Edexcel IGCSE Physics 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: Using a Diagram

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 trapped in the flask cools, so the molecules move more slowly and the pressure inside the flask decreases. The pressure outside the flask is then greater than the pressure inside, producing a resultant force that pushes the egg down into the flask. As the egg moves down, the volume of trapped air decreases, so its pressure increases. Eventually, the pressure inside equals the pressure outside, so there is no resultant force and the egg stops moving.

Mark scheme points

Any four of the following points earn the four marks:

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

Explanation

Initially, the flask contains hot air and the egg seals the opening, so no air escapes. When heating stops, the trapped air cools. Its molecules have less kinetic energy and collide with the flask and egg less strongly, reducing the internal pressure.

The greater atmospheric pressure outside the flask pushes the egg downwards. This reduces the volume available to the trapped air, causing its pressure to rise. The egg stops when the inside and outside pressures balance, so the upward and downward pressure forces have no resultant force.

• Movement down: cooling → lower internal pressure → greater outside pressure → downward resultant force.
• Stopping: smaller volume → increased internal pressure → pressures balance → zero resultant force.

Common mistakes

• Do not say that air escapes from the flask; the egg seals it.
• State which pressure is greater. Cooling makes the pressure inside less than the pressure outside.
• Explain the downward movement using a resultant pressure force, not just by saying that the egg is pulled down.
• Explain why the egg stops: the internal pressure rises as the volume decreases until the pressures balance.
• Examiners reported that many candidates ignored the statement that no air escapes. Treating the flask as sealed is the key idea in this question.

Example 2: Using a Diagram

Question 2

A student blows up two balloons to the same size.

She puts one balloon into a freezer.

After a while, the student compares the two balloons.

The balloon that has been cooled is smaller.

15

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The student decides to investigate the link between temperature and the size of the balloon.

She writes a plan.

There are several faults in the student’s plan.

Identify three of these faults and suggest an improvement to correct each one.

1

2

3

Final answer

• Fault: Putting the balloon in the freezer for different times does not produce a range of different temperatures; it will tend towards the freezer temperature. Improvement: Use water baths or freezers set to different temperatures.
• Fault: It is difficult to measure the temperature of the balloon with a thermometer, and this may not measure the temperature of the gas inside. Improvement: Measure the temperature of the surrounding air or water bath.
• Fault: Lining up the curved balloon next to a ruler is an inaccurate way to measure its size. Improvement: Measure its circumference using a tape measure or string.

Mark scheme points

1. M1 Different times in the freezer do not provide a range of temperatures.
2. M2 Use water baths or freezers set to different temperatures.
3. M3 Measuring the balloon with a thermometer is difficult or does not measure the temperature of the gas inside.
4. M4 Measure the temperature of the surroundings, such as the air or water bath.
5. M5 Measuring the balloon by lining it up next to a ruler is inaccurate or difficult because of its shape.
6. M6 Use a more suitable method, such as measuring circumference with a tape measure or string.

Explanation

Each fault must be clearly identified and matched with a practical improvement. Changing the time in one freezer changes the time allowed for cooling, not the freezer’s set temperature, so different controlled-temperature environments are needed.

The thermometer should not simply be placed on the balloon, because this is difficult and may not represent the gas temperature. The temperature of the surrounding air or water bath can be measured instead.

A balloon is curved and flexible, so placing it beside a ruler gives an imprecise size measurement. Circumference measured with string or tape is a clearly defined quantity and can be compared between temperatures.

Common mistakes

• Saying that different times in the same freezer automatically give different temperatures.
• Writing only “measure the temperature better” without stating what should be measured instead.
• Claiming that repeating the experiment makes it a fair test; repeats improve reliability but do not control variables.
• Using the vague word “size” without specifying a measurable quantity such as circumference, diameter or volume.

Practise Ideal gas molecules questions

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