Water vapour pressure

<p dir="ltr">Water is a relatively volatile substance, meaning that it will readily evaporate at ‘normal’ temperatures. Those water molecules that escape to the gaseous state then exert a pressure above the liquid phase, called a vapour pressure. This vapour pressure conforms approximately to the gas equation, i.e., the greater the number of water molecules per unit volume, and the higher the temperature of those molecules, then the greater the vapour pressure is the volume immediately above the liquid.</p><p dir="ltr">If the liquid is placed in an open container (e.g., a vial without stopper) within a chamber at a controlled pressure (See "vial in a chamber demonstration of vapour pressure") then what happens over time is that the air becomes saturated with water vapour, with each molecule of water displacing approximately the same number of gas molecules from the air (such that the pressure in the chamber remains constant). What is meant by the term <i>saturated</i>, is that the gas can carry no more water molecules than the concentration defined by the vapour pressure at that temperature. The system is then said to have reached equilibrium. This is exactly what happens during the sublimation of ice in the primary drying stage of the lyo cycle. The vapour that is generated from the water inside the vial saturates the air in the chamber, causing some of the air molecules to be vented from the valve. The fan makes sure that the temperature of the gas in the chamber is uniform and so the vial can reach the same temperature as its surroundings. Once the air is saturated then there will be no net vaporisation of water. The gas pressure inside the chamber can be controlled by a vent (i.e., a valve) allowing for molecules to escape (hence maintaining the concentration of gas within the fixed volume of the chamber), or by a frictionless, diaphragm that moves to maintain the pressure equal to the outside pressure, e.g., 1 atm. In the case of the diaphragm-controlled pressure, the volume of the chamber increases to accommodate the extra gas molecules liberated from the water, so that the number of molecules per unit volume and hence pressure remains constant.</p><p dir="ltr"><b>Dynamic equilibria </b>When water molecules in the air and water molecules in the bulk liquid are in dynamic equilibrium, it means that the number of water molecules escaping from the liquid (evaporating) is the same as the number of water molecules re-entering the liquid (condensing) in any finite period. This constant exchange of equal numbers of molecules is why the equilibrium is referred to as a dynamic equilibrium. This concept is illustrated in "water molecule escaping from liquid (vapour pressure schematic" which shows that in the “next instant” there will be two water molecules entering the liquid from the gas phase and two molecules entering the gas phase from the liquid phase. Under such conditions there will be no net evaporation of the liquid, i.e., no drying will take place, nor will there be any net condensation.</p>