Properties of solids, liquids and gases

The three states of matter can be distinguished by their shape, volume, ability to flow and how easily they can be compressed.

A solid has a fixed volume and ordinarily retains its shape. It does not flow to take the shape of its container.

A liquid has a fixed volume, but it can flow. It takes the shape of the part of the container it occupies, rather than retaining a shape of its own.

A gas can flow and spreads to fill its container. It therefore has neither a fixed shape nor a fixed volume: both depend on the container.

Compression means reducing the volume of a substance by squeezing it. Solids and liquids are difficult to compress, whereas gases can be compressed readily.

Changes of state

A change of state occurs when a substance changes between solid, liquid and gas. The name of the change tells us which state the substance starts in and which state it becomes.

• Melting: solid → liquid. The opposite change, liquid → solid, is freezing.
• Vaporisation: liquid → gas. The opposite change, gas → liquid, is condensation.

Vaporisation can occur by evaporation or boiling. Evaporation occurs at the surface of a liquid, while boiling occurs throughout the liquid as gas bubbles form.