When we keep the pressure constant and heat up a quantity of liquid substance
(e.g. water) then its temperature rises.
In the figure below we start at the phase indicate by
1.
The process of rising temperature is indicated by the horizontal line.
At a certain point we arrive at the line which separates the liquid phase
of the substance from its gaseous phase.
At that point, the
boiling point
of the substance, the temperature stops rising
although we continue to supply heat to the quantity of liquid substance.
This heat is used for
evaporating
the liquid into gas.
Only after all substance is evaporated the temperature starts rising again
and we do reach the temperature indicated by 2
in the figure below.
Liquid to gas at constant pressure
The opposite proces, from 2
to 1, happens when we
cool a gas
under constant pressure.
At the boiling point further cooling does not lower the temperature
until all gas is
condensated.
Only after full condensation the temperature starts lowering again
and we do reach the temperature indicated by 1
in the figure above.
