Ideal gas molecules questions

Ideal gas molecules syllabus

Questions on ideal gas molecules can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

• 5.15 explain how molecules in a gas have random motion and that they exert a force, and hence a pressure, on the walls of a container
• 5.16 understand why there is an absolute zero of temperature, which is –273 °C
• 5.17 describe the Kelvin scale of temperature and be able to convert between the Kelvin and Celsius scales
• 5.18 understand why an increase in temperature results in an increase in the average speed of gas molecules
• 5.19 know that the Kelvin temperature of a gas is proportional to the average kinetic energy of its molecules
• 5.20 explain, for a fixed amount of gas, the qualitative relationship between: pressure and volume at constant temperature pressure and Kelvin temperature at constant volume
• 5.21 use the relationship between the pressure and Kelvin temperature of a fixed mass of gas at constant volume: p₁ / T₁ = p₂ / T₂
• 5.22 use the relationship between the pressure and volume of a fixed mass of gas at constant temperature: p₁V₁ = p₂V₂

How to answer ideal gas molecules questions

1. Sketch or describe particle spacing, arrangement and motion before explaining bulk behaviour.
2. Convert volumes and temperatures carefully; gas-law temperatures must be in kelvin.
3. State which variables are fixed when describing pressure-volume or pressure-temperature changes.
4. For practical methods, identify direct measurements, repeated readings and the calculation used.

Ideal gas molecules example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Applying the particle model of a gas

June 2020 (R) · Paper 1P · Question 13(b)

Question 1

This question is about pressure in gases.

Photograph 2 shows a boiled egg, without its shell, placed in the top of the conical flask containing hot air.

The flask is no longer being heated.

The egg seals the flask so that no air escapes.

Photograph 3 shows the egg and the flask a short time later.

Explain why the egg moves down into the flask and then stops moving.

Refer to ideas about pressure in your answer.

Final answer

The air in the flask cools, so its molecules move more slowly and the pressure inside the flask decreases. The greater pressure outside pushes the egg down into the flask. As the egg moves down, the volume of the trapped air decreases and its pressure rises. The egg stops when the pressure inside and outside balance, so there is no resultant force pushing it further down.

Mark scheme points

Any four of the following mark-bearing points are sufficient; together they should explain why the egg moves down and then stops.

1. M1 The air in the flask cools.
2. M2 The molecules slow down, or their kinetic energy decreases.
3. M3 The pressure inside the flask decreases as its temperature decreases.
4. M4 The pressure outside the flask is greater than the pressure inside.
5. M5 The resultant force from the air pushes the egg down the neck of the flask.
6. M6 As the egg moves down, the volume of air in the flask decreases.
7. M7 The pressure inside the flask increases as the volume decreases.
8. M8 Eventually, the pressure inside and outside the flask balance.
9. M9 The resultant force is then zero, so the egg stops moving down.

Explanation

Cooling makes the gas molecules move more slowly. Their collisions with the flask walls produce a lower pressure inside the flask, so the greater pressure outside pushes the egg inward.

As the egg moves farther into the flask, it reduces the volume available to the trapped air, causing the internal pressure to rise. The egg stops when the inside and outside pressures balance and the resultant force on it becomes zero.

Common mistakes

• Saying only that the air cools, without explaining that the pressure inside the flask decreases.
• Reversing the pressure difference: it is the greater pressure outside that pushes the egg down.
• Explaining why the egg moves but not why it stops: include the pressure balance and zero resultant force.

Example 2: Reading a pressure gauge

January 2020 · Paper 1P · Question 7(a)

Question 2

A student uses a syringe containing trapped air to investigate pressure.

Diagram 1 shows the apparatus he uses.

Diagram 2 shows the pressure gauge when the piston is at its initial position.

Determine the reading on the pressure gauge.

pressure = ________ kPa

Final answer

The pressure gauge reads 24 kPa.

Mark scheme points

1. M1 Read the gauge as 24 kPa.

Explanation

Follow the pointer to the scale: it indicates a pressure of 24 kPa.

Common mistakes

• A small minority misread the gauge as 20.4 kPa. Check the pointer against the scale carefully and give the reading in kPa.

Practise Ideal gas molecules questions

Build a focused practice set from this part of the Edexcel IGCSE Physics syllabus.