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When a chemical reaction happens, atoms do not vanish or appear from nothing. They are rearranged into new substances. That is why the mass of the whole system before a reaction equals the mass of the whole system after the reaction.
The key word is system. In a closed container, every substance remains in the part being weighed. In an open container, some gas may leave, so the balance reading can appear to change even though the total mass of the substances is still conserved. OpenStax Chemistry 2e describes conservation of matter as no detectable change in the total amount of matter during chemical or physical changes in Phases and Classification of Matter.
Before modern chemistry, combustion was often explained with phlogiston, as if something left a material when it burned. Antoine Lavoisier challenged that idea through careful weighing. He tracked the mass of reactants and products, including gases, so combustion could be read as a reaction with oxygen, not as the loss of a mysterious substance.
Science History Institute summarizes Lavoisier's role in establishing the law of conservation of mass and connecting combustion with oxygen in Antoine-Laurent Lavoisier.
Switch between a closed and an open system. The correct law always counts every substance, not only the part that is still visible inside the container.
In a closed system, products and leftover reactants stay inside the part being weighed.
A student heats of zinc with of sulfur. After the reaction, of zinc sulfide forms and of zinc remains unreacted.
The simplified equation is:
The product mass does not have to equal the mass of every starting substance if one reactant is left over. First count only the zinc that actually reacted.
So mass conservation is confirmed for the part that actually reacts: the mass of zinc that reacts plus the mass of sulfur equals the mass of zinc sulfide formed.
If the whole container is counted, the mass is also unchanged:
Keep three questions separate.
Britannica explains that chemical equations must be balanced because the same number of atoms of each element appears on both sides, and mass calculations follow conservation of matter in The Conservation of Matter.
Once total mass is protected, the next question becomes sharper: if two elements form the same compound, does their mass ratio stay fixed? That leads directly to the law of constant composition.
