The law of Conservation of Mass

The Law of Conservation of Mass

the principle that in any closed system subjected to no external forces, the mass is constant irrespective of its changes in form; the principle that matter cannot be created or destroyed.

The law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction. For example, when wood burns, the mass of the soot, ashes, and gases, equals the original mass of the charcoal and the oxygen when it first reacted. So the mass of the product equals the mass of the reactant.

https://courses.lumenlearning.com/introchem/chapter/the-law-of-conservation-of-mass/(helping link).