Conservation of Mass and Balanced Equations
Practise Conservation of Mass and Balanced Equations for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Mass is conserved in a reaction, so a symbol equation must be balanced — the same number of each type of atom on both sides.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
The law of conservation of mass states that no atoms are lost or made during a chemical reaction, so the total mass of the products equals the total mass of the reactants.
This is why chemical equations must be balanced. The same number of each type of atom must appear on both sides, and balancing is done by adding large numbers in front of formulae — never by changing the small subscript numbers, which would alter the substance itself.
Sometimes a reaction appears to lose or gain mass in an unsealed container. This is because a gas has escaped or been taken in from the air. The mass is not really lost; it has simply left the container.
Revision notes
The law
No atoms are lost or made during a chemical reaction, so the total mass of products equals the total mass of reactants.
The atoms are simply rearranged into different substances. This is why equations must balance.
Balancing equations
Add large numbers in front of formulae so the number of each type of atom is the same on both sides.
Never change the small subscript numbers — that would change the substance. For example, H₂ + O₂ → H₂O becomes 2H₂ + O₂ → 2H₂O.
Apparent changes in mass
In an unsealed container, mass appears to decrease if a gas is produced and escapes.
Mass appears to increase if a gas from the air, such as oxygen, is taken in during the reaction. In both cases the total mass is conserved; some of it has simply entered or left the container.
Key points
- No atoms are lost or made in a chemical reaction.
- Total mass of products equals total mass of reactants.
- Equations must be balanced.
- Balance by adding large numbers in front of formulae.
- Never change the small subscript numbers.
- Apparent mass changes are due to gases escaping or being taken in.
Worked examples
Example 1
Balance the equation H₂ + O₂ → H₂O. [2 marks]
Model answer
2H₂ + O₂ → 2H₂O. Correct large numbers added (1 mark), giving four hydrogen atoms and two oxygen atoms on each side (1 mark).
Example 2
A reaction in an open flask appears to lose mass. Explain why. [2 marks]
Model answer
A gas has been produced during the reaction (1 mark) and has escaped from the open flask into the surroundings, so it is no longer included when the flask is weighed (1 mark).
Example 3
Magnesium is burned in air and the mass of the product is greater than the mass of the magnesium. Explain why. [2 marks]
Model answer
The magnesium has reacted with oxygen from the air (1 mark), and the mass of that oxygen is now included in the mass of the magnesium oxide produced (1 mark).
Common mistakes
Changing subscript numbers when balancing.
That changes the substance. Only large numbers in front may be added.
Saying mass is destroyed.
Mass is always conserved; a gas has entered or left the container.
Forgetting to count atoms on both sides.
Check each element separately after balancing.
Saying the reaction created new atoms.
Atoms are rearranged, never created or destroyed.
Exam tips
- Count each type of atom on both sides before and after balancing.
- Only ever add large numbers in front of formulae.
- Explain mass changes by identifying the gas involved.
- State that mass is conserved overall, even when it appears not to be.
Key terms
- Conservation of mass
- The principle that mass is unchanged in a chemical reaction.
- Balanced equation
- An equation with equal numbers of each atom on both sides.
- Reactant
- A substance present at the start of a reaction.
- Product
- A substance formed by a reaction.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.