GCSE Chemistry Revision — Conservation of Mass
Revise Conservation of Mass for GCSE Chemistry with a topic explanation, worked example and common mistakes. Check the board notes for specification differences.
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What is Conservation of Mass?
The law of conservation of mass states that no atoms are lost or made during a chemical reaction. This means that the total mass of the reactants at the start of a reaction is equal to the total mass of the products at the end.
Board notes: This is a fundamental principle of chemistry. All boards expect you to understand and be able to apply the law of conservation of mass to chemical reactions, including explaining apparent changes in mass.
Step-by-step explanationWorked examples
Worked example
When 24g of magnesium is burned in excess oxygen, 40g of magnesium oxide is formed. By the law of conservation of mass, the mass of oxygen that reacted must be 40g - 24g = 16g.
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Common mistakes
- 1Thinking that mass has been lost in a reaction when a gas is produced and escapes, if the reaction is not carried out in a closed system.
- 2Believing that mass has been gained when a metal reacts with a gas (like oxygen) if the mass of the gas is not accounted for.
- 3Applying the concept incorrectly to state changes. The mass of a substance remains the same whether it is a solid, liquid, or gas.
Conservation of Mass exam questions
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Frequently asked questions
Can mass ever be lost in a reaction?
According to the law of conservation of mass, mass is not lost or gained in a chemical reaction. However, it might appear to be lost if a gas is produced and allowed to escape.
Does conservation of mass apply to nuclear reactions?
No, the law of conservation of mass does not strictly apply to nuclear reactions, where a small amount of mass can be converted into a large amount of energy, as described by Einstein's equation E=mc².