Law of conservation of mass

The law of conservation of mass states that no atoms are lost or made in a chemical reaction. Instead, the atoms join together in different ways to form products. This is why, in a balanced symbol equation, the number of atoms of each element is the same on both sides of the equation.

Since atoms are not lost or made in a chemical reaction, the total mass of the products is equal to the total mass of the reactants. The sum of the relative formula masses of the reactants is equal to the sum of the relative formula masses of the products.

Two atoms of copper react with two atoms of oxygen to form two molecules of copper oxide.No atoms are created or destroyed when copper reacts with oxygen to form copper oxide

Calculations using the law

The mass of one substance in a reaction can be calculated if the masses of the other substances are known. For example:

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

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

Reactions in closed systems

No substances can enter or leave a closed system, such as a stoppered flask. Sometimes, reactions that happen in open beakers are closed systems, for example acid-alkali neutralisation reactions. Since all the reactants and products stay in the beaker, the total mass of the beaker and the substances in it stay the same during the reaction.

Reactions in non-enclosed systems

Substances can enter or leave a non-enclosed system. These systems include open flasks, boiling tubes or crucibles that let gases enter or leave. For example:

  • copper carbonate decomposes on heating to make solid copper oxide, which stays in the boiling tube, and carbon dioxide gas, which escapes
  • magnesium reacts with oxygen, gained from the air, to produce magnesium oxide

If a gas escapes, the total mass will look as if it has decreased. If a gas is gained, the total mass will look as if it has increased. However, the total mass stays the same if the mass of the gas is included.

Worked example

Question

10.0 g of calcium carbonate, CaCO3, was heated in a thermal decomposition reaction. 5.60 g of solid remained after heating. The equation below represents the reaction:

CaCO3(s) → CaO(s) + CO2(g)

Explain the change in mass.

The mass appears to have decreased because one of the products, carbon dioxide, escapes to the air. The total mass of the solid and gas products will be 10.0 g.