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

Explaining change of state

Melting, evaporating and boiling

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 some of the between particles during
  • overcome the remaining forces of attraction between particles during or

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 and . 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掳颁1413掳颁
WaterSmall molecules0掳颁100掳颁
HydrogenSmall molecules-259掳颁-252掳颁
SubstanceSodium chloride
Bonding typeIonic
Melting point801掳颁
Boiling point1413掳颁
SubstanceWater
Bonding typeSmall molecules
Melting point0掳颁
Boiling point100掳颁
SubstanceHydrogen
Bonding typeSmall molecules
Melting point-259掳颁
Boiling point-252掳颁

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
TemperatureGiven temperature < melting point
Predicted stateSolid
TemperatureGiven temperature is between melting and boiling points
Predicted stateLiquid
TemperatureGiven 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掳颁.

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