CoachConservation of mass is the law that in a chemical reaction, the total mass of the products formed is equal to the total mass of the reactants used up. Nothing is lost and nothing is created; matter only changes form.
Examples
Example 13.2 (The log, weighed properly)
If the burning log and the oxygen it took from the air could be weighed before, and the ash, carbon dioxide and water vapour after, the two totals would be equal. The ash is light because most of the products are gases that go up the chimney.