AQA GCSE Combined Science: Trilogy Physics Paper 1 (Higher), 2020: Question 5

✅ Correct answer

They vibrate about fixed positions.

💡 Key knowledge

• In a solid, particles are packed closely together in a fixed arrangement.
• They cannot move freely from place to place.
• They only vibrate around their fixed positions.

🧠 Exam technique

For 1 mark, choose the option that matches the particle model for a solid. Use the word vibrate — it is the key examiner clue.

❌ Common errors

• Stationary is too absolute — particles still move, but only by vibrating.
• Move freely describes a liquid or gas, not a solid.

✅ Correct answer

Kinetic energy decreases
Potential energy decreases

💡 Key knowledge

• Cooling means particles lose energy, so their kinetic energy decreases.
• When the liquid freezes, particles become more ordered and closer together, so potential energy decreases.

🧠 Exam technique

Think in two parts: cooling lowers kinetic energy, and freezing lowers potential energy. The correct answer must include both changes.

❌ Common errors

• Choosing does not change for either energy is wrong because freezing involves energy loss.
• Mixing up kinetic and potential energy is a very common mistake.

✅ Correct answer

Metal: has a high thermal conductivity, so it transfers energy from the mixture to the cold coolant quickly.

Plastic: has a low thermal conductivity, so it reduces energy transfer from the surroundings into the coolant/mixture.

💡 Key knowledge

• High thermal conductivity = energy passes through quickly.
• Low thermal conductivity = energy passes through slowly.
• The metal inner surface helps the mixture lose energy faster.
• The plastic outer surface helps reduce unwanted warming from outside.

🧠 Exam technique

• To score full marks, link the property to the effect on energy transfer.
• Use phrases like increases the rate and reduces the rate.
• Remember the question is about why the materials are used, not just what they are.

❌ Common errors

• Just writing “metal is a good conductor” may not be enough unless you explain the effect.
• Just saying “plastic is an insulator” is weaker than saying it has low thermal conductivity.
• Do not only talk about “insulation” — the mark scheme accepts references to insulation, but the safest answer is thermal conductivity plus energy transfer.

✅ Correct answer

The coolant should have a high specific heat capacity.

That means it can absorb a large amount of energy with only a small temperature change.

💡 Key knowledge

• Specific heat capacity tells you how much energy is needed to change temperature.
• A high specific heat capacity helps the coolant absorb energy without warming up too quickly.

🧠 Exam technique

For this 2-mark style explanation, give the property and then explain its effect. The key phrase is large amount of energy with only a small temperature change.

❌ Common errors

• Vague answers like “it should stay cold” do not name a science property.
• Do not repeat the freezing point idea — the question asks for one other property.

📐 Calculations — step by step

1. Convert energy to joules: 165 kJ = 165000 J
2. Find the temperature change before freezing:
   ΔT = 20 − (−1.5) = 21.5 °C
3. Energy removed while cooling:
   E = mcΔT
   E = m × 3500 × 21.5
   E = 75250m
4. Energy removed while freezing:
   E = mL
   E = m × 255000
   E = 255000m
5. Total energy removed:
   165000 = 75250m + 255000m
   165000 = 330250m
6. Solve for mass:
   m = 165000 ÷ 330250
   m = 0.4996...
7. Answer to 2 significant figures:
   m = 0.50 kg

✅ Correct answer

Mass = 0.50 kg (2 s.f.)

💡 Key knowledge

• Use E = mcΔT for heating/cooling without a change of state.
• Use E = mL for freezing or melting.
• Total energy is the sum of the energy for cooling and the energy for freezing.

🧠 Exam technique

• Always convert kJ to J.
• Use the correct temperature change: 20 − (−1.5) = 21.5, not 18.5.
• Show substitution clearly to gain method marks.
• Keep units consistent: J, J/kg °C, and kg.
• Round only at the end to the required significant figures.

❌ Common errors

• Forgetting to include both cooling and freezing energies.
• Using 18.5 °C instead of 21.5 °C.
• Not converting 165 kJ to 165000 J.
• Giving the mass in grams instead of kilograms.
• Rounding too early and losing accuracy.

✅ Part (a)

Solid particles vibrate about fixed positions.

✅ Part (b)

Kinetic energy decreases and potential energy decreases.

✅ Part (c)

Metal has high thermal conductivity; plastic has low thermal conductivity.

✅ Part (d)

Coolant should have a high specific heat capacity.

✅ Part (e)

Mass = 0.50 kg