Explain gas pressure and the ideal gas model
Explain pressure through molecular collisions and state the two ideal-gas assumptions used in this syllabus.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Keep pressure, volume, amount and absolute temperature consistent in gas calculations. Written practice is self-checked
Before you begin
- Rearrange a formula and convert between common metric volume units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What produces gas pressure at a container wall?
What you will learn
- Explain gas pressure as molecular collisions with container walls
- State that ideal-gas particles have zero volume and no intermolecular attraction
Molecules colliding with the container wall create pressure
Gas molecules move randomly and collide with the container walls. Each collision changes molecular momentum and exerts a force on the wall. The combined force per unit area produces the gas pressure.
More frequent or more forceful collisions increase the force on a wall and therefore the pressure, provided the wall area is unchanged. Describe the particle-wall collisions rather than saying that pressure is simply caused by molecules 'pushing outward'.
An ideal gas is a simplified model
In the ideal-gas model, particles are treated as having zero volume and no intermolecular forces of attraction. They still move and collide with the walls; the assumptions simplify how the gas is described.
Real gas particles do have finite size and can attract one another. The stated calculation outcomes use the ideal-gas equation; do not mistake the model assumptions for a literal description of every real gas molecule.
Pause and explain
Which pair of assumptions defines an ideal gas here?
Worked example
Explain how gas molecules exert pressure on the walls of a sealed container.
Show the worked solution
- Molecules move randomly through the container.
- They collide with the walls and change momentum.
- The collisions exert force; force per unit area is pressure.
Answer Random molecular collisions with the container walls exert a force. The force per unit area gives the gas pressure.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain the link from a single molecule's wall collision to the measured pressure.
Give me a hint
Connect momentum change to force, then relate force to area.
Compare my reasoning
- A collision changes the molecule's momentum.
- The wall experiences an equal interaction force during the collision.
- Many collisions create force over the wall area; pressure is force per unit area.
Each collision transfers momentum and exerts force on the wall. The combined force per unit area is the measured gas pressure.
Look for these in your work
- Momentum change mentioned
- Wall force described
- Pressure defined using area
State the two ideal-gas assumptions about particles and explain what the model still allows them to do.
Give me a hint
Separate the assumptions about size and attraction from molecular motion.
Compare my reasoning
- Particle volume is treated as zero.
- Intermolecular attraction is treated as absent.
- Particles still move and collide with container walls.
An ideal gas has particles with zero volume and no intermolecular attractions, but the particles still move and collide with the walls.
Look for these in your work
- Zero volume stated
- No attraction stated
- Motion and collisions retained
A student says pressure doubles because molecules attract each other more strongly in an ideal gas. Correct the explanation.
Give me a hint
Use the model's force-on-wall explanation and note the ideal assumption.
Compare my reasoning
- The ideal-gas model assumes no intermolecular attraction.
- Pressure arises from collisions with container walls.
- A pressure change must be explained by collision force or frequency per area, not stronger ideal-gas attraction.
The ideal-gas model has no intermolecular attractions. Pressure comes from wall collisions; a change must affect their force or frequency per unit area.
Look for these in your work
- Ideal assumption applied
- Wall collisions identified
- Unsupported attraction explanation rejected
Common mistakes
- Describing pressure as a force without area
- Forgetting that wall collisions change momentum
- Saying ideal particles are motionless
- Treating zero particle volume as the absence of particles
What can you explain now?
What produces gas pressure at a container wall?
Compare with the explanation
Molecular collisions that exert force on the wall
Repeated impacts transfer momentum and create a force.
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Make it stick
How do particle-wall collisions produce pressure, and what two assumptions make the gas ideal?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.