AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2020: Question 4

Key idea

• The student heats water samples to remove the water and leave dissolved solids behind.
• Masses are compared to find how much dissolved solid is in each sample.
• You need to interpret a method, a results table, and a conversion from 25 cm³ to 1 m³.

How to score well

• Use the correct scientific term for the variable.
• Link errors to what actually happened in the method.
• Show calculation steps clearly, including units and final answer form.

✅ Correct answer

Dependent

💡 Key knowledge

• Independent variable = what you change.
• Dependent variable = what you measure.
• Control variables = what you keep the same.

🧠 Exam technique

For 1 mark, the examiner wants the exact term. If you say “mass” or “result”, that is not specific enough.

❌ Common errors

• Choosing independent because the water sample is changed.
• Choosing control because the volume is kept the same.
• Writing “categorical” — this is not the correct type here.

✅ Correct answer

Error: not all the water had been removed from the sample.

Avoided by: heat to constant mass.

Alternative acceptable points: the bottom of the evaporating basin was wet, or droplets of water were left and should be dried/wiped off before weighing.

💡 Key knowledge

• If water is still present, the measured mass is too high.
• “Constant mass” means heating, cooling, and reweighing until the mass no longer changes.
• A dry evaporating basin is essential for accurate results.

🧠 Exam technique

• Give one clear cause of the error.
• Then give one practical method to prevent it.
• Marks are usually split: one for the cause, one for the fix.

❌ Common errors

• Saying “weighing error” without explaining what was wrong.
• Writing “heat longer” without mentioning constant mass.
• Ignoring the fact that water droplets on the basin can increase the mass.

📐 Calculation steps

1. Use the mean formula: mean = total ÷ number of values
2. For sample A: (0.22 + 0.23 + 0.20 + X) ÷ 4 = 0.21
3. Multiply both sides by 4: 0.22 + 0.23 + 0.20 + X = 0.84
4. Add the known values: 0.65 + X = 0.84
5. Subtract: X = 0.84 - 0.65 = 0.19

X = 0.19 g

✅ Correct answer

0.19 g

🧠 Exam technique

• Always show the mean formula if space is available.
• Keep the unit g.
• Use the given mean of 0.21 exactly.

❌ Common errors

• Adding the three values and forgetting the mean is for four tests.
• Doing 0.21 - 0.22 - 0.23 - 0.20 incorrectly.
• Leaving the answer without units.

✅ Correct answer

Water sample: C

Reason: it has the biggest difference between the maximum and minimum values.

Accepted equivalent idea: it has the greatest range.

📐 Working out the range

• A: 0.23 - 0.19 = 0.04
• B: 0.08 - 0.02 = 0.06
• C: 0.60 - 0.45 = 0.15
• D: 0.91 - 0.79 = 0.12

Largest range = C

🧠 Exam technique

• You need the sample letter and the reason for both marks.
• Using “highest mean” is not enough — the question asks for range.
• If you made a mistake in part (04.3), you can still gain follow-through credit if your comparison is consistent.

❌ Common errors

• Choosing D because it has the largest numbers, rather than the largest spread.
• Writing “largest mass” instead of “largest range”.
• Comparing only the means.

📐 Step-by-step calculation

1. Start with the given data: 25 cm³ contains 0.016 g
2. Convert volume to match the larger amount:
   1 m³ = 1000 dm³
   1 dm³ = 1000 cm³
   1 m³ = 1 × 10⁶ cm³
3. Find how many 25 cm³ portions are in 1 × 10⁶ cm³:
   1 × 10⁶ ÷ 25 = 4 × 10⁴
4. Multiply the mass in one portion:
   0.016 × 4 × 10⁴ = 640 g
5. Write in standard form:
   6.4 × 10² g

✅ Correct answer

6.4 × 10² g

Equivalent form: 640 g

💡 Key knowledge

• 1 m³ = 1 × 10⁶ cm³ is the key conversion.
• Mass scales directly with volume if the concentration is the same.
• Standard form should be written as a × 10ⁿ where 1 ≤ a < 10 .

❌ Common errors and traps

• Using 1 m³ = 1000 cm³ — this is wrong.
• Forgetting to convert the volume before scaling up.
• Missing the unit g.
• Writing the standard form incorrectly, such as 64 × 10¹ instead of 6.4 × 10² .

Top-level responses...

• Used precise terms like dependent variable and constant mass.
• Explained the error clearly, rather than just naming a vague mistake.
• Showed full calculation method, not just the final answer.
• Compared data using the correct idea of range.

How marks were awarded

• Short answer questions usually reward one clear scientific statement.
• Calculation questions reward method as well as the answer.
• For comparison questions, give both the result and the reason.