AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2024: Question 4

✅ Correct answer

A strong acid is completely ionised in aqueous solution.

💡 Key knowledge

• Strong is about how much the acid ionises.
• It is not about concentration.
• A strong acid can still be dilute.

❌ Common errors

• Saying “it has a low pH” — not the definition.
• Saying “it is concentrated” — strong and concentrated are different ideas.
• Missing in water / aqueous solution.

🧠 Exam technique

For 1-mark definition questions, give the exact science phrase. Here, the key words are completely ionised and aqueous solution .

✅ Correct answer

A low concentration of nitric acid dissolved in water.

💡 Key knowledge

• Dilute means not much solute per volume of solution.
• Aqueous means dissolved in water.

❌ Common errors

• Saying “weak acid” — dilute does not mean weak.
• Saying “lots of water” without mentioning low concentration.

✅ Correct answer

The hydrogen ion concentration decreases by a factor of 100.

📐 Calculation idea

1. pH increases from 1 to 3 = change of 2 pH units.
2. Each pH unit means a factor of 10 change in H⁺ concentration.
3. So total change = 10 × 10 = 100.
4. Because pH went up, H⁺ concentration went down.

💡 Key knowledge

As pH increases, the solution becomes less acidic, so the concentration of H⁺ ions decreases.

❌ Common errors

• Choosing factor of 10 instead of 100.
• Saying “increases” instead of “decreases”.
• Forgetting that a change of 2 pH values means two lots of ×10.

🧠 Exam technique

Whenever pH changes by more than 1, count the number of pH steps carefully. Examiner reports often show students lose easy marks by only using one factor of 10.

✅ Correct answer

H⁺ + OH⁻ → H₂O

💡 Key knowledge

This is the ionic equation for neutralisation. Hydrogen ions from the acid react with hydroxide ions from the alkali to form water.

❌ Common errors

• Writing a full symbol equation instead of the ionic equation.
• Forgetting the charges: H⁺ and OH⁻ .
• Writing H₂ + O instead of H₂O.

✅ Correct answer

The ratio of magnesium carbonate to nitric acid is 1 : 2.

📐 How to get it

1. Look at the numbers in front of each substance.
2. MgCO₃ has no number written, so it means 1.
3. HNO₃ has a coefficient of 2.
4. So the mole ratio MgCO₃ : HNO₃ is 1 : 2.

🧠 Exam technique

Use the big balancing numbers in front of formulas, not the small numbers inside formulas.

❌ Common errors

• Using the nitrate subscript and giving the wrong ratio.
• Choosing 2 : 1 because the order of substances was not read carefully.

✅ Correct answer

The solution would be red.

💡 Key knowledge

• The nitric acid is in excess, so after the reaction the mixture is still acidic.
• Universal indicator is red in strongly acidic solutions.

❌ Common errors

• Choosing green because students think “neutralisation” must mean neutral.
• Ignoring the word excess.

🧠 Exam technique

Words like excess, limiting, and complete reaction often decide the answer. Here, excess acid means the final solution is still acidic.

✅ Correct answers

Any two from:

• Repeat the experiment for each mass of magnesium carbonate.
• Discard anomalous results and calculate a mean.
• Measure the highest temperature.

🧠 Exam technique

• The question says method, not apparatus.
• So do not say things like “use a lid” or “use a better thermometer”.
• Best answers focus on how the experiment is carried out.

💡 Why these improvements help

• Repeating and calculating a mean makes results more reliable.
• Measuring the highest temperature gives a more accurate value for the temperature change, because the temperature may keep rising after stirring starts.

❌ Common errors

• Referring to apparatus, for example “use insulation”, “use a digital thermometer”, or “use a lid”. These are not credited here.
• Saying only “do it carefully” — too vague.
• Saying “stir more” without linking to a mark scheme point.

💡 What strong answers did well

• Used precise science vocabulary: completely ionised, low concentration, aqueous.
• Read equations carefully to get the correct mole ratio.
• Spotted that excess acid means the final solution is still acidic.
• Followed the instruction in 04.7 and improved the method, not the apparatus.

❌ Where students lost marks

• Mixing up strong and concentrated.
• Not understanding that a pH change of 2 means a factor of 100.
• Writing a general word equation instead of the required ionic equation.
• Giving sensible practical ideas that were not allowed because they changed the apparatus.