The diagram summarises the common changes of state.
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.
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.
|Substance||Bonding type||Melting point||Boiling point|
Evaporation can take place below the boiling point of a substance.
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.
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.
|Given temperature < melting point||Solid|
|Given temperature is between melting and boiling points||Liquid|
|Given temperature > boiling point||Gas|
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).
The particle model assumes that particles are solid spheres with no forces between them. However: