Understanding Chemical Reactions

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SWBAT model, identify and balance chemical reactions demonstrating the Law of Conservation of Mass.

Big Idea

Explore how to read a chemical equation and count atoms.


This is the third lesson in the unit that covers HS-PS1-7, the uses of mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction; and DCI-PS1.B, the fact that atoms are conserved, together with knowledge of the chemical properties of the elements involved, can be used to describe and predict chemical reactions. Students are introduced to how to write a chemical equation, chemical equation terminology, balancing and the law of conservation of mass. 

This lesson also teaches students how to develop scientific questions by editing questions to be open- and closed-ended questions.  This type of questioning is a useful technique that improves critical-thinking skills, in addition to promoting ownership of learning.  In addition to the Scientific Practice of Questioning (SP1) students will engage in the Scientific Practice of Modeling (SP2) when writing chemical equations. 

During this lesson students will recognize that Energy and Matter is an underlying NGSS Crosscutting Concept involved in all chemical reactions.  This is not the main focus of this lesson, but as students continue to develop an understanding of chemical reactions throughout the unit they will build on the idea that matter and energy are conserved in chemical reactions. 

Group Discussion

20 minutes

At the end of the previous lesson students were introduced to the concept of open- and closed-ended questions.  After observing a demonstration they were asked to write down questions based on their observations and categorize them as either open- or closed-ended.  The purpose of this was to create an understanding that asking questions are an important part of science that can be used to generate curiosity and understanding about a concept or phenomenon.

At the beginning of class students are asked to take out their questions from the previous day.  These questions should have been categorized as an open- or closed-ended question.  Today they will practice changing their questions from open- or closed-ended and vice versa.  The purpose of this is to have students understand that the way a question is phrased can have a significant impact on the amount of information that is obtained. To get them start I provide them with an example of a open- and closde-ended

  • What would happen to the gummy bear if the potassium chlorate wasn’t melted? (open-ended)  The reaction would not have the same net result because the potassium perchlorate did not go through a chemical change.
  • Would the same result be achieved if the potassium chlorate was not heated? (closed-ended) No.

After several minutes of re-writing their questions, I ask each student to pair-up with their lab partner or partners (groups of 2 or 3).   I then ask them to share their original questions with one another and prioritize their questions as to what would be the most effective questions for an investigation.  Upon completion each group will put their top two questions on the board for a class discussion.

During the class discussion we focus on the idea that open-ended questions can be used to obtain more information, but often times can lead to other questions and more information.  I ask students what question would work best for a testable investigation. 

Some student examples I might use:

  1. How does the gummy bear sacrifice reaction work?
  2. Is heating potassium perchlorate necessary for the reaction to occur?

Students should say the question 2 is more testable because it is very specific and can be investigated to achieve a yes or no.  It should also be said that question 1 is a more effective research question that could lead to question 2 after information was obtained. 

Eventually as the discussion progresses, students should understand that as information is obtained, convergent questioning should occur that achieves more specific answers.

To conclude this activity, it is important to have students reflect on what was learned.  After the group discussion I ask students answer the following two questions in their journals: (1) what did you learn? and (2) how did you learn that?  As students are working on this I am walking around seeing what they have learned.  This gives me quick feedback that can be used to assess further teaching about questioning.  

Explain (notes)

20 minutes

Chemical Equation Notes

At some point during the discussion about open- and closed-ended questions I put both unbalanced chemical equations involved in the gummy bear sacrifice on the board. This is easily accomplished because students always ask about the reaction.  I use unbalanced equations because this leads into talking about conservation of mass and balancing later in the lesson.  

___KClO3(s) Δ→ ___ KCl(s) +  ___O2(g)

___C12H22O+   ___ O2  -->     ____CO2   +  ____ H2O       ΔH = -5635 kJ

I use these two equation to provide notes on how to read a chemical equation.  I start out the notes by asking students if they have any questions about the chemical equations that they see on the board.  This leads to student asking about the Δ above the arrow in the decomposition of potassium chlorate. 

At this point I will begin to label the reactants and products on the chemical equations, followed by filling in the table and explaining what each symbol represents. I only spend 10 minutes on this because I want to leave time to let students count atoms and begin to balance equations.


Formulas/symbols left of -->


Formulas/symbols right of -->



Yields (forms)


Used to separate 2 reactants or 2 products 


Reaction is reversible


Solid state


Liquid state


Gas state


Aqueous state (dissolved in water)

Heat OR â →

Heat is applied to reaction


Catalyst used in reaction (speeds up reaction)

ΔH = -5635 kJ 

exothermic (negative value)

ΔH = 5635 kJ 

endothermic (positive value)

Counting and Balancing Notes

While they are taking notes on terminology, a student inevitably notices that the number of atoms are not equal on the reactant and product side, and ask about why this is.  This leads into the law of conservation of mass, counting atoms, coefficients, subscripts and balancing equations. 

At this point I label the subscript and coefficient on the chemical equations and provide the following notes on the board: 

The law of conservation of mass MUST be satisfied.

a. The atoms (mass) on the left side (reactants) must EQUAL the atoms (mass) on the right side (products)

b. You must use coefficients once the chemical formula has been written to change the amount of atoms. NEVER CHANGE A FORMULA (subscripts) YOU KNOW IS RIGHT.

I follow this by asking how many of each atom, both reactants and products, are present in each of the unbalance equations?  I write the answer under the equations on the board


 KClO3(s) →  KCl(s) +  O2(g)

K=1  Cl=1   O=3  à K=1  Cl=1 and O=2

C12H22O11  +    O2  -->     CO2   +  H2O   ΔH = -5635 kJ

C=12  H=22  O=13   à  C=1  H=2  O= 3


I then ask, “is mass (atoms) conserved in these equations?”  Which they clearly see that it is not.

At this point I balance the equations and recount the number of atoms.


K=2  Cl=2   O=6  --> K=2  Cl=2  O=6

C12H22O11   +    12O2  -->     12CO2   +  11H2O   ΔH = -5635 kJ

C=12  H=22  O=35  -->  C=12  H=22  O= 35

I don’t show them how to balance until the next lesson, I just show them that coefficients are used to balance equation to satisfy the Law of Conservation of Mass.  The main objectives here is to show students that subscripts are not changed to balance equation and how to count atoms.  They should understand that coefficients only multiply atoms with that formula not one that is separated by a +.  

Practice (homework)

10 minutes

As a reinforcement (Counting atoms practice) students will count the atoms (reactant and products) in a variety of chemical equations and state whether the Law of Conservation of Mass is satisfied (balanced).  I like using both chemical equations and formulas because it make for an easier transition into balancing. It also exposes them to different types of chemical reactions, even though this is not the main objective.  

Must students understand counting atoms, but I believe that it's important to assist students to make sure that all students understand.  If they are struggling with counting they will more than likely struggle with balancing, so a little assistance goes a long way. They will have the remained of the class to work on this and will be checked in the following class period.