Solid

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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.

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Solid

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A solid object is in the states of matter characterized by resistance to deformation and changes of volume. At the microscopic scale, a solid has these properties :

• The atoms or molecules that compose the solid are packed closely together.
• These constituent elements have fixed positions in space relative to each other. This accounts for the solid's rigidity. In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. A crystal structure is composed of a unit cell, a set of atoms arranged in a particular way; which is periodically repeated in three dimensions on a lattice. The spacing between unit cells in various directions is called its lattice parameters. The symmetry properties of the crystal are embodied in its space group. A crystal's structure and symmetry play a role in determining many of its properties, such as cleavage, electronic band structure, and optical properties.
• If sufficient force is applied, either of these properties can be disrupted, causing permanent deformation.
• Because solids have thermal energy, their atoms vibrate. However, this movement is very small, and cannot be observed or felt under ordinary conditions.

The branch of physics that deals with solids is called solid-state physics, and is a type of condensed matter physics. Materials science is primarily concerned with properties of solids such as strength and phase transformations. It overlaps strongly with solid-state physics. Solid-state chemistry overlaps both of these fields, but is especially concerned with the synthesis of novel materials.

The lightest known solid is aerogel. The lightest aerogel produced has a density of 1.9 mg/cm³ or 1.9 kg/m³ (1/530 the density of water).

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