Melting and solidification

Melting is the change from a solid to a liquid. In a solid, particles vibrate around fixed positions. When the solid melts, its particles are no longer held in fixed positions and can move past one another.

When a pure substance reaches its melting temperature, further energy supplied to it overcomes some of the attractions holding its particles in fixed positions. This changes their arrangement rather than raising the temperature, so the temperature remains constant while melting occurs. At standard atmospheric pressure, pure water melts at $$0,{}^\circ\mathrm{C}$$.

Solidification

Solidification is the reverse change: a liquid becomes a solid. As the liquid loses energy, its particles become arranged in fixed positions. They no longer move freely past one another, but continue to vibrate around those positions.

Boiling

Boiling is the change of a liquid into a gas throughout the liquid. Bubbles of gas form within the liquid, rise through it and escape at the surface.

At a fixed pressure, a pure liquid boils at a particular temperature called its boiling temperature. Once the liquid reaches this temperature, energy supplied to it separates its particles as the liquid changes into a gas. The energy does not raise the temperature, so the temperature remains constant while boiling occurs.

At standard atmospheric pressure, pure water boils at $$100,{}^\circ\mathrm{C}$$.

Condensation

Condensation is the change of a substance from a gas to a liquid. It happens as the gas particles lose energy.

In a gas, particles move rapidly and are far apart. As they lose energy, they move less rapidly, and attractions between them bring them closer together.

The resulting liquid has particles that are close together. They are not fixed in place: they can still move past one another, allowing the liquid to flow.

Evaporation and cooling

Particles in a liquid are constantly moving, but they do not all have the same kinetic energy. Some of the more energetic particles at the surface have enough energy to escape the liquid and become gas particles. This process is called evaporation. It can occur below the liquid’s boiling temperature.

The escaping particles carry energy away from the liquid. Because they are among the more energetic particles, those left behind have a lower average kinetic energy. The temperature of the liquid falls, so the liquid cools.

Factors affecting evaporation rate

Evaporation occurs when particles escape from the surface of a liquid into the air. Its rate depends on how quickly particles leave the liquid and how many return to it.

Liquid temperature

At a higher temperature, a greater proportion of the liquid’s surface particles have enough energy to escape. More particles therefore leave the liquid each second, so evaporation is faster.

Exposed surface area

A larger exposed surface has more particles available at the surface to escape. The liquid therefore evaporates faster, provided the other conditions remain the same.

Air movement

Moving air carries vapour away from the liquid’s surface. Fewer vapour particles then return to the liquid, so the net rate of evaporation increases.

Cooling of objects by evaporation

When a liquid on an object evaporates, particles escape from the liquid’s surface and carry energy away. The particles most able to escape are among the more energetic ones, so the particles left behind have a lower average kinetic energy. The liquid cools.

The object is now warmer than the liquid, so energy is transferred thermally from the object to the liquid. The object loses internal energy and its temperature falls.

For example, when sweat evaporates from skin, the sweat cools and energy is transferred from the skin to the sweat. This cools the skin.