AQA GCSE Combined Science: Trilogy Physics Paper 1 (Higher), 2019: Question 2

✅ Correct answer

Pressure decreased because the air molecules had less kinetic energy, so they moved more slowly and collided with the container walls less often each second.

💡 Key knowledge

• Gas pressure is caused by particles hitting the walls of the container.
• When temperature decreases, particles lose kinetic energy.
• Slower particles mean fewer collisions per second and less force on the walls.
• The volume stayed the same, so the pressure change is due to particle motion, not a change in space.

🧠 Exam technique

To get all 3 marks, make sure you include all three ideas:

1. Pressure decreases.
2. Particles have less kinetic energy / move more slowly.
3. There are fewer collisions with the walls each second.

The mark scheme allowed speed/velocity and less force on the walls as alternatives.

❌ Common errors

• Saying only “temperature decreases so pressure decreases” without explaining why.
• Talking about particles getting smaller or “losing mass” — this is wrong.
• Forgetting to mention collisions with the container walls.
• Saying particles stop moving completely — they still move, just more slowly.

📐 Calculations: step-by-step

Use the equation:

energy = mass × specific latent heat

E = m × L

1. Convert the energy to joules: 0.70 kJ = 700 J
2. Write the values into the equation: 700 = m × 330000
3. Rearrange: m = 700 ÷ 330000
4. Calculate: m = 0.00212... kg
5. Round appropriately: m = 0.0021 kg

Mass of ice = 0.0021 kg

✅ Correct answer

0.0021 kg

This matches the mark scheme’s accepted answer of 0.0021(212121...).

💡 Key knowledge

• Specific latent heat of fusion is the energy needed to change 1 kg of a substance from solid to liquid, or released when it freezes.
• For AQA questions, always check units carefully: kJ must be changed to J.
• The formula uses J, kg, and J/kg.

❌ Common errors

• Using 0.70 instead of 700 J.
• Rearranging incorrectly: the mass must be E ÷ L , not L ÷ E .
• Writing the answer in grams without converting correctly.
• Not giving enough significant figures — 0.0021 kg is the expected form.

💡 Key knowledge

• Oxygen boils at −183 °C, so at −190 °C it is below its boiling point and therefore liquid.
• Nitrogen boils at −195 °C, so at −190 °C it is above its boiling point and therefore gas.
• Carbon dioxide sublimes at −78 °C, so at −190 °C it is solid.

🧠 Exam technique

The key is to compare the temperature to the substance’s change-of-state point:

• If the temperature is below freezing point, it is solid.
• If the temperature is between freezing and boiling point, it is liquid.
• If the temperature is above boiling point, it is gas.

For carbon dioxide, remember the word sublimes means it changes directly from solid to gas.

✅ What a top-level answer includes

• Argon cools, so particles lose kinetic energy and move more slowly.
• In the gas state, particles are far apart and move randomly in all directions.
• As cooling continues, particles come closer together and collisions become more frequent.
• Argon condenses from a gas to a liquid.
• As it cools further, the liquid particles lose more energy.
• The particles become arranged in a more regular pattern and finally form a solid, where they are fixed in position and only vibrate.

💡 Key knowledge

• Internal energy decreases as temperature decreases.
• Kinetic energy decreases with temperature.
• During a change of state, particles become closer together and attractive forces become more important.
• In a solid, particles are in fixed positions and vibrate.
• Attraction between particles is stronger in a solid than in a liquid.

🧠 Exam technique

This is a Level 3 answer if you do two things well:

1. Describe both arrangement and movement of particles.
2. Link the changes clearly to cooling and change of state.

Use a clear sequence like this:

gas → liquid → solid

That helps you show the full journey from 20 °C to −190 °C.

❌ Common errors

• Only describing temperature dropping without mentioning particles.
• Saying particles “stop moving” at low temperature — they still vibrate in a solid.
• Not explaining the change from random motion to vibration in fixed positions.
• Forgetting that particles get closer together as gases condense and liquids freeze.