrast, the compound \(\ce{NO2}\) contains two elements that are both nonmetals (nitrogen, from group 15 [or 5A] , and oxygen, from group 16 [or 6A] . It is not an ionic compound; it belongs to the category of covalent compounds discussed elsewhere. Also note that this combination of nitrogen and oxygen has no electric charge specified, so it is not the nitrite ion.
Method 2
Second, if you recognize the formula of a polyatomic ion in a compound, the compound is ionic. For example, if you see the formula \(\ce{Ba(NO3)2}\), you may recognize the “\(\ce{NO3}\)” part as the nitrate ion, \(\ce{NO3^{-}}\). (Remember that the convention for writing formulas for ionic compounds is not to include the ionic charge.) This is a clue that the other part of the formula, \(\ce{Ba}\), is actually the \(\ce{Ba^{2+}}\) ion, with the 2+ charge balancing the overall 2− charge from the two nitrate ions. Thus, this compound is also ionic.
Example \(\PageIndex{6}\)
Identify each compound as ionic or not ionic.
- \(\ce{Na2O}\)
- \(\ce{PCl3}\)
- \(\ce{NH4Cl}\)
- \(\ce{OF2}\)
Solution
| Explanation | Answer |
|---|---|
| a. Sodium is a metal, and oxygen is a nonmetal. Therefore, \(\ce{Na2O}\) is expected to be ionic via method 1. | \(\ce{Na2O}\), ionic |
| b. Both phosphorus and chlorine are nonmetals. Therefore, \(\ce{PCl3}\) is not ionic via method 1 | \(\ce{PCl3}\), not ionic |
| c. The \(\ce{NH4}\) in the formula represents the ammonium ion, \(\ce{NH4^{+}}\), which indicates that this compound is ionic via method 2 | \(\ce{NH4Cl}\), ionic |
| d. Both oxygen and fluorine are nonmetals. Therefore, \(\ce{OF2}\) is not ionic via method 1 | \(\ce{OF2}\), not ionic |
Exercise \(\PageIndex{6}\)
Identify each compound as ionic or not ionic.
- \(\ce{N2O}\)
- \(\ce{FeCl3}\)
- \(\ce{(NH4)3PO4}\)
- \(\ce{SOCl2}\)
- Answer a:
- not ionic
- Answer b:
- ionic
- Answer c:
- ionic
- Answer d:
- not ionic
Summary
Formulas for ionic compounds contain the symbols and number of each atom present in a compound in the lowest whole number ratio.


7.3: Part 1- Dichotomous Keys
A dichotomous key is an identification key that consists of a series of choices that ultimately lead to the identification of the object in question. In each step of a dichotomous key, the user has two choices and must pick the one that most correctly describes the object. Each choice leads to a new set of choices. To help you understand how a key like this works, consider the following four insects:
After studying the insects, you might classify the insects by wing covering, body shape, and the direction in which the wings point.
To begin the key, you could start separating the four insects based on wing covering— ”flight wings covered by exoskeleton” vs. “flight wings not covered by an exoskeleton.”
 After making a key, you should end up with one less step than the total number of organisms you are trying to identify.
When constructing a key, keep the following in mind:
- Use constant characteristics rather than variable ones.
- Use measurements rather than terms like “large” and “small.”
- Use characteristics that are generally available to the user of the key rather than seasonal characteristics or those seen only in the field.
- Make the choice a positive one—something “is” instead of “is not.”
- If possible, start both choices of a pair with the same word.
- If possible, start different pairs of choices with different words.
When using a key, keep the following in mind:
- Always read both choices, even if the first seems to be the logical one.
- Be sure you understand the meaning of the terms involved. Do Not Guess.
- When measurements are given, use a calibrated scale. Do Not Guess.
- Since living things are always somewhat variable, do not base your conclusion on a single observation. Study several specimens to be sure your specimen is typical.
- If the choice is not clear, for whatever reason, try both divisions. If you end up with two possible answers, read descriptions of the two choices to help you decide.
