大象传媒

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 without becoming a liquid in between. This is called . Solid carbon dioxide ('dry ice') can .

Changes of state are . 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 to happen. During these changes the gain energy, which is used to break or overcome:

  • some of the between particles during
  • all the remaining bonds between particles during or

Evaporation can take place below the 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 and 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 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
ComparisonGiven temperature < melting point
Predicted stateSolid
ComparisonGiven temperature is between melting and boiling points
Predicted stateLiquid
ComparisonGiven temperature > boiling point
Predicted stateGas

Question

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