Summary
Matter comes in three main states: solid, liquid and gas. What tells them apart is how their particles are joined. In a solid they are close together and ordered, so it has a fixed shape and volume. In a liquid they are close but can move: it keeps its volume but takes the shape of its container. In a gas they are far apart and move freely: no fixed shape or volume, filling all the space available.
The same material can change state if we add or remove heat. On heating, a solid melts and a liquid evaporates; on cooling, a gas condenses and a liquid solidifies. These changes happen at set temperatures —the melting point and the boiling point— and, while the change lasts, the temperature does not rise: all the heat goes into separating the particles. There is a fourth state, plasma, which makes up the stars.
Key points
- Three main states: solid, liquid and gas (plus plasma, uncommon).
- What changes is how the particles are joined and moving, not the particles.
- Solid: fixed shape and volume. Liquid: fixed volume, container's shape. Gas: neither fixed.
- Melting (solid→liquid), evaporation (liquid→gas), condensation (gas→liquid), freezing (liquid→solid).
- Sublimation goes straight from solid to gas (dry ice).
- The changes happen at set temperatures: the melting and boiling points.
- During a change of state, the temperature stays constant.