Melting and boiling points

A substance changes state at characteristic temperatures:

At its melting point, a solid melts to form a liquid, or a liquid freezes to form a solid.

At its boiling point, a liquid boils to form a gas, or a gas condenses to form a liquid.

These temperatures describe when the bulk material changes state. The melting and boiling points used must be for the pressure being considered.

Predicting the state of a pure substance

At a given pressure, compare the substance’s temperature with its melting and boiling points:

Below the melting point: it is solid.

Between the melting and boiling points: it is liquid.

Above the boiling point: it is a gas.

At a melting or boiling point, the two states involved can coexist while the substance changes state.

For example, at standard atmospheric pressure, pure water has a melting point of

0,^{\circ}\mathrm{C}

and a boiling point of

100,^{\circ}\mathrm{C}

. It is solid at

-5,^{\circ}\mathrm{C}

, liquid at

25,^{\circ}\mathrm{C}

and a gas at

110,^{\circ}\mathrm{C}

Particle model of solids, liquids and gases

A material contains many particles. The particle model represents them as small solid spheres so that their arrangement and movement are easier to picture. The model helps explain why the same material behaves differently as a solid, liquid or gas.

Particle arrangement and movement

Solid: The particles are close together in a fixed arrangement. They vibrate about fixed positions but do not move past one another. A solid therefore keeps its shape.
Liquid: The particles are close together but are not fixed in place. They move past one another, so a liquid can flow and take the shape of its container.
Gas: The particles are far apart and move freely in all directions. A gas spreads out to fill its container.

Energy and changes of state

When energy is transferred to a material, its particles can move more and their arrangement can change. A solid can melt to form a liquid, and a liquid can become a gas. During a change of state, the transferred energy changes the particle arrangement rather than necessarily increasing the particles’ speed.

When energy is transferred out of a material, its particles can move less and their arrangement can change. A gas can condense to form a liquid, and a liquid can freeze to form a solid.

Particles and bulk properties

Properties such as flowing or keeping a shape describe the behaviour of the material as a whole. They arise from how many particles are arranged and move together; a single atom or particle does not itself have these bulk properties.