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.

Changes of state are physical changes. Unlike chemical reactions, no new substances are formed during changes of state.

Explaining change of state

Melting, evaporating and boiling

Energy must be transferred, by heating, to a substance for these changes of state to happen. During these changes the particles gain energy, which is used to break or 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 condensation and freezing to happen. During these changes of state, the particles lose energy as bonds 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.

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

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).

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