AQA GCSE Chemistry Chemistry Paper 2 (Higher), 2023: Question 10

10 marks · Standard Demand difficulty · Short Answer

Evaluate the use of three different compounds as fertilisers for a specific soil, and answer questions on the sourcing and chemical production of NPK fertilisers.

Practise this question

Question

Question text

10 This question is about fertilisers.

Compounds of nitrogen (N), phosphorus (P) and potassium (K) are used as fertilisers

to improve agricultural productivity.

Table 4 shows information about three compounds, A, B and C, that can be used

as fertilisers.

Table 4

Compound A Compound B Compound C

diammonium

Name potassium chloride ammonium nitrate

hydrogen phosphate

Formula KCl NH4NO3 (NH4)2HPO4

Percentage (%) of N,

K: 52% N: 35% N: 21% P: 23%

P and K by mass

Cost in £/kg 0.24 0.23 0.35

10.1 A scientist analysed the percentages of nitrogen, phosphorus and potassium in a soil.

The percentages of nitrogen and of potassium in the soil were lower than the

percentages needed for high agricultural productivity.

There was sufficient phosphorus in the soil for high agricultural productivity.

Evaluate the use of the compounds in Table 4 to improve the agricultural productivity

of this soil.

[4 marks]

10.2 How is potassium chloride (compound A) obtained from the Earth?

[1 mark]

10.3 Name one other compound that could be used instead of potassium chloride

(compound A) to give a similar improvement in agricultural productivity.

[1 mark]

10.4 Nitric acid is needed to produce ammonium nitrate (compound B).

Name a compound needed to produce nitric acid.

[1 mark]

10.5 Phosphate rock contains phosphorus compounds.

Plants absorb phosphorus from compounds dissolved in rainwater.

Suggest why phosphate rock cannot be used directly as a fertiliser.

[1 mark]

10.6 Phosphate rock can be treated with different acids to produce salts useful

as fertilisers.

Name the salts which are produced by treating phosphate rock with:

• sulfuric acid

• phosphoric acid.

[2 marks]

Sulfuric acid

Phosphoric acid

Mark scheme

Show the mark scheme

Question 10

AO/

Answers Mark

Question Spec. Ref

Level 3: A judgement, strongly linked and logically supported by a AO3

10.1 3–4

sufficient range of correct reasons, is given. 4.10.1.1

4.10.2.2

Level 2: Some logically linked reasons are given. There may also 4.10.4.2

1–2

be a simple judgement.

No relevant content 0

Indicative content

reasons

• compound A (potassium chloride) only contains potassium

• compound A (potassium chloride) is the only source of

potassium so is needed.

• compound B (ammonium nitrate) only contains nitrogen

• compound B (ammonium nitrate) contains more nitrogen than

compound C (diammonium hydrogen phosphate) so is preferable

• compound B (ammonium nitrate) contains more nitrogen and is

cheaper than compound C (diammonium hydrogen phosphate)

and so is more cost effective

• compound C (diammonium hydrogen phosphate) contains

phosphorus which is not needed

judgement

• none of the compounds contain both nitrogen and potassium so

a mixture is needed

• (both) compound A (potassium chloride) and B (ammonium

nitrate) should be used

• (both) compound A (potassium chloride) and C (diammonium

phosphate) could be used

AO /

Question Answers Extra information Mark

Spec. Ref.

10.2 mining allow quarrying 1 AO1

4.10.4.2

AO /

Spec. Ref.

10.3 ignore potassium chloride AO1

4.10.4.2

potassium sulfate allow potassium nitrate 1

allow any other named

potassium salt

AO /

Spec. Ref.

10.4 ammonia allow water 1 AO1

4.10.4.2

AO /

Spec. Ref.

10.5 (phosphate rock is) insoluble (in allow (phosphate rock) cannot 1 AO3

water) be absorbed as a solid 4.10.4.2

AO /

Spec. Ref.

10.6 (sulfuric acid) AO1

calcium sulfate allow single superphosphate 1

4.10.4.2

allow calcium phosphate

(phosphoric acid)

calcium phosphate allow triple superphosphate 1

Total Question 10 10

How to answer it

NPK Fertilisers & Chemical Production

What this question tests

This question assesses your understanding of NPK fertilisers (nitrogen, phosphorus, and potassium), how their raw materials are sourced, and how they are chemically processed. Key skills include:

Evaluating data from a table to select the best combination of fertilisers for a specific soil.
Recalling the sources of potassium salts and phosphate rock.
Knowing the chemical reactions used to convert insoluble phosphate rock into soluble, usable salts.

