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Liquid-Vapour Equilibrium

For any liquid, there exists equilibrium of the type
liquid vapour
Taking any example of water, we have
H2O(1) H2O(g)

The pressure caused by the vapour in equilibrium with its liquid is known as vapour pressure of liquid.
Let an evacuated vessel fitted with a movable piston be placed in a thermostat whose temperature is a little above 0 C (say, 10 C) such as shown in Fig. The position of the piston can be held anywhere against a set of stops. Let liquid water be introduced in the vessel. The main characteristics of the above system at various stages of temperature and pressure are described in the following.

System exhibiting liquid-vapour equilibrium of water

  1. When the system has attained equilibrium, it is found that the pressure within the vessel attains a constant value. This pressure is known as vapour pressure of liquid water at the given temperature. At this stage, the rates of transfer of water molecules from liquid to vapour and vice versa are identical.
  2. For a given temperature, the liquid-vapour equilibrium exists only at a fixed pressure of vapours. If this pressure is increased or decreased, the equilibrium adjusts to have the same vapour pressure. This may be demonstrated as follows. If the volume of a system is increased at constant temperature by moving the position of the piston outward it will cause a decrease in the pressure of vapour phase. Since at a given temperature the vapour pressure of liquid water has a fixed value, a little water will evaporate to keep the vapour pressure to a constant value. If the above process of expansion is continued, more and more liquid water evaporates till the entire liquid phase disappears. At this stage, the system contains only the gaseous phase. The expansion of the latter merely causes a decrease in its pressure. Similarly, if the volume of the system is gradually decreased, more and more vapour is condensed in order to keep the vapour pressure of liquid water to a constant value. If the compression is continued, eventually a stage is reached where the entire vapour phase is condensed and the system contains only the liquid phase. The piston of the vessel will be directly in contact with the liquid phase. Applying more pressure on the piston can increase the pressure on the liquid phase.
  3. The vapour pressure of a liquid is found to increase with increasing temperature of the system Fig. The expression connecting these two has the form given below.

    where Δ Hm, vap is the molar enthalpy of vaporization, p is the standard unit pressure* and I is a constant factor.

Variation of vapour pressure with temperature

  1. Boiling point of a liquid On continuous increase of temperature, a stage is reached where the vapour pressure of the liquid becomes equal to the external pressure on the liquid. At this stage, vapours start forming within the liquid and these on rising within the liquid disappear in the surroundings. The liquid is said to have started boiling. The temperature at which this happens is known as the boiling point of the liquid. The temperature remains constant till the entire liquid is converted into vapour phase. Obviously, changing the external pressure can change the boiling point of a liquid.
  2. Normal boiling point of a liquid When the external pressure on the liquid is the atmospheric pressure (1 atm), the boiling point is known as the normal boiling point. Obviously, a liquid with comparative low vapour pressure would require more heating so as to make its vapour pressure equal to the atmospheric pressure and thus such a liquid will have higher boiling point. For example, comparing ethanol (vapour pressure 5.85 kPa at 293 K) and acetone (vapour pressure 12.36 kPa at 293 K), ethanol is expected to have higher boiling point.
  3. Process of evaporation When the liquid is exposed to atmosphere the vapours formed are dispersed to the larger volume of air. In order to keep the vapour pressure constant at the surface of liquid, more and more liquid is converted into vapour that, in turn, continues to disperse in air. This process of continuous conversion of liquid into vapour is known as evaporation. During this process, cooling of liquid is observed as the energy required to convert liquid molecule into vapour molecule comes from the liquid itself. The phenomenon of evaporation is faster in hot and airy weather as these factors help in evaporating and dispersing the gaseous molecules at a faster rate.

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