AQA GCSE Combined Science: Trilogy Physics Paper 1 (Higher), 2018: Question 3

✅ Correct answers

• They move in random directions.
• They move with a range of different speeds.

💡 Key knowledge

• Gas particles are constantly moving.
• Their motion is not predictable.
• Different particles have different energies, so speeds vary.

🧠 Exam technique

• Look for gas particle ideas: random motion and different speeds.
• “Vibrate about a fixed position” is for solids, not gases.
• “Circular paths” is not the particle model for gases.

❌ Common errors

• Ticking predictable motion — this is wrong.
• Choosing vibrate about a fixed position — that describes solids.
• Thinking gas particles all move at the same speed.

✅ Correct answer

The mean speed of the particles would increase.

Also accepted: kinetic energy increases.

💡 Key knowledge

• Heating a gas gives particles more energy.
• This makes them move faster on average.
• In particle language: temperature increase → kinetic energy increase → higher mean speed.

🧠 Exam technique

• Use the word mean if you can — it shows you know it is an average.
• For 1 mark, a short statement is enough.

❌ Common errors

• Saying only “temperature increases” without linking to particle motion.
• Saying particles “expand” — particles themselves do not get bigger.
• Talking about pressure here instead of motion.

✅ Correct answer

If the temperature increases, the pressure increases, so it could explode.

Acceptable extra wording: large pressure difference.

💡 Key knowledge

• Higher temperature means faster particles.
• Faster particles collide more often and with more force.
• This raises the gas pressure inside the canister.
• If pressure becomes too high, the container may burst/explode.

🧠 Exam technique

• To get both marks, give a cause and result.
• Good structure: temperature ↑ → pressure ↑ → explode

❌ Common errors

• Saying only “it gets hot” — not enough.
• Not linking temperature to pressure.
• Writing “it would melt” instead of explode/burst.

✅ Correct answer

Atmospheric pressure = 0.1 MPa

💡 Key knowledge

• When the canister empties, the measured pressure approaches atmospheric pressure.
• The flat part of the graph shows the lowest pressure reached.

🧠 Exam technique

• Read from the plateau of the graph.
• Use the correct unit: MPa.

❌ Common errors

• Using 0.2 MPa or 0.0 MPa instead of the settled value.
• Forgetting the unit.

📐 Calculations: step by step

1. Find the original pressure from the graph: about 2.25 MPa.
2. Calculate 25% of 2.25 MPa:
   2.25 × 25/100 = 0.5625 MPa
3. Estimate on the graph where the pressure is about 0.56 MPa.
4. Read the time: about 27 minutes.

✅ Correct answer

Time = 27 minutes

🧠 Exam technique

• Show the percentage calculation clearly for method marks.
• Use the graph carefully and quote a sensible estimate.
• Keep units with pressure: MPa.
• If your pressure is slightly different, your time can still score if it matches your value.

❌ Common errors

• Using 25% of 0.1 MPa instead of 25% of the original pressure.
• Reading the wrong point on the graph.
• Writing 0.56 with no unit.
• Giving a time far from the graph value instead of an estimate.

✅ Correct answer

The volume of the air increases.

💡 Key knowledge

• Gas spreads out when pressure is reduced.
• When released from the canister, the air expands.

🧠 Exam technique

• For 1 mark, a direct statement is enough.
• Use the word increases or expands.

❌ Common errors

• Saying the volume decreases.
• Mixing up pressure and volume.

💡 What distinguished strong answers

• They used the gas particle model correctly.
• They linked temperature increase to higher particle speed and pressure.
• They read the graph carefully and used the correct unit.
• They showed the percentage working clearly in the calculation.

🧠 Quick memory phrases

• Gas particles: random directions, different speeds
• Heating gas: mean speed increases
• Danger: pressure increases, can explode
• Graph skill: read the plateau and estimate carefully
• Volume: gas expands when released