States of Matter

The same particles can stand shoulder-to-shoulder in a marching block (solid), shuffle past each other in a moving crowd (liquid), or scatter and fly across an empty hall (gas): heat is what loosens their grip on one another.

Everything around us is made of matter, and matter is built from extremely tiny particles. These particles are always moving, they have empty spaces between them, and they pull on each other with forces of attraction. When potassium permanganate spreads through water, or a smell drifts across a room, you are seeing these moving particles in action. How strongly the particles are held together decides whether a substance is a solid, a liquid or a gas.

In a solid the particles are packed closely in a fixed, regular pattern and only vibrate in place, so a solid keeps its shape and volume. In a liquid the particles are still close but can slide past one another, so a liquid flows and takes the shape of its container while keeping a fixed volume. In a gas the particles are far apart and move freely at high speed, so a gas has neither a fixed shape nor a fixed volume and can be compressed. The force of attraction is strongest in solids and weakest in gases.

Heating a substance gives its particles more energy, so they break free of one another: a solid melts into a liquid at its melting point, and a liquid boils into a gas at its boiling point. Cooling reverses this. Strikingly, while a substance is changing state its temperature does not rise even though heat is being supplied. That hidden heat, used to break the forces between particles rather than to raise temperature, is called latent heat: the latent heat of fusion for melting and the latent heat of vaporisation for boiling.

A liquid does not have to boil to become vapour. At its surface, the fastest particles can escape into the air at any temperature below the boiling point: this is evaporation. Because the most energetic particles leave, the average energy of those left behind falls, so the liquid cools. That is why sweat cools your skin and why water in an earthen pot stays cool. Evaporation is faster when the temperature is higher, the surface area is larger, the wind is stronger and the air is drier.