AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Conservation of mass and balanced chemical equations: The law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants.
During a chemical reaction, bonds break and new bonds form, but the atoms themselves are simply rearranged.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
During a chemical reaction, bonds break and new bonds form, but the atoms themselves are simply rearranged. No atom disappears and no new atom appears, so the total mass of the products must equal the total mass of the reactants. For example, when 12 g of carbon reacts completely with 32 g of oxygen, the carbon dioxide formed has a mass of 44 g. This idea is the law of conservation of mass. It only appears to fail when a reactant or product is a gas that escapes from or enters the reaction vessel. In a sealed container, the total mass stays constant. The law allows you to calculate an unknown mass in a reaction by subtraction or addition, and it is the reason symbol equations must be balanced.
What earns the marks
- States that atoms are not created or destroyed in a chemical reaction, only rearranged.
- Explains that the total mass of reactants equals the total mass of products in a closed system.
- Applies the law to a calculation, for example finding the mass of a product from the masses of reactants.
- Recognises that an apparent mass change is caused by a gas entering or leaving the reaction vessel.
- Links the law to the need for balanced symbol equations.
Common mistakes and how to put them right
Thinking mass is lost because a gas escapes.
Put it right: The gas still exists and must be included or the vessel sealed.
Believing atoms change into energy during ordinary chemical reactions.
Put it right: Atoms are conserved and only rearranged.
Adding the masses of reactants and products together.
Put it right: Reactant mass equals product mass, so compare the two totals.
Exam tips
- Show the subtraction or addition clearly when calculating an unknown mass.
- Mention the sealed container or trapped gas when explaining an apparent mass change.
- Use the phrase 'atoms are rearranged' rather than 'atoms are used up'.
Try the lesson
The start of the Conservation of mass and balanced chemical equations: The law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants. lesson, made from checked teaching notes for this specification point.
Have a go first
In a chemical reaction, 12 g of magnesium reacts completely with 8 g of oxygen. What mass of magnesium oxide is produced?
Listen
During a chemical reaction, bonds break and new bonds form, but the atoms themselves are simply rearranged. No atom disappears and no new atom appears, so the total mass of the products must equal the total mass of the reactants. For example, when 12 g of carbon reacts completely with 32 g of oxygen, the carbon dioxide formed has a mass of 44 g. This idea is the law of conservation of mass. It only appears to fail when a reactant or product is a gas that escapes from or enters the reaction vessel. In a sealed container, the total mass stays constant. The law allows you to calculate an unknown mass in a reaction by subtraction or addition, and it is the reason symbol equations must be balanced.
Key words
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Conservation of mass
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Rearrangement of atoms
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Closed systems
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Mass calculations
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See every definition
- Conservation of mass
- No atoms are lost or made during a chemical reaction, so the mass of the products equals the mass of the reactants.
- Rearrangement of atoms
- During a chemical reaction, bonds break and new bonds form, but the atoms themselves are simply rearranged.
- Closed systems
- In a sealed container, the total mass stays constant.
- Mass calculations
- The law allows you to calculate an unknown mass in a reaction by subtraction or addition.
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