AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Foundation), 2024: Question 5

AQA GCSE Combined Science: Trilogy · Question 5

✅ Correct answer

Hydrogen

💡 Key knowledge

• Most metals reacting with acids produce a salt + hydrogen.
• Magnesium is above hydrogen in the reactivity series, so it reacts with hydrochloric acid.

🧠 Exam technique

• For a 1-mark multiple choice question, you only need the correct tick.
• If writing the equation, remember hydrogen is a diatomic molecule: H₂.

❌ Common errors

• Choosing nitrogen or oxygen because they are common gases.
• Forgetting that metal + acid gives hydrogen, not a metal oxide gas.

✅ Correct answer

MgCl₂(l)

💡 Key knowledge

• (l) means liquid / molten.
• (g) means gas, and (s) means solid.
• “Molten” means the substance has been melted, so it is a liquid.

🧠 Exam technique

State symbols are easy marks. Read the keyword carefully: molten = liquid.

❌ Common errors

• Ticking MgCl₂(s) because salts are often solids at room temperature.
• Ticking MgCl₂(g) because electrolysis uses electricity, but the compound itself is not a gas.

✅ Correct answers

Negative electrode: magnesium

Positive electrode: chlorine

💡 Key knowledge

• Molten MgCl₂ contains Mg²⁺ and Cl⁻ ions only.
• Positive ions go to the negative electrode and gain electrons.
• Negative ions go to the positive electrode and lose electrons.
• At the cathode: Mg²⁺ + 2e⁻ → Mg
• At the anode: 2Cl⁻ → Cl₂ + 2e⁻

🧠 Exam technique

Mark schemes often award one mark per electrode. If you reverse the names, you may still get credit for identifying both substances, but the best answer is to match each ion to the correct electrode.

❌ Common errors

• Writing magnesium chloride instead of the element formed.
• Putting chlorine at the negative electrode.
• Forgetting that in electrolysis of a molten ionic compound, the products are the elements.

✅ Correct answer

Most reactive: magnesium

Then: iron

Least reactive: copper

💡 Key knowledge

• A more reactive metal will displace a less reactive metal from its salt solution.
• If there is no reaction, the solid metal is less reactive than the metal in the solution.
• Magnesium reacts with both copper chloride and iron chloride, so it is the most reactive.
• Iron reacts with copper chloride but not magnesium chloride, so iron is between magnesium and copper.
• Copper does not react with either magnesium chloride or iron chloride, so copper is the least reactive.

🧠 Exam technique

• Give two reasons for 3 marks: one for the order, two supporting statements.
• Use evidence from the table in clear sentences, e.g. “Magnesium reacts with both solutions.”
• Top responses compare all three metals, not just one reaction.

❌ Common errors

• Thinking “no reaction” means the metal is more reactive.
• Only stating the order without using the table as evidence.
• Mixing up the meaning of displacement reactions.

✅ Correct answer

1.6 g

💡 Key knowledge

Use conservation of mass:

mass of reactants = mass of products

🧠 Exam technique

• This is a quick subtraction question, but you still need to show the calculation clearly for full confidence.
• Use the numbers given exactly as shown and include the unit g.
• Watch out for whether the question asks for the mass of a reactant or a product.

❌ Common errors

• Adding the two masses instead of subtracting.
• Using 2.32 − 0.72 = 1.60 but forgetting the unit.
• Writing 1.6 without understanding it comes from conservation of mass.

✅ Correct answer

5.4 g

💡 Key knowledge

• 1 dm³ = 1000 cm³
• The mass in a smaller volume is found by using a proportion or ratio.
• Mass is directly proportional to volume for the same solution.

Alternative ratio method: 1000 cm³ contains 180 g, so 30 cm³ contains (180 ÷ 1000) × 30 = 5.4 g

🧠 Exam technique

• Always check whether you need to convert cm³ to dm³.
• In GCSE chemistry, cm³ to dm³ conversion is a very common trap.
• Use the correct number of significant figures if the question suggests it; here 5.4 g is suitable.

❌ Common errors

• Using 30 × 180 without converting units.
• Dividing the wrong way round.
• Writing 5.40 g is not wrong, but the key is to show the method clearly.

✅ Answers in order

1. Hydrogen
2. MgCl₂(l)
3. Negative electrode: magnesium; Positive electrode: chlorine
4. Magnesium, iron, copper
5. 1.6 g
6. 5.4 g

🧠 What top-level responses did well

• Used precise science terms: hydrogen, molten, displacement, reactivity.
• Matched answers directly to evidence in the table.
• Showed calculations clearly and included units.