Conservation of mass

Mass is never lost or gained in chemical reactions. It is said that mass is always conserved. In other words, the total mass of products at the end of the reaction is equal to the total mass of the reactants at the beginning. This is because no atoms are created or destroyed during chemical reactions.

Diagram showing 16 atoms of element X (16 X) reacting with 8 atoms of element Y (8 Y) to form 8 compounds of element X and Y (8 X2Y).

The principle of conservation of mass allows you to work out the mass of one substance in a reaction if the masses of the other substances are known.

Calcium carbonate is made up of 28 grams of calcium oxide and 22 grams of carbon dioxide.

Calcium carbonate is made up of 28 grams of calcium oxide and 22 grams of carbon dioxide

In practice, it is not always possible to get the entire calculated amount of product from a reaction. This is because:

  • reversible reactions may not be completed
  • some product may be lost when it is removed from the reaction mixture
  • some of the reactants may react in an unexpected way