Solid

A solid is one of the fundamental states of matter, characterized by a definite shape and volume. The constituent particles—atoms, ions, or molecules—are closely bound and arranged in a fixed structure, either a crystalline lattice with long-range order or an amorphous network lacking periodicity. In solids, particles vibrate around fixed positions, which gives rise to rigidity and resistance to compression compared with liquids and gases.

Crystalline solids exhibit periodic order and a well-defined unit cell that repeats in three dimensions, forming distinct crystal systems. Amorphous solids lack long-range order, and their properties are generally more isotropic at macroscopic scales, though local variations can occur. Common examples include metals, minerals, and polymers; glass is a widely cited amorphous solid.

Solids have high density and low compressibility. Their mechanical properties include elasticity (the ability to return to original shape after small deformations), stiffness (quantified by Young’s modulus), yield strength, hardness, and the potential for plastic deformation under larger stresses. Anisotropy is common in crystalline materials, meaning properties can vary with direction. Thermal properties include expansion with temperature, heat capacity, and thermal conductivity. Electrical behavior ranges from good conductors, such as metals, to insulators and semiconductors.

Phase transitions involving solids include melting to a liquid at a melting point and, under certain conditions, sublimation directly to a gas. Phase behavior is described by phase diagrams that depend on temperature and pressure. Defects such as vacancies, interstitials, and dislocations influence mechanical and transport properties.

Examples include iron and copper (metals), quartz (a mineral), diamond (rigid and hard with strong covalent bonds), graphite (anisotropic conductivity), and ice (solid water).