AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2020: Question 3

6 marks · High Demand difficulty · Extended Answer

Plan a method to test the hypothesis that the same mass of any metal carbonate produces the same mass of carbon dioxide when thermally decomposed, using mass measurements before and after heating.

Practise this question

Question

The question states that students investigated the thermal decomposition of metal carbonates and gives the word equation: metal carbonate arrow metal oxide plus carbon dioxide. It provides a hypothesis that when heated, the same mass of any metal carbonate produces the same mass of carbon dioxide, then says students heated a test tube containing copper carbonate. A table shows heating time in minutes of 0, 2, 4 and 6, with corresponding masses of the test tube and contents of 17.7 g, 17.1 g, 17.0 g and 17.0 g. The task asks for a planned method to test the hypothesis, including how results would be used, says safety precautions are not needed, and the question is worth 6 marks with lined space for an extended written answer.
Question text

03 Some students investigated the thermal decomposition of metal carbonates.

The word equation for the reaction is:

metal carbonate → metal oxide + carbon dioxide

The students made the following hypothesis:

‘When heated the same mass of any metal carbonate produces

the same mass of carbon dioxide.’

The students heated a test tube containing copper carbonate.

Table 1 shows their results.

Table 1

Time the test tube containing copper

02 4 6

carbonate was heated in mins

Mass of test tube and contents in g9 17.7 17.1 17.0 17.0

Plan a method the students could use to test their hypothesis.

You should show how the students use their results to test the hypothesis.

You do not need to write about safety precautions.

[6 marks]

Mark scheme

Show the mark scheme The mark scheme is a level-of-response table for question 03 with Level 3 worth 5 to 6 marks, Level 2 worth 3 to 4 marks, Level 1 worth 1 to 2 marks, and 0 for no relevant content. It says high-level answers identify logically sequenced steps leading to a valid outcome. Indicative content includes weighing the empty test tube, adding metal carbonate, weighing test tube and carbonate, heating, allowing to cool, reweighing the test tube and metal oxide, repeating heat-cool-weigh until no change in mass, determining the mass of carbonate used and carbon dioxide produced, and repeating with different metal carbonates; it also allows an alternative method using any starting mass and dividing to find mass of carbon dioxide per gram of metal carbonate.

AO/

Question Answers Mark

Spec. Ref

Level 3: The method would lead to the production of a valid 5–6 AO3

outcome. The key steps are identified and logically sequenced.

Level 2: The method would not necessarily lead to a valid 3–4 5.3.1.3

outcome. Most steps are identified, but the method is not fully 5.4.2.2

logically sequenced.

Level 1: The method would not lead to a valid outcome. 1–2

Some relevant steps are identified, but links are not made clear.

No relevant content 0

Indicative content:

• weigh test tube

• add metal carbonate

• weigh test tube and metal carbonate

• heat

• allow to cool

• weigh test tube and metal oxide

• repeat (heat, cool and weigh) until no change in mass

• determine mass of metal carbonate used

• determine mass of carbon dioxide produced

• repeat with different metal carbonate(s)

an alternative method can be based on any mass of metal

carbonates and at end divide by this mass to find

mass carbon dioxide per gram metal carbonate

level 3 change in mass is determined for at least one other

carbonate

Total 6

How to answer it

GCSE AQA Combined Science: Trilogy • Question 03

Testing a hypothesis about thermal decomposition of metal carbonates

What this question tests

You need to plan a valid practical method, use mass measurements correctly, and show how to compare results for different metal carbonates. The examiner is looking for a clear sequence: weigh → heat → cool → reweigh → repeat to constant mass → compare the mass of carbon dioxide produced.

Question focus

Hypothesis: “When heated the same mass of any metal carbonate produces the same mass of carbon dioxide.”

This is a 6-mark practical planning question. Full marks are awarded for a method that would actually test the hypothesis, not just describe heating one sample once.

Part (a) — Plan a method to test the hypothesis [6 marks]

What a full-mark answer needs

✅ Correct answers

  • Weigh the empty test tube.
  • Add a known mass of a metal carbonate.
  • Weigh the test tube + carbonate.
  • Heat the test tube.
  • Allow it to cool.
  • Weigh the test tube + contents again.
  • Repeat the heat–cool–weigh cycle until the mass stops changing.
  • Calculate the mass lost, which is the mass of CO₂ produced.
  • Repeat the experiment with a different metal carbonate.
  • Use the same starting mass for each carbonate so the comparison is fair.

💡 Key knowledge

  • Thermal decomposition of a metal carbonate makes a metal oxide + CO₂.
  • Mass decreases because carbon dioxide leaves the test tube as a gas.
  • A valid test needs at least two different carbonates to compare.
  • “Same mass of any metal carbonate” means the starting mass must be controlled.

🧠 Exam technique

  • Write the method in a logical order.
  • Make it clear how the results will test the hypothesis.
  • Use words like constant mass, difference in mass, and repeat for another carbonate.
  • State what is being measured and what is being compared.

❌ Common errors

  • Heating once and stopping before constant mass.
  • Not saying to cool before weighing.
  • Only using one carbonate, so the hypothesis is not tested.
  • Forgetting to compare the mass of CO₂ from different carbonates.
  • Not making clear that the same mass of each carbonate should be used.

📐 How to use the results

The students should find the change in mass:

mass of CO₂ = initial mass of test tube + carbonate − final constant mass of test tube + oxide

Then they should repeat this for another metal carbonate and compare the masses of CO₂ produced from the same starting mass. If the masses are the same, the hypothesis is supported. If they are different, the hypothesis is not supported.

How marks are awarded

Level 3 response: 5–6 marks

Clear, logical method with all key steps included. The answer explains how to obtain a valid comparison and shows how the results test the hypothesis.

What distinguished top answers

Students who gained top marks did not just list actions — they linked them: heat → cool → weigh → repeat → calculate CO₂ lost → compare with another carbonate.

Model full-mark answer

Example answer:

Weigh the empty test tube, then add a known mass of copper carbonate and weigh again. Heat the test tube, allow it to cool, and weigh it again. Repeat the heating, cooling and weighing until the mass no longer changes. The loss in mass is the mass of CO₂ produced. Repeat the experiment using the same mass of a different metal carbonate. Compare the mass of CO₂ produced by each carbonate. If the same mass of each carbonate gives the same mass of CO₂, the hypothesis is supported.

Common misconceptions examiner comments

❌ Mistakes that lost marks

  • Not repeating until constant mass.
  • Not explaining how the mass of CO₂ is found.
  • Not mentioning a second carbonate.
  • Giving safety precautions instead of method, when the question said they were not needed.

💡 Why the method is valid

  • Constant mass means all decomposition has finished.
  • Mass lost from the test tube is the gas that escaped.
  • Using the same starting mass makes the comparison fair.

Best exam answer structure

  1. State the starting mass is measured.
  2. Heat the carbonate.
  3. Cool before weighing.
  4. Repeat until the mass is constant.
  5. Work out the mass lost = mass of CO₂.
  6. Repeat for a different carbonate using the same starting mass.
  7. Compare the results to test the hypothesis.

Topics

Chemistry · C3: Quantitative Chemistry · C5: Energy Changes

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 1 (Higher), 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.