- Having arrived at an answer in a key, do not accept this as absolutely reliable. Check a description of the organism to see if it agrees with the unknown specimen. If not, there is an error somewhere, either in the key or in its use. For a scientist, the ultimate check of identification is a comparison of the unknown with an authentically named “Type Specimen.”
Procedure
- Examine the 6 specimens from school collections or organisms you collected and use them to construct your own dichotomous key here (use the example above as a template):
-
Use someone else’s key to identify 2 specimens. List your path to identification here (letters are fine):
- Specimen 1
- Specimen 2


8.2: Gas Pressure
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Learning Objectives
By the end of this section, you will be able to:
- Define the property of pressure
- Define and convert among the units of pressure measurements
- Describe the operation of common tools for measuring gas pressure
The earth’s atmosphere exerts a pressure, as does any other gas. Although we do not normally notice atmospheric pressure, we are sensitive to pressure changes—for example, when your ears “pop” during take-off and landing while flying, or when you dive underwater. Gas pressure is caused by the force exerted by gas molecules colliding with the surfaces of objects (Figure \(\PageIndex{1}\)). Although the force of each collision is very small, any surface of appreciable area experiences a large number of collisions in a short time, which can result in a high pressure. In fact, normal air pressure is strong enough to crush a metal container when not balanced by equal pressure from inside the container.
Link to Learning
A dramatic illustration of atmospheric pressure is provided in this brief video, which shows a railway tanker car imploding when its internal pressure is decreased.
A smaller scale demonstration of this phenomenon is briefly explained.
Atmospheric pressure is caused by the weight of the column of air molecules in the atmosphere above an object, such as the tanker car. At sea level, this pressure is roughly the same as that exerted by a full-grown African elephant standing on a doormat, or a typical bowling ball resting on your thumbnail. These may seem like huge amounts, and they are, but life on earth has evolved under such atmospheric pressure. If you actually perch a bowling ball on your thumbnail, the pressure experienced is twice the usual pressure, and the sensation is unpleasant.
In general, pressure is defined as the force exerted on a given area: \(P=\frac{F}{A}). Note that pressure is directly proportional to force and inversely proportional to area. Thus, pressure can be increased either by increasing the amount of force or by decreasing the area over which it is applied; pressure can be decreased by decreasing the force or increasing the area.
Let’s apply this concept to determine which exerts a greater pressure in Figure \(\PageIndex{2}\) —the elephant or the figure skater? A large African elephant can weigh 7 tons, supported on four feet, each with a diameter of about 1.5 ft (footprint area of 250 in 2 ), so the pressure exerted by each foot is about 14 lb/in 2 :
\[\text { pressure per elephant foot }=14,000 \frac{ lb }{\text { elephant }} \times \frac{1 \text { elephant }}{4 \text { feet }} \times \frac{1 \text { foot }}{250 in^2}=14 lb / in^2 \nonumber \]
The figure skater weighs about 120 lbs, supported on two skate blades, each with an area of about 2 in 2 , so the pressure exerted by each blade is about 30 lb/in 2 :
\[\text { pressure per skate blade }=120 \frac{ lb }{\text { skater }} \times \frac{1 \text { skater }}{2 \text { blades }} \times \frac{1 \text { blade }}{2 \text { in }^2}=30 lb / in^2 \nonumber \]
Even though the elephant is more than one hundred-times heavier than the skater, it exerts less than one-half of the pressure. On the other hand, if the skater removes their skates and stands with bare feet (or regular footwear) on the ice, the larger area over which their weight is applied greatly reduces the pressure exerted:
\[\text { pressure per human foot }=120 \frac{ lb }{\text { skater }} \times \frac{1 \text { skater }}{2 \text { feet }} \times \frac{1 \text { foot }}{30 in^2}=2 lb / in^2 \nonumber \]
