AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Foundation), 2020: Question 8

6 marks · Standard Demand difficulty · Extended Answer

Plan a method to test whether equal masses of different metal carbonates produce the same mass of carbon dioxide when heated.

Practise this question

Question

The question asks students to investigate the thermal decomposition of metal carbonates. It shows the word equation metal carbonate → metal oxide + carbon dioxide and a hypothesis stating that when the same mass of any metal carbonate is heated, it produces the same mass of carbon dioxide. A results table for copper carbonate is shown with time heated in minutes 0, 2, 4, and 6, and the mass of the test tube and contents in grams changing from 17.7 to 17.1 to 17.0 to 17.0. The question asks the student to plan a method to test the hypothesis using the results, without discussing safety precautions, and is worth 6 marks.
Question text

08 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 4 shows their results.

Table 4

Time the test tube containing copper

02 4 6

carbonate was heated in mins

Mass of test tube and contents in g27 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 uses levels. Level 3 describes a method that would produce a valid outcome with key steps identified and logically sequenced, for 5 to 6 marks. Level 2 gives 3 to 4 marks for a method that may not necessarily lead to a valid outcome but includes most steps. Level 1 gives 1 to 2 marks for some relevant steps without clear links. Indicative content includes weighing the test tube, adding a metal carbonate, weighing test tube and carbonate, heating, allowing to cool, weighing test tube and metal oxide, repeating heat-cool-weigh until the mass is constant, determining the mass of metal carbonate used and the mass of carbon dioxide produced, and repeating with different metal carbonates. It also says an alternative method can use any mass of carbonate and divide by this mass to find carbon dioxide per gram, and level 3 requires a change in mass for at least one other 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

Planning an Investigation: Thermal Decomposition

What this question tests: This is an AO3 (Analysis and Evaluation) question. You are being tested on your ability to plan a scientific investigation, ensure data validity, and compare results to test a hypothesis.

The Task

You need to describe a method to test the hypothesis: "When heated, the same mass of any metal carbonate produces the same mass of carbon dioxide."

🧠 Exam Technique: The "Method" Checklist

For a 6-mark "Plan a method" question, think like a scientist. You need to be logical and precise:

  • Preparation: Weigh the empty test tube, add the carbonate, and weigh again to find the exact mass of the carbonate.
  • Execution: Heat the substance, allow it to cool, and weigh it.
  • Validity: Repeat the heating, cooling, and weighing process until the mass stops changing (this ensures the reaction is complete).
  • Calculation: Calculate the mass of CO₂ lost (Initial mass - Final mass).
  • Comparison: Repeat the entire process with at least one other metal carbonate using the same starting mass.

How to Score Full Marks

✅ The Ideal Response

To achieve Level 3 (5-6 marks), your answer must be logically sequenced:

  1. Weigh the empty test tube.
  2. Add a specific mass of metal carbonate and re-weigh to find the exact mass of the carbonate.
  3. Heat the test tube and contents.
  4. Allow the test tube to cool before weighing it again.
  5. Repeat the heating, cooling, and weighing until the mass remains constant (this confirms all CO₂ has been driven off).
  6. Calculate the mass of CO₂ produced (Initial mass of carbonate - Final mass of residue).
  7. Repeat the experiment using the same mass of a different metal carbonate.
  8. Compare the mass of CO₂ produced for each carbonate to test the hypothesis.

Key Knowledge & Common Pitfalls

💡 Key Knowledge

Thermal decomposition is the breaking down of a substance using heat. In this case: Metal Carbonate → Metal Oxide + CO₂. Since CO₂ is a gas, it escapes into the air, causing the mass of the test tube contents to decrease.

❌ Common Errors

  • Forgetting to cool: Weighing a hot test tube is inaccurate due to convection currents affecting the balance reading.
  • Incomplete reaction: Not heating until "constant mass" means some carbonate may remain, leading to incorrect CO₂ measurements.
  • Vague steps: Saying "heat it" without mentioning the need to weigh before and after.

📐 Calculation Logic

The hypothesis is about the mass of CO₂. You must show how to find this:

Mass of CO₂ = (Mass of test tube + carbonate) - (Mass of test tube + metal oxide)

Note: If you use different starting masses, you must divide the mass of CO₂ by the mass of the carbonate used to find the "mass of CO₂ per gram of carbonate" to make a fair comparison.

Topics

Chemistry · C4: Chemical Changes

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