AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2022: Question 7

✅ Correct Answer

nitric acid

💡 Key Knowledge

Fertiliser salts are produced by neutralisation reactions between ammonia and an acid:

• Ammonia + nitric acid → ammonium nitrate
• Ammonia + sulfuric acid → ammonium sulfate
• Ammonia + hydrochloric acid → ammonium chloride

🧠 Exam Technique

Look at the name of the salt to deduce the acid: any salt ending in nitrate must come from nitric acid.

❌ Common Errors

Selecting sulfuric acid because students vaguely remember it is used in industry, without matching the "nitrate" part of the product name.

🧠 Exam Technique

Always check the units required on the answer line. Here it asks for kg/m² , matching the quantities given in the question (kg and m²), so no unit conversions (e.g. into grams) are needed.

❌ Common Calculation Traps

• Forgetting to multiply by 40 bags (calculating for only 1 bag: 600 ÷ 800 000).
• Inverting the division (dividing area by mass: 800 000 ÷ 24 000 = 33.3).
• Miscounting zeros on your calculator when entering 800 000.

✅ Correct Answer (Any one of)

• (The scientist might be) biased
• (There was) no peer review
• The investigation was not repeated by other scientists / independently

💡 Key Knowledge

Commercial scientists work for a company that sells products. They have a financial incentive for their company's product to look best. To be accepted as fully valid, scientific claims must be:

• Subject to independent peer review.
• Reproduced and verified by independent researchers.

❌ Common Errors

Vague answers such as "they might have lied" or "they didn't test all crops". Use the precise scientific terms: bias or lack of peer review.

🧠 Exam Technique

Whenever a question states that a scientist "works for a company that makes/sells" the product under test, the examiner is prompting you to identify commercial bias.

✅ Correct Answer

1 (or one)

💡 Formula Breakdown: K₂NH₄PO₄

• K₂ = 2 Potassium atoms
• N = 1 Nitrogen atom (no subscript means 1)
• H₄ = 4 Hydrogen atoms
• P = 1 Phosphorus atom
• O₄ = 4 Oxygen atoms

❌ Common Errors

Writing 4 because students see the subscript 4 directly following NH₄ and assume it applies to the nitrogen rather than the hydrogen.

🧠 Exam Technique

Subscripts apply ONLY to the element directly before them. If there is no subscript written after a capital letter, the count is always 1.

✅ Correct Answer

[ ] Hydrogen
[ ] Oxygen
[✓] Phosphorus

💡 Key Knowledge: The N-P-K Elements

NPK fertilisers provide the three main mineral nutrients plants require for healthy growth:

• N = Nitrogen (for healthy leaves and growth)
• P = Phosphorus (for root development and flowers)
• K = Potassium (for disease resistance and fruit growth)

❌ Common Errors

Ticking oxygen or hydrogen because they are present in the formula. While plants need them, they are supplied via air and water ( CO₂ and H₂O ), not as the key nutrient supplied by NPK fertilisers.

✅ Correct Answer

formulation

💡 Definition to Memorise

A formulation is a mixture that has been designed as a useful product. In a formulation, each chemical component is present in carefully measured, fixed quantities to ensure the product has the required properties.

Examples from the GCSE course: Fertilisers, medicines, paints, cleaning agents, fuels, and foods.

❌ Common Errors

Writing "compound", "molecule", or "alloy". While an alloy is a type of formulation, the universal scientific term tested across Paper 2 for a designed mixture is formulation.

🧠 Exam Technique

The clue is right in the question prompt: "a mixture of different compounds in fixed proportions". Learn this definition word-for-word, as it appears regularly on Paper 2!