The mass of a substance – solid, liquid or gas – is measured with a balance. This mass balance ... when the chemical reaction produces a gas, eg metal reacting with acid to produce hydrogen.

Using the results of experiments like these, the average rate of the reaction can be calculated. No matter which quantity is measured during the course of a reaction, the average rate of reaction ...

The Law of Conservation of Mass dates from Antoine Lavoisier's 1789 discovery that mass is neither created nor destroyed in chemical reactions ... by conducting a mass balance.