Conservation of Mass

This lesson covers the law of conservation of mass as required by the AQA GCSE Combined Science Trilogy specification (8464). You will understand why mass is conserved in chemical reactions, how to explain apparent mass changes in open systems, and how to perform calculations using this law.

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The Law of Conservation of Mass

No atoms are created or destroyed in a chemical reaction. The total mass of the products is always equal to the total mass of the reactants.

This law was established by Antoine Lavoisier in the 18th century. It is one of the fundamental principles of chemistry.

Why Is Mass Conserved?

In a chemical reaction:

• Bonds between atoms break and new bonds form.
• The atoms are rearranged into new substances.
• But the number and type of atoms stays exactly the same.

Since no atoms are gained or lost, the total mass cannot change.

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Demonstrating Conservation of Mass

Experiment in a Sealed Container