Final answer:
The law of conservation of mass dictates that the mass of substances remains constant before and after a chemical change; mass is neither created nor destroyed. So, the mass of substance remains constant.
Step-by-step explanation:
The mass of all substances before a chemical change is equal to the mass of all substances after a chemical change. This principle is known as the law of conservation of mass, which is a fundamental concept in chemistry stating that mass cannot be created or destroyed in a chemical reaction. Instead, matter can change forms, like from solids to liquids or gases, but the total mass remains the same. The number of atoms also remains consistent, meaning that during a reaction, atoms are simply rearranged to form new substances.