A vessel with a movable piston maintained at a constant temperature by a thermostat contains a certain amount of liquid in equilibrium with its vapours. Do these vapours obey Boyle’ s law? In other words, what happens when volume of vapours is decreased? Does the vapour pressure increase?

Solution

Here, the liquid and vapours are in equilibrium.
Therefore, the vapours are saturated vapours and pressure of vapours is equal to saturated vapour pressure at that temperature.
Saturated vapours do not obey Boyle’s law. Saturated vapour pressure is the maximum pressure and when volume is decreased at saturation, the vapours condense maintaining the same pressure.