AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Foundation), 2024: Question 3

💡 Key knowledge

• The independent variable is the one you change.
• Zinc reacts with copper sulfate to make zinc sulfate and copper.
• When copper is displaced, the blue solution becomes colourless and copper metal appears.
• Temperature increase = highest temperature − starting temperature.

🧠 Exam technique

• Use the exact science term the mark scheme wants.
• For means, add the values and divide by the number of results.
• For thermometer readings, read the scale carefully and use the correct unit: °C.

❌ Common errors

• Calling the changed variable dependent instead of independent.
• Naming the product as “copper sulfate” instead of zinc sulfate.
• Forgetting that a mean needs all relevant results divided by 3, or averaging the wrong numbers.
• Giving the range as a single number instead of a lower-to-higher interval.

✅ Correct answer

Independent

💡 Key knowledge

• Independent variable = the factor deliberately changed.
• Dependent variable = what is measured, here the temperature increase.
• Control variable = kept the same, such as volume of copper sulfate solution.

🧠 Exam technique

The question says the student varied the mass of zinc, so that is your clue. Always match the answer to the wording of the method.

❌ Common errors

Students often picked dependent because the temperature changes, but the mass of zinc is what is being changed, not measured.

✅ Correct answer

zinc sulfate (allow ZnSO₄)

💡 Key knowledge

This is a displacement reaction. Zinc is more reactive than copper, so zinc replaces copper in copper sulfate.

zinc + copper sulfate → zinc sulfate + copper

🧠 Exam technique

If you know the general pattern, you can predict the product by swapping the metal in the sulfate compound.

❌ Common errors

Do not write copper sulfate again. The word sulfate stays, but the metal changes to zinc.

✅ Correct answers

• The solution becomes colourless.
• A brown / orange solid is produced.

💡 Key knowledge

• Copper sulfate solution is blue at the start.
• As copper ions are removed, the solution loses its blue colour.
• Copper metal forms as a reddish-brown solid.

🧠 Exam technique

Describe what you can see. Examiner guidance says to ignore mentions of temperature or fizzing because they are not the required observation marks.

❌ Common errors

• Writing “it gets hotter” — this was already stated in the question.
• Writing “gas is made” — not a correct observation here.
• Describing the copper as “black” instead of brown/orange.

✅ Correct answers

• Highest temperature reached = 36 °C
• Temperature increase = 36 − 21 = 15 °C

📐 Calculations

1. Read the thermometer scale carefully: the top of the liquid is at 36 °C.
2. Use the formula: temperature increase = highest temperature − starting temperature.
3. Substitute the values: 36 − 21.
4. Calculate: 15 °C.

Unit: degrees Celsius, °C

🧠 Exam technique

Marks are awarded for the method as well as the answer. If your highest temperature is slightly wrong, you can still gain the calculation mark if you subtract correctly from your own value.

❌ Common traps

• Reading the scale one division out.
• Using 35 °C or 37 °C by not checking the intervals.
• Forgetting to subtract the starting temperature.
• Writing a minus sign in the increase when the question wants the size of the change.

✅ Correct answer

B = 7.6 °C

Accepted working:
(7.8 + 7.3 + 7.7) ÷ 3 = 22.8 ÷ 3 = 7.6

Alternative valid method using the two consistent results:
(7.8 + 7.7) ÷ 2 = 7.75 °C

📐 Calculations

1. Add the three results: 7.8 + 7.3 + 7.7 = 22.8
2. Divide by 3: 22.8 ÷ 3 = 7.6
3. Give the answer to a sensible decimal place: 7.6 °C

Important: keep the unit °C .

🧠 Exam technique

If one value is suspicious, you may be allowed to use the two matching results to find a mean. The mark scheme accepts 7.75 °C if you clearly show the method using 7.8 and 7.7.

❌ Common errors

• Adding only two values and dividing by three.
• Rounding too early.
• Forgetting the unit.
• Writing 76 or 7.06 because of place-value mistakes.

✅ Correct answer

Range = 13.1 °C to 13.8 °C

📐 Calculations

Results for 2.0 g zinc are:

• 13.1 °C
• 13.8 °C
• 13.3 °C

The minimum is 13.1 °C and the maximum is 13.8 °C.

🧠 Exam technique

In this question, “range” means the smallest to largest value, not the difference between them.

❌ Common errors

• Writing 0.7 °C instead of the interval.
• Using the mean instead of the range.
• Mixing up the minimum and maximum values.

✅ Correct answer

Anomalous result: 12.9 °C

One reason: the mixture was not stirred / less than 3 g of zinc was used / more than 50 cm³ of copper sulfate solution was used.

💡 Key knowledge

An anomalous result is one that is very different from the others and does not fit the pattern. The other 3.0 g results are 20.4 °C and 20.2 °C, so 12.9 °C is much lower.

🧠 Exam technique

Top answers do two things:

• identify the odd result,
• explain why it is suspicious compared with the other repeats.

The examiner noted that the explanation must be linked to the experiment, not just “it is wrong”.

❌ Common errors

• Picking 20.4 °C or 20.2 °C because they are larger numbers.
• Giving a vague reason like “human error” without detail.
• Forgetting that the reason must be something that could happen in the experiment.

💡 What to remember

• Independent variable = mass of zinc.
• Product salt = zinc sulfate.
• Observations = solution becomes colourless; brown/orange solid forms.
• Temperature increase = highest − start.
• Mean = add and divide by number of results.
• Range = lowest to highest result.
• Anomalous result = value that does not fit the pattern.

🧠 How to secure full marks

• Use the exact scientific terms.
• Show working in calculations.
• Include units.
• Link explanations to the data in the table.

❌ Last-minute traps

• Using the wrong variable type.
• Mixing up zinc sulfate and copper sulfate.
• Not reading the thermometer carefully.
• Confusing range with difference.