A liquid is a state of matter intermediate between a solid and a gas. A liquid, at constant temperature can readily change its shape (like a gas but unlike a solid) but it is incapable of changing its volume (like a solid but unlike a gas).

A substance can be said to be a liquid when its physical properties are that it has a fixed volume, a moderate to high density, an indefinite shape (it takes the shape of its container), and it generally flows easily.

The change of state from a solid to a liquid is melting, from a liquid to a solid is freezing. The change from a liquid to a gas is boiling and from a gas to a liquid is condensing. With a pure substance each of these changes of state occurs at a specific temperature. This is an indication of a pure substance...