The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. This means that the total mass of the reactants (the substances that undergo the reaction) is always equal to the total mass of the products (the substances formed after the reaction).
In other words, during any physical or chemical change, the amount of matter remains constant, although it may change its form. Atoms are simply rearranged to form new substances, but their total number (and hence total mass) stays the same.
🔹 Example:
When hydrogen gas burns in oxygen to form water:
Hydrogen (2 g) + Oxygen (16 g) → Water (18 g)
Here, the mass of reactants (2 g + 16 g = 18 g) equals the mass of products (18 g).
This principle, first established by Antoine Lavoisier in 1789, is a fundamental concept in chemistry and forms the basis for writing and balancing chemical equations.