The three states of matter are solid, liquid and gas.

The idea

Matter can exist in three physical states: solid, liquid and gas. The state depends on the arrangement, spacing and movement of the particles, and on the strength of the forces between them. In a solid, particles are close together in a regular arrangement and vibrate about fixed positions, so a solid has a fixed shape and volume. In a liquid, particles are close together but randomly arranged and can slide past one another, so a liquid has a fixed volume but takes the shape of its container. In a gas, particles are far apart, randomly arranged and move quickly in all directions, so a gas fills its container and is easily compressed. For example, ice, water and steam are the solid, liquid and gas states of the same substance. Changes of state are physical changes and do not alter the chemical identity of the substance.

What earns the marks

• Names the three states of matter as solid, liquid and gas.
• Describes the arrangement and spacing of particles in each state.
• Describes the movement of particles in each state.
• Links the particle model to properties such as shape, volume or compressibility.
• Recognises that changes of state are physical changes and are reversible.

Common mistakes and how to put them right

• Stating that particles in a gas do not move.

  Put it right by saying gas particles move quickly and randomly.

• Saying that particles in a liquid are far apart.

  Put it right by stating that liquid particles are close together but can slide past one another.

• Describing changes of state as chemical changes.

  Put it right by stating that they are physical changes.

Exam tips

• Use a simple particle diagram for each state and label spacing and arrangement.
• Compare the three states in a table to make differences clear.
• When explaining a property, always refer to both particle arrangement and particle movement.