States of matter and changes of state

Everything around us is a solid, a liquid or a gas. Solids keep their shape, liquids flow and take the shape of their container, and gases spread out to fill any space. Heating and cooling change one state into another, and these changes drive the water cycle.

Key facts

  • Solids: fixed shape and volume. Liquids: fixed volume, flow, take the container’s shape. Gases: fill the container and can be squashed.
  • Melting: solid → liquid. Freezing: liquid → solid. Evaporating/boiling: liquid → gas. Condensing: gas → liquid.
  • A substance melts at its melting point and boils at its boiling point; water: 0°C and 100°C.
  • During a change of state the temperature stays the same.
  • Evaporation happens at the surface at any temperature; it is faster when warm, windy, and with a large surface area.
  • Mass is conserved when a substance changes state or dissolves.

Worked examples

Example 1

Sort these into solid, liquid or gas: a stone, orange juice, the air in a balloon.

  1. A stone keeps its own shape: solid.
  2. Orange juice flows and takes the shape of its cup: liquid.
  3. Air spreads out to fill the whole balloon: gas.

Answer: stone: solid; orange juice: liquid; air: gas

Example 2

Name the change of state when a bathroom mirror steams up.

  1. Water vapour particles in the warm air hit the cold mirror and lose energy.
  2. They slow down and stick together as tiny drops of liquid water: condensation.

Answer: condensation

Example 3

Ethanol melts at −114°C and boils at 78°C. What state is it in at −120°C, 20°C and 90°C?

  1. Below the melting point it is solid. −120 < −114: solid
  2. Between the melting and boiling points it is liquid. −114 < 20 < 78: liquid
  3. Above the boiling point it is a gas. 90 > 78: gas

Answer: solid, liquid, gas

Example 4

Explain how rain forms in the water cycle.

  1. The Sun heats the sea and water evaporates into water vapour.
  2. The vapour rises, cools and condenses into tiny droplets: clouds.
  3. Droplets join and fall as rain; rivers carry it back to the sea.

Answer: evaporation, then condensation into clouds, then precipitation