Definition · Plain-language
States of matter
The states of matter are the distinct forms a substance can take — solid, liquid and gas, and also plasma — set by how its particles are arranged and move.
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Solids, liquids and gases
The everyday states differ in how their particles are packed and how they move. In a solid, particles are held closely in a fixed, regular arrangement and can only vibrate in place, so a solid keeps a definite shape and volume. In a liquid, particles are still close together but can move past one another, so a liquid keeps a definite volume but takes the shape of its container and can flow. In a gas, particles are far apart and move freely and rapidly, so a gas has neither a fixed shape nor a fixed volume and spreads to fill any space.
Plasma and other states
Beyond the familiar three, plasma is often called the fourth state of matter. It forms when a gas is given so much energy that electrons are stripped from atoms, leaving a hot mix of charged particles; stars, lightning and neon signs all involve plasma. Although rare on Earth, plasma is the most common state of ordinary matter in the universe. Scientists also study more exotic states under extreme conditions, such as the Bose–Einstein condensate produced at temperatures close to absolute zero.
Changing state
A substance moves between states when heating or cooling changes the energy of its particles, but its chemical identity stays the same — these are physical changes. Adding energy drives melting (solid to liquid), boiling or evaporation (liquid to gas), and sublimation (solid straight to gas). Removing energy drives condensation (gas to liquid) and freezing (liquid to solid). Each substance changes state at characteristic temperatures, its melting point and boiling point, which is why water melts at 0 °C and boils at 100 °C at normal atmospheric pressure.
Key facts
At a glance
- Definition: the forms a substance takes, set by particle arrangement and motion
- Solid: fixed shape and volume; particles vibrate in place
- Liquid: fixed volume, takes container’s shape; particles flow
- Gas: no fixed shape or volume; particles move freely
- Plasma: ionised gas; the fourth state, common in stars
- Changes of state: physical changes driven by heating or cooling
Common misconceptions
What people often get wrong
Often heard: There are only three states of matter.
Actually: Solid, liquid and gas are the everyday three, but plasma is a widely recognised fourth state, and exotic states such as Bose–Einstein condensates exist under extreme conditions.
Often heard: Changing state, such as melting ice, is a chemical change.
Actually: Changes of state are physical changes. The substance’s chemical identity is unchanged — melted ice is still water — only the arrangement and energy of the particles differ.
Often heard: Particles in a solid do not move at all.
Actually: Particles in a solid are fixed in position but still vibrate. They simply cannot move past one another the way particles in a liquid or gas can.
Going deeper
