Predicting physical states

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:

  • some of the bonds between particles during melting
  • all of the remaining bonds between particles during evaporating or boiling

Evaporation can take place below the boiling point of a substance. The rate of evaporation is at its maximum at the boiling point.

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 pointsLiquid
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).