State of matter

Lore & Background

The four classical states of matter are solid, liquid, gas, and plasma. In a solid, the constituent particles—atoms, molecules, or ions—are packed closely together, held by forces so strong that they cannot move freely, only vibrate. This gives a solid a stable, definite shape and a definite volume; it can only change shape through an outside force like breaking or cutting. Many solids are crystalline, with particles arranged in a regularly ordered, repeating pattern, and the same substance can have multiple solid phases with different crystal structures—for example, iron takes a body-centered cubic structure below 912 °C and a face-centered cubic structure at higher temperatures, while ice has fifteen known crystal structures at various pressures and temperatures. Solids can become liquids through melting, or transform directly into gases via sublimation. A liquid is a nearly incompressible fluid that conforms to its container’s shape while retaining a nearly constant volume, independent of pressure. Its particles remain close together but have enough energy to move relative to one another, making the structure mobile. The volume of a liquid is usually greater than that of its solid form, with water being a notable exception. A gas is a compressible fluid that expands to fill both the shape and volume of its container. Its molecules possess enough kinetic energy that intermolecular forces are negligible, and the typical distance between neighboring molecules far exceeds molecular size. Below its critical temperature, a gas is also called a vapor and can be liquefied by compression alone. A supercritical fluid occurs above the critical temperature and pressure, where the distinction between liquid and gas disappears, giving it gas-like physical properties but high density that can confer solvent properties. Plasma resembles a gas but contains charged particles—ions and free electrons—that move independently and respond to electric and magnetic fields.

Reader's Guide

The concept of state of matter is fundamental to physics and chemistry, describing how matter organizes under different conditions. The four classical states—solid, liquid, gas, and plasma—are observable in everyday life, with plasma being the most abundant form in the universe, composing all stars. Beyond these, additional states exist, such as liquid crystals, which exhibit properties of both solids and liquids, and magnetic states like ferromagnetism, which depend on spin alignment rather than spatial arrangement. Some states occur only under extreme conditions, including Bose–Einstein condensates (extreme cold), neutron-degenerate matter (extreme density), and quark–gluon plasma (extremely high energy). The term 'phase' is sometimes used synonymously with state of matter, but a single compound can form multiple phases in the same state, as with ice having multiple crystal structures. Phase transitions mark changes between states, often with abrupt property changes, and intermediate steps are called mesophases, exploited in liquid crystal technology.

Did You Know?

• Plasma is by far the most abundant of the four fundamental states, composing 99% of all ordinary matter in the universe.
• Ice has fifteen known crystal structures, or solid phases, which exist at various temperatures and pressures.
• A supercritical fluid has the physical properties of a gas but its high density confers solvent properties in some cases.
• Magnetic states such as ferromagnetism do not depend on the spatial arrangement of atoms, but on the alignment of their intrinsic magnetic moments (spins).