AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2021: Question 3

✅ Correct answer

• Add the sample of water to anhydrous copper sulfate.
• The copper sulfate changes colour from white to blue.

How marks are awarded: 3 separate marking points were available: the substance used, and the colour change.

💡 Key knowledge

• Anhydrous means “without water”.
• It is used because it is white when dry and blue when hydrated.
• If water is present, the anhydrous copper sulfate becomes hydrated.

🧠 Exam technique

• Say the chemical name clearly: anhydrous copper sulfate .
• Always include the colour change if the question asks for a test.
• Best exam wording: “Add water to anhydrous copper sulfate; it turns from white to blue.”

❌ Common errors

• Saying just “copper sulfate” without anhydrous.
• Getting the colour change backwards.
• Describing the test as a chemical equation instead of a simple observation.

✅ Correct answer

A pure substance is a single element or a single compound.

The mark scheme also accepts: not mixed with any other substance.

💡 Key knowledge

• Pure substances have a fixed melting point and fixed boiling point.
• Impurities usually change these values.
• Water can be pure or impure depending on what is dissolved in it.

🧠 Exam technique

• For 1 mark, keep it short and exact.
• If unsure, write: “only one substance” but it is safer to name element or compound.

❌ Common errors

• Writing “only one type of atom” — this is too narrow because compounds are also pure substances.
• Writing “clean” or “safe to drink” — that is not the definition of pure.

📐 Calculations: step-by-step

1. Find the temperature increase:
   100.60 °C − 100.00 °C = 0.60 °C
2. Work out the percentage impurity:
   Each 1% impurity raises boiling point by 0.12 °C
   0.60 ÷ 0.12 = 5%
3. Find 5% of 250 g:
   5/100 × 250 = 12.5 g

Mass of impurity = 12.5 g

✅ Correct answer

12.5 g

The answer gets full marks only if the method is correct. The final value must match the given data.

🧠 Exam technique

• Always use the difference in boiling point first.
• Then convert that difference into % impurity.
• Finally, calculate the mass from the percentage.
• Write units carefully: °C and g.

❌ Common errors

• Using 100.60 instead of the difference 0.60.
• Dividing 250 by 5 instead of finding 5% of 250.
• Forgetting that the question asks for mass of impurity, not percentage.
• Dropping the decimal and writing 12 g instead of 12.5 g.

✅ Correct answer

1. Heat the salty water.
2. The water evaporates and becomes water vapour.
3. Cool the water vapour.
4. The vapour condenses into liquid water, which is collected as potable water.

Four marking points were available, so the best answers clearly describe each stage in order.

💡 Key knowledge

• Salt does not evaporate with the water.
• Distillation is used when you need to separate a solvent from dissolved solids.
• Condensation means changing from gas/vapour to liquid.

🧠 Exam technique

• Use the process words in the correct order: heat → evaporate → cool → condense.
• If asked to “explain”, do not just name the method — describe what happens.
• Make it clear that the final liquid is fresh/potable water.

❌ Common errors

• Saying the salt “melts” or “disappears”.
• Forgetting the condensation stage.
• Mixing up evaporation and condensation.
• Writing “filtering” instead of distillation — filtering cannot remove dissolved salt.

✅ Correct answer

• Distillation requires energy to boil the salty water.
• Ground water only needs filtering and sterilising, so it is cheaper.

This was a 2-mark explanation: one mark for the energy needed in distillation, one for the simpler treatment of ground water.

💡 Key knowledge

• Boiling water needs fuel or electricity, so it costs money.
• Ground water often already contains less salt, so it can be treated by simpler methods.
• Filtering removes solids; sterilising kills microbes.

🧠 Exam technique

• Make a comparison: salty water vs ground water.
• Use the phrase requires energy or requires fuel.
• Link cost to the process used, not just to “being more difficult”.

❌ Common errors

• Saying salty water is expensive because it is “dirty”.
• Not mentioning energy or fuel.
• Forgetting to compare with ground water treatment.

Top-level responses

• Use the exact science wording.
• Give answers in the right sequence for process questions.
• Show calculation steps clearly, not just the final number.
• Use comparisons when asked “why” something is more expensive.

Where students often lose marks

• Vague wording like “it changes colour” without naming the colours.
• Confusing pure substances with clean or safe substances.
• Skipping the unit or giving an answer without working.
• Describing distillation too briefly, missing condensation.

✅ Must-know answers

• Water test: anhydrous copper sulfate turns from white to blue.
• Pure substance: one element or one compound.
• Impurity mass: 12.5 g.
• Distillation: heat → evaporate → cool → condense.
• Cost: distillation needs energy; ground water needs only filtering and sterilising.