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No atoms are created or destroyed in a chemical reaction. Instead, they just join together in a different way than they were before the reaction, and form products. This means that the total mass of the products in a chemical reaction will be the same as the total mass of the reactants.

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:

Reactions in closed systems

No substances can enter or leave a closed system. A simple closed system could be a sealed container such as a stoppered flask. Sometimes reactions that happen in open beakers are closed systems. These include:

• acid-alkali neutralisation reactions, which produce salt solutions
• precipitation reactions, which produce an insolubleprecipitate from two reacting solutions

In both examples, the reactants and products stay in the beaker. The total mass of the beaker and the substances it contains stays the same during the reaction.

Reactions in non-enclosed systems

Substances can enter or leave a non-enclosed system. These systems are often open flasks or crucibles that let gases enter or leave. For example:

• zinc reacts with dilute hydrochloric acid to produce zinc chloride solution and hydrogen gas, which then 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.