AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2023: Question 4

💡 Key knowledge

• To measure dissolved solids by evaporation, heat until all water has evaporated and the mass is constant.
• Pure water has a fixed boiling point of 100 °C or freezing point of 0 °C.
• The symbol ⇌ means a reversible reaction.
• For an exothermic forward reaction, increasing temperature shifts equilibrium to the left.
• Chlorine bleaches damp litmus paper.
• Filtration removes insoluble solids, not dissolved substances.

🧠 Exam technique

• Use the exact wording of the science idea being tested.
• For 2-mark questions, give both the method and the result/explanation.
• In calculation questions, show working even if the answer is simple.
• Read carefully: “pure” and “potable” are not the same thing.

✅ Correct answer

Heat the evaporating basin and contents to constant mass.

Acceptable idea: continue heating until all of the water has evaporated.

💡 Key knowledge

If the student only heated for 2 minutes, some water might still be present. That means the mass measured would not be just the dissolved solids.

Constant mass means heat, cool, weigh, then repeat until the mass no longer changes.

🧠 Exam technique

Do not just say heat it longer . That is too vague for full credit. The marking point is the precise idea of constant mass or all water evaporated.

❌ Common errors

• Saying “repeat it more times” – repeats improve reliability, not validity here.
• Saying “use more water” – this does not fix the method problem.
• Saying “use a lid” – not relevant to measuring dissolved solids after evaporation.

✅ Correct answer

Mass X = 0.31 g

📐 Calculations

1. Use the mean formula:
   mean = total of 3 results ÷ 3
2. Find the total mass for 3 tests:
   0.29 × 3 = 0.87 g
3. Add the two known results:
   0.30 + 0.26 = 0.56 g
4. Subtract to find X:
   X = 0.87 − 0.56 = 0.31 g

You could also write:

(0.30 + X + 0.26) ÷ 3 = 0.29

🧠 Exam technique

Always show the equation you are using. Even if you make an arithmetic slip, you may still get the method mark.

Include the unit: g.

❌ Common calculation traps

• Adding the known values and then dividing by 3 again.
• Forgetting the mean is for 3 tests.
• Writing 0.031 g or 0.3 g from poor decimal handling.
• Leaving off the unit.

✅ Correct answer

Test: determine the boiling point.

Result: pure water boils at 100 °C.

OR

Test: determine the freezing point.

Result: pure water freezes at 0 °C.

💡 Key knowledge

A pure substance has a sharp, fixed melting/freezing point and a sharp, fixed boiling point.

This question is about identifying purity using physical properties.

🧠 Exam technique

For 2 marks, you must give both parts:

• what you would measure
• what result shows purity

Top answers are short and precise.

❌ Common errors

• Saying “test the pH” – this does not confirm purity.
• Saying “it should be colourless” – many impure liquids are also colourless.
• Not giving the actual temperature value.

✅ Correct answer

Reversible reaction

Acceptable wording: a reaction that can go both ways.

💡 Key knowledge

In a reversible reaction, products can react to form the original reactants again.

❌ Common errors

• Saying “balanced equation” – that is not what the symbol means.
• Saying “the reaction stops and starts” – equilibrium is not about stopping.

✅ Correct answer

Shifts towards the left-hand side

💡 Key knowledge

If the forward reaction is exothermic, heat acts like a product.

Increasing temperature adds heat, so equilibrium moves in the direction that uses up that extra heat: the endothermic direction.

That means it shifts to the left.

🧠 Exam technique

When a multiple-choice style question mentions exothermic and increase in temperature, think:

increase temperature → favour endothermic direction

❌ Common errors

• Choosing right because “higher temperature makes reactions faster”. Rate is different from equilibrium position.
• Saying “stays the same” – temperature does change equilibrium position.

✅ Correct answer

Test: use damp litmus paper.

Result: the litmus paper is bleached (turns white).

💡 Key knowledge

The gas used to sterilise water here is chlorine, Cl₂.

Chlorine first turns damp litmus paper red because it is acidic, then it bleaches it white. The mark scheme credits the bleaching result.

🧠 Exam technique

Because the question says describe a test, your answer should have two parts:

• what you do
• what you see

Examiner insight: students often lost a mark by naming chlorine but not giving the actual test result.

❌ Common errors

• Writing “glowing splint relights” – that is oxygen.
• Writing “popping sound” – that is hydrogen.
• Writing only “litmus turns red” – the credited identifying feature is that it is bleached.

✅ Correct answer

Dissolved solids pass through the filter paper because the dissolved solid particles are very small.

💡 Key knowledge

Filtering removes insoluble solid particles suspended in water.

Dissolved substances are present as particles small enough to go through the gaps in the filter paper.

This is why filtered water can still contain dissolved salts and minerals.

🧠 Exam technique

To get both marks, make sure your answer includes:

• what happens at the filter paper
• why it happens

A strong 2-mark answer links the process and the particle explanation in one sentence.

❌ Common errors

• Saying “the filter paper has holes in it” without linking to the tiny dissolved particles.
• Saying “potable means pure” – potable means safe to drink, not chemically pure.
• Saying “the solids evaporate” – this is about filtration, not evaporation.

🧠 What distinguished top answers

• Using exact scientific phrases such as constant mass, reversible reaction, and bleached litmus paper.
• Giving both parts of 2-mark responses, not just one.
• Showing clear calculation working for the mean question.
• Explaining filtration using particle size.

❌ Big misconceptions to avoid

• Pure is not the same as potable.
• Heating for a fixed time is not as good as heating to constant mass.
• Equilibrium position is not the same thing as reaction rate.
• Filtration does not remove dissolved substances.