In general, liquids show expansion on heating, contraction on cooling; water, however, does not follow the rule exactly. A liquid changes at its boiling point to a gas and at its freezing point, or melting point, to a solid. The boiling point is especially important because, since liquids change their states at different temperatures, those in a mixture can be separated from one another by raising the temperature of the mixture gradually so that each component in turn undergoes vaporization at its boiling point. This process is known as fractional distillation.
Liquids, like gases, exhibit the property of diffusion. When two miscible liquids (i.e., they mix without separation) are poured carefully into a container so that the denser one forms a separate layer on the bottom, each will diffuse slowly into the other until they are thoroughly mixed. Liquids, like gases, differ from solids in that they are fluids, that is, they flow into the shape of a containing vessel. Liquids exert pressure on the sides of a containing vessel and on any body immersed in them, and pressure is transmitted through a liquid undiminished and in all directions. Liquids exert a buoyant force on an immersed body equal to the weight of the liquid displaced by the body (see Archimedes' principle and specific gravity). Unlike gases, liquids are very nearly incompressible, and for that reason are useful in such devices as the hydraulic press. Liquids are useful as solvents. No one liquid can dissolve all substances; each takes into solution only certain specific substances.