Change of state

The diagram summarises the common changes of state.

Image showing the molecular changes from a solid, to  liquid, to gas.

Some substances can change directly from solid to gas, or from gas to solid, without becoming a liquid in between. This is called sublimation. Solid carbon dioxide ('dry ice') and iodine can sublime.

Explaining change of state

Melting, evaporating and boiling

Energy must be transferred, by heating, to a substance for these changes of state to happen. The energy is used to overcome:

Evaporation can take place below the boiling point of a substance. This is why damp clothes dry when they are hung from a washing line. Boiling happens at the boiling point, when the rate of evaporation is at its maximum.

Condensing and freezing

Energy must be transferred from a substance to the environment for these changes of state to happen. This is because forces of attraction form between the particles.

Predicting a physical state

The state of a substance at a given temperature can be predicted if its melting point and boiling point are known. The table summarises how to work this out.

TemperaturePredicted state
Given temperature < melting pointSolid
Given temperature is between melting and boiling points Liquid
Given temperature > boiling pointGas
Question

The melting point of oxygen is -218°C and its boiling point is -183°C. Predict the state of oxygen at -200°C.

Oxygen will be in the liquid state at -200°C (because this is between its melting and boiling points).

Question

The element bromine has a melting point of -7°C and a boiling point of 59°C. Predict the state of bromine at room temperature.

Bromine will be in the liquid state at room temperature, because this is between its melting and boiling points (room temperature is usually around 20°C).