Ideal Gases (Edexcel IGCSE Physics): Revision notes

Section 1

What causes gas pressure?

Gas molecules are in constant, random motion, moving in straight lines at high speed until they collide with each other or the walls of their container.

Each time a molecule hits a wall, it exerts a tiny force on that wall as it changes direction (bounces back). With billions of molecules colliding every second, these tiny forces add up to a steady, measurable pressure on the container walls.

Pressure = total force from molecular collisions ÷ area of the wall
More frequent or harder collisions mean higher pressure

Key termsrandom motionpressure

Think of it like this

Think of gas molecules as a swarm of tiny balls constantly pelting the inside of a balloon — each impact is a little push, and together they keep the balloon inflated.

Section 2

What is the Kelvin scale and absolute zero?

Temperature can be measured in degrees Celsius (°C) or in kelvin (K). The Kelvin scale starts at absolute zero, the lowest temperature theoretically possible, equal to −273 °C.

At absolute zero, particles have the minimum possible kinetic energy — they are not moving at all (or have the least motion allowed).

To convert between scales:

Kelvin = Celsius + 273
Celsius = Kelvin − 273

A change of 1 °C is the same size as a change of 1 K, so temperature differences are identical on both scales.

Key termsabsolute zeroKelvin scale

Exam tip

Always convert temperatures to kelvin before using them in any gas equation — Celsius values will give wrong answers because 0 °C is not the zero point of molecular motion.

Example

Convert 27 °C to kelvin: 27 + 273 = 300 K.

Section 3

How does temperature affect molecular speed and energy?

As the temperature of a gas increases, the average speed of its molecules increases — the molecules move faster and collide with the walls more often and more forcefully.

The Kelvin temperature of a gas is directly proportional to the average kinetic energy of its molecules. Doubling the Kelvin temperature doubles the average kinetic energy of the molecules.

This link between temperature and kinetic energy explains why heating a gas (at constant volume) increases its pressure — faster molecules hit the walls harder and more often.

Key termskinetic energyproportional

Section 4

How are pressure and volume related at constant temperature?

For a fixed mass of gas at constant temperature, squeezing the gas into a smaller volume forces the same number of molecules into a smaller space. This means more frequent collisions with the walls, so the pressure increases as the volume decreases.

This relationship is described by:

$p_1 V_1 = p_2 V_2$

where $p_1, V_1$ are the initial pressure and volume, and $p_2, V_2$ are the final pressure and volume (temperature and mass of gas unchanged).

Key termsconstant temperature

Example

A gas at 100 kPa occupies 4 m³. It is compressed to 2 m³ at the same temperature. New pressure: p₂ = p₁V₁/V₂ = (100 × 4)/2 = 200 kPa.

Common mistake

Students often forget this relationship only applies when temperature is constant — if temperature also changes, p₁V₁ = p₂V₂ cannot be used on its own.

Section 5

How are pressure and temperature related at constant volume?

For a fixed mass of gas in a sealed, rigid container (constant volume), increasing the Kelvin temperature increases the average speed and kinetic energy of the molecules. Faster molecules strike the walls more often and with greater force, so the pressure increases as Kelvin temperature increases.

This relationship is described by:

$\dfrac{p_1}{T_1} = \dfrac{p_2}{T_2}$

where temperatures $T_1$ and $T_2$ must be in kelvin.

Key termsconstant volume

Example

A gas at 200 kPa and 300 K is heated at constant volume to 450 K. New pressure: p₂ = p₁T₂/T₁ = (200 × 450)/300 = 300 kPa.

Must Know

Gas pressure is caused by molecules in random motion colliding with the container walls
Absolute zero is −273 °C = 0 K; always convert to kelvin before calculating: K = °C + 273
Kelvin temperature is directly proportional to the average kinetic energy of gas molecules
Increasing temperature increases average molecular speed and hence pressure/collision rate
At constant temperature: p₁V₁ = p₂V₂ (pressure increases as volume decreases)
At constant volume: p₁/T₁ = p₂/T₂ (pressure increases as Kelvin temperature increases)

That's the notes covered.

Carry on to the next subtopic.