Conservation of mass and balanced chemical equations - AQA GCSE Chemistry Exam Questions

Practise 20 exam-style questions on Conservation of mass and balanced chemical equations for AQA GCSE Chemistry. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.

20 questions
1–6 marks each
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20 questions, marked as you go

Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 1

2 marks

Hydrogen gas reacts with oxygen gas to form water.

Write a balanced symbol equation for this reaction.

Model answer

2H₂ + O₂ → 2H₂O

Explanation

Hydrogen and oxygen are both diatomic molecules, so they are written H₂ and O₂. One O₂ molecule provides two oxygen atoms, so two water molecules form, which need four hydrogen atoms, provided by two H₂ molecules.

Mark scheme

1 mark
correct formulae: H₂ + O₂ → H₂O
1 mark
balanced: 2H₂ + O₂ → 2H₂O
allow multiples, e.g. 4H₂ + 2O₂ → 4H₂O; allow H₂ + ½O₂ → H₂O; ignore state symbols

Question 2

2 marks

When sodium hydroxide solution is added to a solution containing iron(III) ions, a brown precipitate of iron(III) hydroxide, Fe(OH)₃, forms.

Write a balanced ionic equation for this reaction. Include state symbols.

Model answer

Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s)

Explanation

Iron(III) hydroxide contains three hydroxide ions for each Fe³⁺ ion, so three OH⁻ are needed. The equation is balanced for atoms and for charge: 3+ and 3- on the left give a neutral compound. The precipitate is a solid, so it is (s).

Mark scheme

1 mark
Fe³⁺ + 3OH⁻ → Fe(OH)₃
ignore spectator ions (Na⁺) if shown on both sides
1 mark
state symbols: Fe³⁺(aq), OH⁻(aq), Fe(OH)₃(s)

Question 3

3 marks

Magnesium metal reacts with hydrochloric acid to produce magnesium chloride solution and hydrogen gas.

Write a balanced symbol equation for this reaction. Include state symbols.

Model answer

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Explanation

Magnesium chloride is MgCl₂ because Mg²⁺ needs two Cl⁻ ions, so two HCl are needed to supply two chlorine atoms; this also gives two hydrogen atoms for one H₂ molecule. Magnesium is a solid, the acid and the salt are dissolved in water (aq) and hydrogen is a gas.

Mark scheme

1 mark
correct formulae: Mg + HCl → MgCl₂ + H₂
1 mark
balanced: Mg + 2HCl → MgCl₂ + H₂
allow multiples
1 mark
state symbols: Mg(s), HCl(aq), MgCl₂(aq), H₂(g)
all four correct; state symbol mark can be awarded if formulae are correct but unbalanced

Question 4

4 marks

A student investigated whether mass is conserved when copper sulfate solution reacts with sodium hydroxide solution to form a blue precipitate. No gas is produced.

The student weighed both beakers of solution, poured one solution into the other, and then weighed both beakers again. The table shows the results.

Do the results support the law of conservation of mass? Use the data in your answer and suggest a reason for the result of trial 2.

Table with columns "Trial", "Mass before mixing in g", "Mass after mixing in g". Trial: 1; Mass before mixing in g: 112.46; Mass after mixing in g: 112.46 | Trial: 2; Mass before mixing in g: 113.05; Mass after mixing in g: 112.70 | Trial: 3; Mass before mixing in g: 112.81; Mass after mixing in g: 112.80.

Model answer

Yes. In trial 1 the mass did not change, and in trial 3 it changed by only 0.01 g, which is within the uncertainty of the balance. This supports the law of conservation of mass. Trial 2 is anomalous: the mass fell by 0.35 g. This was probably because some solution was spilled or splashed out when it was poured.

Explanation

Because no gas is produced, all the products stay in the beakers, so the mass should not change. A result that does not fit the pattern is anomalous and is best explained by an experimental error such as a spill, not by mass being destroyed.

Mark scheme

1 mark
in trials 1 and 3 the mass did not change / changed by only 0.01 g
1 mark
so the results support the law (of conservation of mass)
allow the 0.01 g change is within the resolution / uncertainty of the balance
1 mark
trial 2 is anomalous / the mass decreased by 0.35 g
1 mark
some solution was spilled / splashed out / lost when pouring
allow error in weighing / not all apparatus reweighed; do not accept a gas was given off

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