State changes

Changes of state are:

  • solids melting into liquids
  • liquids boiling into gases
  • gases condensing into liquids
  • liquids freezing or solidifying into solids

Evaporation is sometimes confused with boiling. They both involve liquids turning to gases, but evaporation is different because:

  • it occurs at any temperature - not just the boiling point
  • it only happens at the surface of the liquid - not throughout like boiling
  • boiling requires an energy input - whereas evaporation is the release of the molecules with the highest energy

Evaporation cools liquids as a result of this energy loss. Evaporation is increased by higher temperatures, a greater surface area or a draft over this surface area.

A substance must absorb heat energy so that it can melt or boil. The temperature of the substance does not change during melting, boiling or freezing - even though energy is still being transferred.

Heating curves and cooling curves

A heating curve is a graph showing the temperature of a substance plotted against the amount of energy it has absorbed. You may also see a cooling curve, which is obtained when a substance cools down and changes state.

The temperature stays the same when a solid is melting or a liquid is boiling (changing state) during a change of state, even though heat energy is being absorbed.

The temperature also stays the same while a liquid freezes, even though heat energy is still being released to the surroundings.