The SI unit of pressure is the pascal (Pa) , with 1 Pa = 1 N/m 2 , where N is the newton, a unit of force defined as 1 kg m/s 2 . One pascal is a small pressure; in many cases, it is more convenient to use units of kilopascal (1 kPa = 1000 Pa) or bar (1 bar = 100,000 Pa). In the United States, pressure is often measured in pounds of force on an area of one square inch— pounds per square inch (psi) —for example, in car tires. Pressure can also be measured using the unit atmosphere (atm) , which originally represented the average sea level air pressure at the approximate latitude of Paris (45°). Table \(\PageIndex{1}\) provides some information on these and a few other common units for pressure measurements
| Unit Name and Abbreviation | Definition or Relation to Other Unit |
|---|---|
| pascal (Pa) |
1 Pa = 1 N/m
2
recommended IUPAC unit |
| kilopascal (kPa) | 1 kPa = 1000 Pa |
| pounds per square inch (psi) | air pressure at sea level is ~14.7 psi |
| atmosphere (atm) |
1 atm = 101,325 Pa = 760 torr
air pressure at sea level is ~1 atm |
| bar (bar, or b) |
1 bar = 100,000 Pa (exactly)
commonly used in meteorology |
| millibar (mbar, or mb) | 1000 mbar = 1 bar |
| inches of mercury (in. Hg) |
1 in. Hg = 3386 Pa
used by aviation industry, also some weather reports |
| torr |
named after Evangelista Torricelli, inventor of the barometer |
| millimeters of mercury (mm Hg) | 1 mm Hg ~1 torr |
Example \(\PageIndex{1}\): Conversion of Pressure Units
The United States National Weather Service reports pressure in both inches of Hg and millibars. Convert a pressure of 29.2 in. Hg into:
- torr
- atm
- kPa
- mbar
Solution
This is a unit conversion problem. The relationships between the various pressure units are given in Table \(\PageIndex{1}\).
- \[29.2 \text { in } Hg_{ g } \times \frac{25.4 mm }{1 im } \times \frac{1 \text { torr }}{1 mm Hg_{ g }}=742 \text { torr } \nonumber \]
- \[742 \text { torr } \times \frac{1 atm }{760 \text { torr }}=0.976 atm \nonumber \]
- \[742 \text { torr } \times \frac{101.325 kPa }{760 \text { torr }}=98.9 kPa \nonumber \]
- \[98.9 kPa \times \frac{1000 Pa }{1 kPa } \times \frac{1 bar }{100,000 Pa } \times \frac{1000 mbar }{1 bar }=989 mbar \nonumber \]
Exercise \(\PageIndex{1}\)
A typical barometric pressure in Kansas City is 740 torr. What is this pressure in atmospheres, in millimeters of mercury, in kilopascals, and in bar?
- Answer
-
0.974 atm; 740 mm Hg; 98.7 kPa; 0.987 bar
We can measure atmospheric pressure, the force exerted by the atmosphere on the earth’s surface, with a barometer (Figure \(\PageIndex{3}\)). A barometer is a glass tube that is closed at one end, filled with a nonvolatile liquid such as mercury, and then inverted and immersed in a container of that liquid. The atmosphere exerts pressure on the liquid outside the tube, the column of liquid exerts pressure inside the tube, and the pressure at the liquid surface is the same inside and outside the tube. The height of the liquid in the tube is therefore proportional to the pressure exerted by the atmosphere.
If the liquid is water, normal atmospheric pressure will support a column of water over 10 meters high, which is rather inconvenient for making (and reading) a barometer. Because mercury (Hg) is about 13.6-times denser than water, a mercury barometer only needs to be \(\frac{1}{13.6}}\) as tall as a water barometer—a more suitable size. Standard atmospheric pressure of 1 atm at sea level (101,325 Pa) corresponds to a column of mercury that is about 760 mm (29.92 in.) high. The torr was originally intended to be a unit equal to one millimeter of mercury, but it no longer corresponds exactly. The pressure exerted by a fluid due to gravity is known as hydrostatic pressure , \(p\):
\[p=h \rho g \nonumber \]
where h is the height of the fluid, ρ (lowercase Greek letter rho) is the density of the fluid, and g is acceleration due to gravity.
Example \(\PageIndex{2}\): Calculation of Barometric Pressure
Show the calculation supporting the claim that atmospheric pressure near sea level corresponds to the pressure exerted by a column of mercury that is about 760 mm high. The density of mercury = 13.6 g/cm 3 .
