The three states of matter: The three states of matter are solid, liquid and gas. Melting and freezing take place at the melting point, boiling and condensing take place at the boiling point.

The idea

Matter exists as solid, liquid or gas depending on the energy of its particles and the strength of the forces between them. In a solid, particles are closely packed in a regular arrangement and can only vibrate. In a liquid, particles are close but can move past one another. In a gas, particles are far apart and move rapidly in all directions. Melting is the change from solid to liquid and occurs at the melting point; freezing is the reverse change, liquid to solid, at the same temperature. Boiling is the change from liquid to gas and occurs at the boiling point; condensing is the reverse change, gas to liquid, at the same temperature. For example, ice melts at 0 °C and water boils at 100 °C at standard atmospheric pressure. These temperatures are fixed for a pure substance at a given pressure.

What earns the marks

• The three states of matter are solid, liquid and gas, distinguished by the arrangement, separation and motion of their particles.
• Melting is the change of state from solid to liquid, and freezing is the change from liquid to solid; both occur at the melting point of a pure substance.
• Boiling is the change of state from liquid to gas, and condensing is the change from gas to liquid; both occur at the boiling point of a pure substance.
• The temperature remains constant during a change of state because the energy supplied is used to overcome forces between particles rather than to increase kinetic energy.
• Impurities or changes in pressure can alter the melting and boiling points of a substance.

Common mistakes and how to put them right

• Confusing boiling with evaporation.

  Put it right: Boiling occurs throughout the liquid at a specific boiling point, while evaporation occurs at the surface at temperatures below the boiling point.

• Thinking that temperature increases during melting or boiling.

  Put it right: The temperature stays constant during a change of state for a pure substance.

• Mixing up melting point and boiling point.

  Put it right: Melting point is the temperature at which a solid changes to a liquid; boiling point is the temperature at which a liquid changes to a gas.

Exam tips

• When describing changes of state, name both the starting and finishing states, for example 'solid to liquid' for melting.
• Use the correct terms 'melting point' and 'boiling point' rather than vague phrases like 'melting temperature'.
• If a question asks about the effect of impurities, state that impurities lower the melting point and raise the boiling point.