How the law of conservation of mass applies to chemical changes?

As in all conservation laws, it means that mass (in this case) won't increase or decrease.In chemical reactions there is a stronger law, which can be quite useful: the total number of atoms OF EACH KIND won't change; thus, if at the start of a reaction you have, for example, three carbon atoms, two oxygen atoms, and four hydrogen atoms in total, at the end you'll have the same combination (three carbon, etc.).