The particle model pictures every substance as a collection of tiny particles. In a solid the particles are packed closely together in a regular pattern. They are held in place by strong forces of attraction, so they can only vibrate about fixed positions. A solid keeps its shape and cannot be poured.
In a liquid the particles are still close together and touching, but they are arranged randomly and can move past each other. This is why a liquid flows and takes the shape of its container, yet has a fixed volume. In a gas the particles are far apart, move quickly in random directions and have very weak forces between them. A gas fills any container and can be squashed.
A change of state happens when energy is transferred to or from the particles. Heating a solid makes the particles vibrate more until they break free from their fixed positions and the solid melts. Further heating makes particles escape from the liquid and it boils or evaporates to a gas. Cooling reverses this: a gas condenses to a liquid and a liquid freezes to a solid. The substance is the same in every state, only the arrangement and movement of its particles change.
