4PH1

Ideal Gases

Solids, Liquids & Gases · 1 question type

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 7% of your exam marks.

stable
Low
Stable7%

Gas law calculations (Boyle's Law, pressure-temperature) and kinetic theory explanations appear regularly.

Random motion of gas molecules

  • Gas molecules never sit still; they fly about ceaselessly in random directions at high speed
  • "Random" means no preferred direction: every molecule is constantly changing direction as it bounces off other molecules and off the walls of the container
  • A typical air molecule at room temperature moves at hundreds of metres per second between collisions
Gas molecules moving in random directions inside a sealed container, each travelling in a straight line until it collides
Source: Kinetic Theory by Save My Exams

How collisions produce pressure

  • Each time a gas molecule strikes the wall of its container, it bounces back. By Newton's third law, the wall pushes the molecule back, and the molecule pushes the wall
  • Over countless collisions, the millions of tiny pushes add up to a net force on every wall, acting at right angles to the surface
  • Pressure is force per unit area:

P = F / A

  • Two things control how big the pressure is:
    • How often the molecules hit the wall each second (more collisions per second means more force per second)
    • How hard each collision is (faster molecules mean a harder impact)
  • Anything that changes either factor (heating the gas, compressing it, adding more molecules) changes the pressure
Molecules striking a container wall; each bounce exerts a force on the wall, and the many collisions add up to a net outward force
Source: Kinetic Theory by Save My Exams
Exam tip

Explaining how gas molecules produce pressure

What comes up: A 3-mark question asking you to explain, in terms of particles, how a gas exerts a pressure on the walls of its container.

Write (three marks): (1) Gas molecules (or particles) collide with the walls of the container. (2) Each collision exerts a force on the wall. (3) Pressure is force per unit area, so the total force acting across the wall area gives the gas pressure.

Watch out: The mark scheme ignores answers where collisions are described between molecules with no mention of the walls. You must explicitly reference the walls at least once. Writing "molecules collide (with each other)" does not score the collision mark.