Final answer:
According to the law of conservation of mass, in a chemical reaction, the mass of the reactants is equal to the mass of the product; matter cannot be created or destroyed.
Step-by-step explanation:
When two substances react chemically to produce a new substance, the law of conservation of mass states that the mass of the reactants (the substances that are reacting) equals the mass of the product (the new substance formed). In other words, the total mass of the substances before the reaction is equal to the total mass after the reaction has occurred. This principle was established through careful quantitative analysis of chemical reactions in the 1790s, and it is a fundamental concept in chemistry that indicates matter cannot be created or destroyed in a chemical reaction; it can only change forms.