Test for carbon dioxide - AQA GCSE Chemistry Exam Questions

Practise 12 exam-style questions on Test for carbon dioxide 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.

12 questions
1–6 marks each
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12 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

Describe how to test a gas to show that it is carbon dioxide. Give the result of the test.

Model answer

Bubble the gas through limewater (or shake the gas with limewater in a stoppered tube). If the gas is carbon dioxide, the limewater turns milky (cloudy).

Explanation

Limewater is the only reliable test for carbon dioxide in the specification. 'Milky' or 'cloudy' is the expected wording; the cloudiness is a white precipitate.

Mark scheme

1 mark
bubble the gas through limewater / shake the gas with limewater
allow calcium hydroxide solution
1 mark
limewater turns milky / cloudy
dependent on limewater (or calcium hydroxide solution) being used; allow white precipitate forms; do not accept 'turns white' alone or 'goes clear'

Question 2

2 marks

Baking powder contains sodium hydrogencarbonate (NaHCO₃). When it is heated it decomposes to form sodium carbonate (Na₂CO₃), water and carbon dioxide.

Write a balanced symbol equation for this reaction.

Model answer

2NaHCO₃ → Na₂CO₃ + H₂O + CO₂

Explanation

Sodium carbonate contains two sodium atoms, so two NaHCO₃ are needed. Check each element: Na 2, H 2, C 2 and O 6 on both sides. The carbon dioxide released makes cakes rise and would turn limewater milky.

Mark scheme

1 mark
correct formulae: NaHCO₃ → Na₂CO₃ + H₂O + CO₂
ignore state symbols
1 mark
balanced: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
allow multiples

Question 3

3 marks

A student heats green copper(II) carbonate (CuCO₃) in a boiling tube. A delivery tube carries any gas produced into a test tube of limewater. The copper carbonate decomposes to form copper(II) oxide and carbon dioxide.

Describe two observations the student would make, and write a balanced symbol equation for the reaction.

Model answer

The green solid turns black (copper oxide is black). The limewater turns milky because carbon dioxide is given off.
CuCO₃ → CuO + CO₂

Explanation

This is thermal decomposition: one substance breaks down into two on heating. The colour change shows a new solid has formed, and the milky limewater confirms the gas is carbon dioxide.

Mark scheme

1 mark
the solid changes from green to black
allow turns black
1 mark
limewater turns milky / cloudy
allow bubbles in the limewater only if milky / cloudy also stated
1 mark
CuCO₃ → CuO + CO₂
ignore state symbols

Question 4

4 marks

A student bubbles 0.48 dm³ of carbon dioxide through excess limewater. All of the carbon dioxide reacts.

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

Relative atomic masses (Ar): C = 12, O = 16, Ca = 40
The volume of one mole of any gas at room temperature and pressure is 24 dm³.

Calculate the mass of calcium carbonate precipitate formed.

Model answer

moles of CO₂ = volume ÷ 24 = 0.48 ÷ 24 = 0.020 mol
1 mol CO₂ forms 1 mol CaCO₃, so moles of CaCO₃ = 0.020 mol
Mr of CaCO₃ = 40 + 12 + (3 × 16) = 100
mass of CaCO₃ = 0.020 × 100 = 2.0 g

Explanation

Gas volume ÷ 24 dm³ gives moles of gas. The equation shows a 1 : 1 ratio between carbon dioxide and calcium carbonate, so multiply the moles by the Mr of calcium carbonate to get the mass of precipitate.

Mark scheme

1 mark
moles of CO₂ = 0.48 ÷ 24 = 0.02(0) mol
1 mark
moles of CaCO₃ = 0.02(0) mol
allow ecf; 1:1 ratio
1 mark
Mr of CaCO₃ = 100
1 mark
mass = 0.020 × 100 = 2(.0) g
allow ecf; correct answer with no working scores 4 marks

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