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.

A thermometer with labels showing substance will be a gas above the boiling point, a solid below the melting point and a liquid in between.
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).

Explaining melting points and boiling points

Melting

Particles in a substance have forces of attraction between them. When a substance in the solid state is heated:

  • energy is transferred to the particles
  • the particles move faster and have more kinetic energy
  • some of the bonds are overcome and particles can slide round each other

Boiling

  • energy is transferred to the particles
  • particles eventually gain enough kinetic energy to overcome the forces of attraction
  • the particles can then move apart

The strength of the forces of attraction between particles varies between substances.

The stronger the forces of attraction between particles, the more energy is needed to overcome them, and so the higher the boiling point and the higher the melting point.

Condensing and freezing

Condensing and freezing are the opposite of boiling and melting. During these processes energy is transferred to the surroundings.