In a chemical reaction, mass is never gained or lost. This is known as the Law of Conservation of Mass.

Here's why:

* Atoms are not created or destroyed: Chemical reactions involve rearranging atoms, not creating or destroying them.

* Total mass remains constant: The total mass of the reactants (starting materials) must equal the total mass of the products (substances formed).

Example:

Imagine burning a piece of wood. The wood reacts with oxygen in the air, producing ash, smoke, and carbon dioxide.

* Reactants: Wood + Oxygen

* Products: Ash + Smoke + Carbon Dioxide

Although the substances change, the total mass of the reactants (wood + oxygen) will be the same as the total mass of the products (ash + smoke + carbon dioxide).