States of matter

All matter is made of tiny particles: atoms, molecules or ions. These particles are always moving, and the faster they move on average, the higher the temperature. The state of a substance depends on the balance between that motion and the attractive forces pulling particles together.

When attractions dominate, particles stay locked in place as a solid. When motion and attraction are more evenly matched, particles slide past one another as a liquid. When motion wins, particles fly apart as a gas.

• Solids have a fixed shape and volume; particles vibrate in place, often in a regular crystal lattice.
• Liquids have a fixed volume but take the shape of their container; particles are close together but can move around.
• Gases have neither fixed shape nor fixed volume; particles are far apart and spread to fill any space.
• Gases are easily compressed because of the space between particles, while liquids and solids hardly compress at all.

Adding heat makes particles move faster. At the melting point a solid becomes a liquid, and at the boiling point a liquid becomes a gas. Removing heat reverses these changes: gases condense into liquids and liquids freeze into solids.

During a change of state, temperature stays constant even as heat is added. The energy goes into breaking attractions between particles rather than speeding them up. This hidden energy is called latent heat, and it is why sweat cools the body: evaporating water carries heat away.

Boiling point depends on pressure. At lower pressure, particles escape a liquid more easily, so water boils at a lower temperature at high altitude, around 70 degrees Celsius near the summit of Mount Everest. That is why cooking takes longer in the mountains, and why pressure cookers, which raise pressure, cook faster.

Some substances skip the liquid state. Dry ice, solid carbon dioxide, turns directly into gas at normal pressure, a process called sublimation. Frost forming directly from water vapour is the reverse, called deposition.

Water is unusual. Most substances are denser as solids than as liquids, but ice is less dense than liquid water, because hydrogen bonds hold ice molecules in an open hexagonal structure. That is why ice floats.

This matters enormously for life. Lakes freeze from the top down, and the floating ice insulates the water below, allowing fish and other organisms to survive winter. Liquid water is densest at about 4 degrees Celsius.

Heat a gas enough and electrons are stripped from its atoms, producing plasma, a mixture of charged particles. Plasma is often called the fourth state of matter. It is the most common visible state in the universe, making up stars, and on Earth it appears in lightning, auroras and neon signs.

Scientists have created other exotic states in laboratories. A Bose-Einstein condensate, first produced in 1995 by cooling atoms to within a tiny fraction of absolute zero, behaves like a single quantum object. Materials such as glass and gels also blur the neat boundaries between solid and liquid.