Conservation of Mass

This lesson covers the law of conservation of mass as required by the Edexcel GCSE Combined Science specification (1SC0). You will learn why mass is conserved in chemical reactions, how balanced equations demonstrate this, and how to explain apparent changes in mass using practical evidence.

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The Law of Conservation of Mass

No atoms are created or destroyed during a chemical reaction.

This fundamental law, first clearly stated by Antoine Lavoisier in the 18th century, means:

$\text{Total mass of reactants} = \text{Total mass of products}$Total mass of reactants=Total mass of products

The atoms present at the start are simply rearranged to form new substances — they do not appear from nowhere or vanish.

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Why Is Mass Conserved?

In a chemical reaction:

1. Bonds between atoms in the reactants are broken.
2. New bonds are formed to make the products.
3. The same atoms are present before and after — just arranged differently.

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

Demonstration With a Balanced Equation

Consider the reaction between magnesium and oxygen:

$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$2Mg+O2​→2MgO