Changes 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. During these changes the particles gain energy, which is used to:

In evaporation, particles leave a liquid from its surface only. In boiling, bubbles of gas form throughout the liquid. They rise to the surface and escape to the surroundings, forming a gas.

The amount of energy needed to change state from solid to liquid, and from liquid to gas, depends on the strength of the forces between the particles of a substance. The stronger the forces of attraction, the more energy is required.

Every substance has its own melting point and boiling point. The stronger the forces between particles, the higher its melting and boiling points.

The strength of the forces between particles depends on the particles involved. For example, the forces between ions in an ionic solid are stronger than those between molecules in water or hydrogen. This explains the melting and boiling point data in the table.

SubstanceBonding typeMelting pointBoiling point
Sodium chlorideIonic801°C1413°C
WaterSmall molecules0°C100°C
HydrogenSmall molecules-259°C-252°C

Evaporation can take place below the boiling point of a substance.

Condensing and freezing

Energy is transferred from a substance to the surroundings when a substance condenses or freezes. This is because the forces of attraction between the particles get stronger.

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).

Limitations of the particle model

The particle model assumes that particles are solid spheres with no forces between them. However:

  • particles are not solid, since atoms are mostly empty space
  • many particles are not spherical
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