Observed in Illinois on the first Monday of March every year, the holiday ostensibly honors Polish-born Revolutionary War cavalry officer Casimir Pulaski, though for most school kids with a four-day week, itâs probably just another sweet, sweet sleep-in day.
Yes, in chemistry a pure substance means either an element or a compound. Thus, hydrogen, oxygen, sodium, iron, copper etc. are pure substances; and so are compounds such as water, sugar, ammonia, sodium chloride, methane, ethanol etc. A compound is composed of two or more elements combined in a fixed ratio by mass. Like an element, a compound also has a distinct chemical identity. A mixture is not a pure substance as it is a collection of two or more elements/compounds in varying ratio. Air, sea water, gasoline, vegetable oils, steel etc. are mixtures.
Free ammonia is a chemical compound of nitrogen and hydrogen with the formula NH3. Technically ammonia in water is called Ammonium hydroxide. In the aquarium hobby the term ammonia also refers to an ionized form called ammonium (NH4+). These are held in equilibrium in the water [Citation needed]. 'Total ammonia' (TA) or 'Total ammonia nitrogen' (TAN) refers to the total concentration of both NH3 and NH4+ in the aquarium water.