Solid

A solid is one of the states of matter. Solid matter has a definite shape and volume, unlike a liquid, which has a definite volume but takes the shape of its container, and a gas, which changes both. This is because the atoms and molecules that make up a solid are tightly bound together by strong forces and cannot move freely—they only vibrate in their fixed positions.

The science that studies the properties of solid matter is called solid-state physics, which is part of the broader field of condensed matter physics. It examines properties such as hardness, heat conductivity, electrical conductivity, and how matter responds when subjected to pressure or temperature changes.

Internal Structure

Solids are divided into two main types. Crystalline solids have a regular, repeating internal arrangement, such as salt, most metals, and diamonds. Amorphous solids have an arrangement that does not repeat over long distances, for example glass and some plastics. Crystalline solids typically have a sharp melting point, whereas amorphous solids gradually soften as temperature increases.

Bonding Forces

The forces that hold atoms in place are of different kinds: ionic bonding (as in salt), covalent bonding (as in diamonds and silicon), metallic bonding in which electrons move freely between atoms (which is why metals conduct electricity), and weaker forces between molecules in some solids. The type of bond determines hardness, melting point, and whether the material conducts electricity.

Change of State

A solid can change into a liquid when heat is applied, a process called melting; the reverse is freezing. In some cases, a solid can change directly into a gas without passing through the liquid state, a process known as sublimation, as in the case of dry ice (solid carbon dioxide). These changes depend on temperature and pressure.

Significance

The properties of solids are fundamental to construction engineering, metallurgy, and electronics technology. For example, crystalline silicon—a solid—is the basis of computer chips. Study of solid structure is commonly carried out using X-ray diffraction to determine how atoms are arranged.