Solution
The hydrostatic pressure is given by p = hρg , with h = 760 mm, ρ = 13.6 g/cm 3 , and g = 9.81 m/s 2 . Plugging these values into the equation and doing the necessary unit conversions will give us the value we seek. (Note: We are expecting to find a pressure of ~101,325 Pa.)
\[101,325 N / m^2=101,325 \frac{ kg \cdot m / s^2}{ m^2}=101,325 \frac{ kg }{ m \cdot s^2} \nonumber \]
\[\begin{gathered}
p=\left(760 mm \times \frac{1 m }{1000 mm }\right) \times\left(\frac{13.6 g }{1 cm^3} \times \frac{1 kg }{1000 g } \times \frac{(100 cm )^3}{(1 m )^3}\right) \times\left(\frac{9.81 m }{1 s^2}\right) \\[4pt]
=(0.760 m )\left(13,600 kg / m^3\right)\left(9.81 m / s^2\right)=1.01 \times 10^5 kg / ms^2=1.01 \times 10^5 N / m^2 \\[4pt]
=1.01 \times 10^5 Pa
\end{gathered} \nonumber \]
Exercise \(\PageIndex{2}\)
Calculate the height of a column of water at 25 °C that corresponds to normal atmospheric pressure. The density of water at this temperature is 1.0 g/cm 3 .
- Answer
-
10.3 m
A manometer is a device similar to a barometer that can be used to measure the pressure of a gas trapped in a container. A closed-end manometer is a U-shaped tube with one closed arm, one arm that connects to the gas to be measured, and a nonvolatile liquid (usually mercury) in between. As with a barometer, the distance between the liquid levels in the two arms of the tube ( h in the diagram) is proportional to the pressure of the gas in the container. An open-end manometer (Figure \(\PageIndex{4}\)) is the same as a closed-end manometer, but one of its arms is open to the atmosphere. In this case, the distance between the liquid levels corresponds to the difference in pressure between the gas in the container and the atmosphere.
Example \(\PageIndex{3}\): Calculation of Pressure Using a Closed-End Manometer
The pressure of a sample of gas is measured with a closed-end manometer, as shown to the right. The liquid in the manometer is mercury. Determine the pressure of the gas in:
- torr
- Pa
- bar
Solution
The pressure of the gas is equal to a column of mercury of height 26.4 cm. (The pressure at the bottom horizontal line is equal on both sides of the tube. The pressure on the left is due to the gas and the pressure on the right is due to 26.4 cm Hg, or mercury.) We could use the equation \(p = hρg\) as in Example \(\PageIndex{2}\), but it is simpler to just convert between units using Table \(\PageIndex{1}\).
- \[26.4 em Hg_{ g } \times \frac{10 mm Hg_{ g }}{1 emHg } \times \frac{1 \text { torr }}{1 mm \Pi Hg }=264 \text { torr } \nonumber \]
- \[264 \text { torr } \times \frac{1 \text { atm }}{760 \text { torr }} \times \frac{101,325 Pa }{1 \text { atm }}=35,200 Pa \nonumber \]
- \[35,200 Pa \times \frac{1 bar }{100,000 Pa }=0.352 bar \nonumber \]
Exercise \(\PageIndex{3}\)
The pressure of a sample of gas is measured with a closed-end manometer. The liquid in the manometer is mercury. Determine the pressure of the gas in:
- torr
- Pa
- bar
- Answer
-
(a) ~150 torr; (b) ~20,000 Pa; (c) ~0.20 bar
Example \(\PageIndex{4}\): Calculation of Pressure Using an Open-End Manometer
The pressure of a sample of gas is measured at sea level with an open-end Hg (mercury) manometer, as shown to the right. Determine the pressure of the gas in:
- mm Hg
- atm
- kPa
Solution
The pressure of the gas equals the hydrostatic pressure due to a column of mercury of height 13.7 cm plus the pressure of the atmosphere at sea level. (The pressure at the bottom horizontal line is equal on both sides of the tube. The pressure on the left is due to the gas and the pressure on the right is due to 13.7 cm of Hg plus atmospheric pressure.)
