When we keep the temperature constant,
but increase the pressure on a quantity of gaseous substance
(e.g. water vapor), then we may observe the transition
from gas to liquid at a certain pressure.
In the figure below we start at the gas phase indicate by
1
to end up at the liquid phase indicated by 2.
Gas to liquid at constant temperature
The opposite proces, from 2
to 1, happens when we
decrease the pressure
on a liquid under constant temperature.
How the boiling point of water depends on temperature
is shown here.
Further reading on gas-liquid isothermal processes:
