AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2018: Question 3

Mark scheme

Show the mark scheme

AO /

Question Answers Mark

Spec. Ref.

03 Level 3: Relevant points (reasons / causes) are identified, given in 5–6 AO3

detail and logically linked to form a clear account.

Level 2: Relevant points (reasons / causes) are identified, and 3–4 AO3

there are attempts at logical linking. The resulting account is not

fully clear.

Level 1: Points are identified and stated simply, but their relevance 1–2 AO1

is not clear and there is no attempt at logical linking.

No relevant content 0

Indicative content: 5.4.2.3

• uses sulfuric acid not hydrochloric acid

or sulfuric acid needed

• uses copper carbonate / oxide not calcium carbonate

or copper carbonate / oxide needed

• add solid until solid remains or is in excess or no more

reacts / dissolves

so that most / all of the acid reacts

• filter

to remove excess or unreacted carbonate / oxide / solid

• heat gently or partially evaporate or leave

until crystals appear or to crystallise

for level 3 the correct chemicals must have been selected

Total 6

How to answer it

Copper Sulfate Crystals: fixing the method

What this question tests

You need to recognise the correct method for making a soluble salt by reacting an acid with an insoluble base, then explain how to get pure crystals. The marks are for selecting the right acid and solid, making sure the acid is fully reacted, filtering off excess solid, and then gently evaporating and crystallising the solution.

Question title

Making pure copper sulfate crystals from an acid and insoluble base

Exam focus: Explain the improvements to a flawed preparation method. This is a 6-mark extended response, so you need several correct points linked together logically.

Part (a) / whole question

How should the student improve the method?

✅ Correct answers to include

Use sulfuric acid, not hydrochloric acid.
Use a copper compound such as copper oxide or copper carbonate, not calcium carbonate.
Add the solid until no more reacts or some solid remains so the acid is fully used up.
Filter the mixture to remove excess unreacted solid.
Gently heat the filtrate, or partially evaporate it, until crystals start to form.
Leave it to cool and crystallise, then filter and dry the crystals.

💡 Key knowledge

Copper sulfate is made from sulfuric acid.
A soluble salt is usually made by reacting an acid with an insoluble base.
The insoluble base must contain copper ions if the salt is copper sulfate.
Common correct solids: copper oxide, copper carbonate.
Reaction idea: sulfuric acid + copper oxide → copper sulfate + water
If copper carbonate is used: sulfuric acid + copper carbonate → copper sulfate + carbon dioxide + water

🧠 Exam technique

For top marks, give the steps in the correct order.
Link each improvement to why it matters: e.g. “filter to remove excess solid”.
Use the right chemistry: the mark scheme said Level 3 requires the correct chemicals.
A clear answer often sounds like a method, not a list of disconnected facts.

Best responses were detailed and logically linked, not just a few isolated words.

❌ Common errors

Using hydrochloric acid and expecting copper sulfate.
Using calcium carbonate instead of a copper compound.
Heating until all the liquid is gone — this can decompose or ruin the crystals.
Forgetting to filter after excess solid is added.
Saying “add a lot of solid” without explaining that some must remain unreacted.

Mark scheme insight

What earned Level 3?

📐 Full-mark answer structure

Use sulfuric acid and a copper oxide / copper carbonate.
Add the copper compound until it is in excess, so all the acid reacts.
Filter to remove the excess solid.
Gently heat the filtrate to partially evaporate it.
Leave to cool so crystals form.
Remove and dry the crystals.

💡 Why these steps matter

Using the wrong acid gives the wrong salt.
Using calcium carbonate gives calcium sulfate, not copper sulfate.
Adding excess solid ensures no acid is left behind.
Filtering removes unreacted solid so the crystals are pure.
Gentle evaporation avoids making the solution too concentrated too quickly.

Model 6-mark answer

What a top-level response could say

The student should use sulfuric acid instead of hydrochloric acid and react it with an insoluble copper compound such as copper oxide or copper carbonate, not calcium carbonate. Add the copper compound until it is in excess so that all the acid reacts. Then filter the mixture to remove any unreacted solid. Gently heat the filtrate to partially evaporate the water until crystals start to form, then leave it to cool so pure copper sulfate crystals can be collected and dried.

This response would access the higher level because it selects the correct chemicals, gives the full method, and explains the purpose of each step.

Part (b): how marks are awarded

How to think like an examiner

🧠 For 1–2 marks

Simple correct points, but not clearly linked. Example: “Use sulfuric acid and filter it.”

🧠 For 3–4 marks

Several relevant points with some linking, but the explanation may be incomplete.

🧠 For 5–6 marks

Correct chemicals chosen, steps in order, and a clear explanation of why each step is done.

Quick exam checklist

Before you move on, make sure you can say all of these

✅ Must include

sulfuric acid
copper oxide or copper carbonate
excess solid / no more reacts
filter
gently heat / partially evaporate
cool to crystallise

❌ Avoid saying

heat until all liquid is gone
use hydrochloric acid
use calcium carbonate
skip filtration

Topics

Chemistry · C4: Chemical Changes