Elements, Compounds and Mixtures
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.
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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