solid, one of the three commonly recognized states in which matter occurs, i.e., that state, as distinguished from liquid and gas, in which a substance has both a definite shape and a definite volume. Solids resemble liquids in having a definite volume, but differ from both liquids and gases in having a definite shape. The molecules of a solid, like those of a liquid, are very close together, but whereas the molecules of a liquid are free to move around, those of a solid have less thermal energy and are held fixed in their places by intermolecular forces. Their only movement is a vibration about a fixed position. A solid changes to a liquid when its temperature is raised to its melting point. A definite quantity of heat (called the heat of fusion) is needed to change each gram of the substance from solid to liquid. Some substances change directly from solid to gas without passing through the liquid state (see sublimation), but most change from solid to liquid before becoming gaseous. Solids are of various types. Metals, their alloys, some nonmetals, and ionic chemical compounds are crystalline in form. Some solids, e.g., chalk and clay, have no regular structure and are called amorphous. Substances such as pitch and resin are called semisolids; these are actually very viscid liquids, but their flow or change of shape is so slow at ordinary temperatures as to be scarcely discernible by the human eye (see viscosity). Properties in which solids differ from one another include density, hardness, malleability, ductility, elasticity, brittleness, and tensile strength.