SOLUTION

Vapour pressure

When a liquid is placed in a closed container such as a bottle with a stopper, the space above the liquid fills up with vapor. Liquid molecules evaporate into space above the liquid, whether it contains air or not. 

The vapor molecules collide with the walls of the container, bounce off, and cause pressure. This pressure, called the vapor pressure, is added to the air pressure already present (due to the air molecules). The maximum vapor pressure that can be developed by a liquid is called the saturated vapor pressure of that liquid. Since the fraction of molecules with high energy increases with temperature, evaporation increases as the temperature rises, and the saturated vapor pressure increase with temperature. The magnitude of the saturated vapor pressure depends on the identity of the liquid and the temperature; it does not depend on the amount of liquid present. 

The vapor pressure is directly related to the temperature of the liquid, higher water temperatures produce higher vapor pressures. Hence to decrease the vapor pressure we must decrease the temperature of the water.