Particle Model of Gases

A gas is made of tiny particles (atoms or molecules) that are far apart. They move in straight lines in random directions until they collide with each other or the walls of a container. There are very weak forces between gas particles, so they spread out to fill any space.

Temperature and particle motion

Temperature measures the average kinetic energy (energy of motion) of the particles. Heating a gas makes its particles move faster. There is a lowest possible temperature, absolute zero, at −273 °C. This is 0 K on the kelvin scale. Convert using: $T(\text{K}) = \theta(^{\circ}\text{C}) + 273$.

Gas pressure

When fast-moving particles hit the container walls, they exert forces. Many collisions each second create a steady force on an area, which is the gas pressure. Faster particles or more collisions mean higher pressure.

Changing temperature or volume

• Constant volume: Increase temperature → particles move faster → pressure increases.
• Constant temperature: Decrease volume → particles hit walls more often → pressure increases.

Boyle’s law (fixed mass, constant temperature)

Pressure and volume are inversely related. Doubling the volume halves the pressure. Mathematically:

$$pV = \text{constant}$$

or for two states: $$p_1V_1 = p_2V_2.$$

Evidence: Brownian motion

Tiny smoke or pollen particles seen under a microscope jiggle randomly. They are being knocked by invisible, fast-moving gas (or liquid) molecules. This supports the particle model.

Real-world links

• Balloons expand when warmed (pressure or volume rises).
• Aerosol cans get higher pressure if heated (dangerous).
• Car tyres warm up during driving, raising pressure.