GCSE CHEMISTRY - What Happens to the Temperature of a Liquid when it Boils and becomes a Gas?

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What happens to the Temperature of a Liquid
as it Boils to become a Gas?

The graph below shows how the temperature changes with time
as a substance is heated at a constant rate.

Graph of Temperature against Time for a Substance

Below the boiling point the substance is a liquid.
Above the boiling point the substance is a gas.

At the boiling point, although the substance is still being heated,
the temperature does not change for a time and the graph
becomes horizontal. During this time the extra heat that
is being added goes to overcome the force of attraction
between the particles of the liquid as it turns into a gas.
The process of boiling needs extra energy - it is endothermic.

It is not possible to raise the temperature of a liquid above
its boiling point at atmospheric pressure. When all of the
liquid has boiled and the liquid has become a gas, then the
temperature of the gas will start to rise with constant heating.

The same graph as that shown above occurs on cooling
a gas to a liquid or a liquid to a solid. The process of
freezing or condensing gives out energy - it is exothermic.

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