Part (a) — Question 10.1

4 Marks

Evaluate the use of the compounds in Table 4 to improve the agricultural productivity of this soil.

💡 Key Knowledge

Soil Needs: The soil is deficient in Nitrogen (N) and Potassium (K), but has plenty of Phosphorus (P).
Compound A (KCl): Contains potassium ( K: 52% ). It is the only source of potassium available.
Compound B (NH₄NO₃): Contains nitrogen ( N: 35% ). It contains no phosphorus.
Compound C ((NH₄)₂HPO₄): Contains nitrogen ( N: 21% ) and phosphorus ( P: 23% ).

🧠 Exam Technique

For an evaluate question, you must compare the options using the data provided (percentages and costs) and make a clear, justified judgement at the end.

Structure your answer by analyzing what the soil needs, comparing the compounds, and stating which ones to mix.

✅ Model Answer & Mark Breakdown

Reasoning for Compound A: Compound A (potassium chloride) must be used because it is the only source of potassium ( K ). [1 Mark]
Reasoning for Compound B vs C: Compound B (ammonium nitrate) is better than Compound C because it has a higher percentage of nitrogen ( 35% vs 21% ) and is cheaper ( £0.23/kg vs £0.35/kg ). [1 Mark]
Reasoning against Compound C: Compound C contains phosphorus ( P ) which is not needed by this soil. [1 Mark]
Final Judgement: Therefore, a mixture of Compound A and Compound B should be used to provide both N and K without wasting money on unnecessary phosphorus. [1 Mark]

❌ Common Errors

No Judgement: Students often list facts about the table but forget to write a concluding sentence stating exactly which compounds should be used.
Ignoring the Soil Requirements: Recommending Compound C because "it has more nutrients" while ignoring that the soil already has sufficient phosphorus.

Part (b) — Question 10.2

1 Mark

How is potassium chloride (compound A) obtained from the Earth?

✅ Correct Answer

Mining (or quarrying)

💡 Key Knowledge

Potassium chloride ( KCl ) is a soluble salt found in underground rock deposits. It is extracted directly from the ground by mining, just like common table salt ( NaCl ).

Part (c) — Question 10.3

1 Mark

Name one other compound that could be used instead of potassium chloride (compound A) to give a similar improvement in agricultural productivity.

✅ Correct Answer

Potassium sulfate (or potassium nitrate)

Accept any other named soluble potassium salt. Do not write potassium chloride.

🧠 Exam Technique

The question asks for a compound that gives a similar improvement. Since Compound A provides potassium ( K ), you must name another potassium compound commonly used in agriculture. Simply swap "chloride" for another common anion like "sulfate" or "nitrate".

Part (d) — Question 10.4

1 Mark

Nitric acid is needed to produce ammonium nitrate (compound B). Name a compound needed to produce nitric acid.

✅ Correct Answer

Ammonia (or water)

💡 Key Knowledge

Nitric acid ( HNO₃ ) is manufactured from ammonia ( NH₃ ) in a multi-step industrial process (the Ostwald process) where ammonia is oxidised in the presence of a catalyst.

❌ Common Errors

Writing "nitrogen" or "hydrogen". While these elements make up ammonia, they are not the direct compound used to manufacture nitric acid in industrial chemistry.

Part (e) — Question 10.5

1 Mark

Suggest why phosphate rock cannot be used directly as a fertiliser.

✅ Correct Answer

It is insoluble in water (so plants cannot absorb it).

Also accept: "It cannot be absorbed as a solid".

💡 Key Knowledge

Plants can only absorb mineral ions through their roots if those ions are dissolved in water (rainwater/soil moisture). Because raw phosphate rock is insoluble, the phosphorus is locked away and unavailable to the plant.

Part (f) — Question 10.6

2 Marks

Name the salts which are produced by treating phosphate rock with: sulfuric acid and phosphoric acid.

✅ Correct Answer

Sulfuric acid: Calcium sulfate [1 Mark]
(also accept: single superphosphate / calcium phosphate)
Phosphoric acid: Calcium phosphate [1 Mark]
(also accept: triple superphosphate)

🧠 Exam Technique

Memorise this simple summary of phosphate rock reactions:

Acid Added	Product Name
Nitric acid	Calcium nitrate + Phosphoric acid
Sulfuric acid	Single superphosphate (calcium phosphate & calcium sulfate)
Phosphoric acid	Triple superphosphate (calcium phosphate)

Topics

Chemistry · C10: Using Resources

Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Higher), 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.