The mass of the reactants must always equal the mass of the products.  However, please note that this law works only when the matter is not approaching the speed of light.  Once the mass goes faster it will transform into forms of energy, which we will discuss in later chapters (Law of Conservation of Energy (or First Law of Thermodynamics).   Hence, in nuclear reactions seen in large astronomical objects throughout the Universe this law may not conform.

In this lab, you will test this rule by performing several chemical reactions and see if the end results are still the same mass.   Key in these experiments is that the system is closed, especially if gas is being created.  Measuring the mass precisely is also important, so depending on the precision of your scales will depend on how you can compare your data.