- In mm Hg, this is: 137 mm Hg + 760 mm Hg = 897 mm Hg
- \[897 mm Hg_{ g } \times \frac{1 atm }{760 mm Hg_{ g }}=1.18 atm \nonumber \]
- \[1.18 \text { atm } \times \frac{101.325 kPa }{1-20 \times 10^2 kPa }=1.20 \nonumber \]
Exercise \(\PageIndex{4}\)
The pressure of a sample of gas is measured at sea level with an open-end Hg manometer, as shown to the right. Determine the pressure of the gas in:
- mm Hg
- atm
- kPa
- Answer
-
(a) 642 mm Hg; (b) 0.845 atm; (c) 85.6 kPa
Chemistry in Everyday Life: Measuring Blood Pressure
Blood pressure is measured using a device called a sphygmomanometer (Greek sphygmos = “pulse”). It consists of an inflatable cuff to restrict blood flow, a manometer to measure the pressure, and a method of determining when blood flow begins and when it becomes impeded (Figure \(\PageIndex{5}\)). Since its invention in 1881, it has been an essential medical device. There are many types of sphygmomanometers: manual ones that require a stethoscope and are used by medical professionals; mercury ones, used when the most accuracy is required; less accurate mechanical ones; and digital ones that can be used with little training but that have limitations. When using a sphygmomanometer, the cuff is placed around the upper arm and inflated until blood flow is completely blocked, then slowly released. As the heart beats, blood forced through the arteries causes a rise in pressure. This rise in pressure at which blood flow begins is the systolic pressure— the peak pressure in the cardiac cycle. When the cuff’s pressure equals the arterial systolic pressure, blood flows past the cuff, creating audible sounds that can be heard using a stethoscope. This is followed by a decrease in pressure as the heart’s ventricles prepare for another beat. As cuff pressure continues to decrease, eventually sound is no longer heard; this is the diastolic pressure— the lowest pressure (resting phase) in the cardiac cycle. Blood pressure units from a sphygmomanometer are in terms of millimeters of mercury (mm Hg).
How Sciences Interconnect: Meteorology, Climatology, and Atmospheric Science
Throughout the ages, people have observed clouds, winds, and precipitation, trying to discern patterns and make predictions: when it is best to plant and harvest; whether it is safe to set out on a sea voyage; and much more. We now face complex weather and atmosphere-related challenges that will have a major impact on our civilization and the ecosystem. Several different scientific disciplines use chemical principles to help us better understand weather, the atmosphere, and climate. These are meteorology, climatology, and atmospheric science. Meteorology is the study of the atmosphere, atmospheric phenomena, and atmospheric effects on earth’s weather. Meteorologists seek to understand and predict the weather in the short term, which can save lives and benefit the economy. Weather forecasts (Figure \(\PageIndex{6}\)) are the result of thousands of measurements of air pressure, temperature, and the like, which are compiled, modeled, and analyzed in weather centers worldwide.
In terms of weather, low-pressure systems occur when the earth’s surface atmospheric pressure is lower than the surrounding environment: Moist air rises and condenses, producing clouds. Movement of moisture and air within various weather fronts instigates most weather events.
The atmosphere is the gaseous layer that surrounds a planet. Earth’s atmosphere, which is roughly 100–125 km thick, consists of roughly 78.1% nitrogen and 21.0% oxygen, and can be subdivided further into the regions shown in Figure \(\PageIndex{7}\) the exosphere (furthest from earth, > 700 km above sea level), the thermosphere (80–700 km), the mesosphere (50–80 km), the stratosphere (second lowest level of our atmosphere, 12–50 km above sea level), and the troposphere (up to 12 km above sea level, roughly 80% of the earth’s atmosphere by mass and the layer where most weather events originate). As you go higher in the troposphere, air density and temperature both decrease.
Climatology is the study of the climate, averaged weather conditions over long time periods, using atmospheric data. However, climatologists study patterns and effects that occur over decades, centuries, and millennia, rather than shorter time frames of hours, days, and weeks like meteorologists. Atmospheric science is an even broader field, combining meteorology, climatology, and other scientific disciplines that study the atmosphere.