The kinetic theory explains the behaviour of solids, liquids and gases in terms of the movement of their particles. It accounts for changes of state, expansion, evaporation and the gas laws. The central idea is simple: all matter is made of tiny particles that are constantly moving, and the hotter the substance, the faster they move.
The three states of matter
| State | Arrangement | Movement | Forces between particles |
|---|---|---|---|
| Solid | regular, closely packed | vibrate about fixed positions | strong |
| Liquid | close, irregular | move around each other | moderate |
| Gas | far apart, random | move quickly in all directions | very weak |
This explains their properties: a solid has fixed shape and volume; a liquid has fixed volume but takes the shape of its container; a gas fills any container and is easily compressed (because of the large spaces between particles).
Evidence for moving particles
Brownian motion — smoke particles viewed under a microscope are seen to move in random, jerky paths. This is because they are constantly bombarded by even smaller, fast-moving air molecules, providing direct evidence that gas particles move randomly. Diffusion (the spreading of a smell across a room, or a coloured gas through air) is further evidence.
Temperature and internal energy
Heating a substance gives its particles more kinetic energy, so they move faster. Temperature is a measure of the average kinetic energy of the particles. The total energy of all the particles (kinetic + potential) is the internal energy.
Temperature is measured in degrees Celsius (°C) or kelvin (K), where 0 K (−273 °C) is absolute zero, the lowest possible temperature, at which particle motion is at a minimum.
Changes of state
When a solid is heated it melts; a liquid boils/evaporates to a gas; cooling reverses these (condensing, freezing). During a change of state the temperature stays constant even though heat is still supplied — the energy is used to break the forces between particles, not to raise the temperature. This absorbed energy is the latent heat.
Evaporation happens from the surface of a liquid at any temperature: the fastest-moving particles escape, so the average energy of those left falls and the liquid cools. This is why sweating cools the body. Evaporation is faster at higher temperature, with greater surface area, and in moving air.
The gas laws
The pressure, volume and temperature of a fixed mass of gas are linked.
- Boyle's law: at constant temperature, pressure is inversely proportional to volume (P × V = constant). Squeeze a gas into half the volume and its pressure doubles — because the particles hit the walls twice as often.
- The pressure law: at constant volume, pressure is proportional to the kelvin temperature. Heat a gas in a sealed container and its pressure rises, because the particles move faster and hit the walls harder and more often.
- Charles's law: at constant pressure, volume is proportional to the kelvin temperature.
For all gas-law calculations, temperature must be in kelvin (K = °C + 273).
Explaining gas pressure with the theory
A gas exerts pressure because its particles are constantly colliding with the walls of the container. Each collision exerts a tiny force; millions of collisions per second give a steady pressure. Heating the gas, or compressing it, makes the collisions more frequent and/or harder, raising the pressure.
Worked gas-law example
A fixed mass of gas has a volume of 300 cm³ at a pressure of 100 kPa. If it is compressed at constant temperature to a pressure of 150 kPa, what is the new volume? Boyle's law says P₁V₁ = P₂V₂, so:
100 × 300 = 150 × V₂ V₂ = 30000 ÷ 150 = 200 cm³
The volume falls because squeezing the gas forces the same number of particles into a smaller space, so they hit the walls more often and the pressure rises. For the pressure law and Charles's law, remember to convert any Celsius temperature to kelvin first (K = °C + 273); using Celsius directly is the single most common error in these calculations.
Thermal expansion
Because heating makes particles move faster and spread out a little, most substances expand when heated and contract when cooled. Solids expand least, liquids more, and gases most of all. This has practical consequences you should know:
- gaps are left between sections of railway line, and rollers placed under bridges, to allow for expansion in hot weather;
- a bimetallic strip (two metals that expand by different amounts) bends when heated and is used in thermostats and fire alarms;
- liquid-in-glass thermometers work because the liquid expands up the tube as the temperature rises.
Thermal expansion is simply the large-scale result of the same particle movement that the kinetic theory describes, which is why it sits naturally within this topic.
Common exam mistakes
- Forgetting to convert temperature to kelvin in gas-law problems.
- Saying temperature stays constant during a change of state "because heating stops" — heating continues; the energy goes into breaking inter-particle forces (latent heat).
- Confusing evaporation (surface, any temperature) with boiling (throughout, at a fixed temperature).
- Saying particles "expand" when heated — the particles do not get bigger; they move faster and further apart.
Key terms to remember
- Kinetic theory — the idea that all matter is made of constantly moving particles.
- Brownian motion — the random movement of small particles caused by collisions with molecules.
- Diffusion — the spreading of particles from high to low concentration.
- Internal energy — the total kinetic and potential energy of all the particles.
- Absolute zero — 0 K (−273 °C), the lowest possible temperature.
- Latent heat — energy absorbed or released during a change of state at constant temperature.
- Evaporation — escape of fast particles from a liquid's surface at any temperature; it cools the liquid.
- Boyle's law — at constant temperature, P × V = constant.
- Thermal expansion — the increase in size of a substance when heated.
Quick recap
- All matter is made of moving particles; solids vibrate in place, liquids flow, gases move fast and freely.
- Brownian motion and diffusion are evidence of random particle movement.
- Temperature measures average particle kinetic energy; absolute zero is 0 K (−273 °C).
- During a change of state the temperature is constant — energy goes to breaking forces (latent heat); evaporation cools.
- Gas laws: Boyle (P ∝ 1/V), pressure law and Charles's law — always use kelvin.