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Vapour Pressure

Origin of Vapour Pressure
Suppose a beaker containing a liquid is placed in an evacuated vessel see fig.  Let the latter be connected to a manometer so that any pressure that is developed in the free space can be measured. After sometime, it is found that the manometer records a constant pressure. This pressure is known as the vapour pressure of the liquid.
Before making an attempt to understand how the vapour pressure arises, we will have to consider the following two facts.
  1. The molecules of a liquid, like those of gases, have different kinetic energies. The distribution of speeds amongst molecules follows Maxwell-Boltzmann distribution. Figure shows such a distribution at two different temperatures. The most important point to be noted in these distribution curves is that the fraction of molecules having higher speeds increases with the increase in temperature.
  1. If we consider a molecule in the bulk of a liquid, it will be surrounded by other molecules in a symmetrical manner. Thus, the forces of attraction on this molecule by the molecules present on one side are completely balanced by the molecules present on the opposite side. Hence, the net force of attraction experienced by this molecule will, of the whole, be zero. It will move as if there existed no forces of attraction on it. However, the situation is altogether different at the surface of the liquid (fig). There are a large number of molecules towards the liquid side of a molecule than towards the open space above it, with the result that this molecule experiences a net force of attraction in the downward direction.
The force of attraction between the molecules of a liquid is of a stronger nature and is larger than the average thermal energy of the molecules. However, because of the Maxwell-Boltzmann distribution, some of the molecules can have thermal energies equal to or greater than the characteristic energy which-is just sufficient to overcome the forces of attraction.

If such a molecule happens to be at the surface, it will overcome the net downward forces of attraction and will immediately leave the surface and escape to the empty space above. If the space above the surface is an open one, then the molecules will continue to escape resulting in the phenomenon of evaporation. Since molecules of higher thermal energies are leaving the surface of the liquid, it follows, therefore, that the average thermal energy of molecules in the liquid will decrease. Consequently, the temperature of the liquid is reduced and, hence, cooling is observed. However, if the space above the liquid is a closed one, then the molecules escaping from the surface of the liquid (referred to as vapour molecules) will go on collecting in the empty space. After sometime, it is observed that a constant pressure is registered in the space above the liquid. This pressure is due to vapour molecules of the liquid and, hence, is known as the vapour pressure of the liquid. 

Since this pressure is constant, it follows that there must be a constant number of molecules in the space above the liquid. This can be true only if the molecules in the space are also returning to the liquid; otherwise, the pressure in the space would continue to increase. In fact, when a vapour molecule with a comparatively smaller thermal energy collides with the surface if the liquid, it sticks to the latter. Thus, there is a two-way process; the molecules are leaving the liquid and are simultaneously coming back to it. We get a state of dynamic equilibrium when the rate of evaporation of liquid molecules is equal to the rate of condensation of the vapour molecules. Thus, the vapour pressure of a liquid may be defined as the pressure of the vapour in equilibrium with the liquid.

Effect of Temperature on Vapour Pressure  
On raising the temperature, more and more molecules of a liquid will have energies equal to or greater than the critical energy which is just sufficient to overcome the forces of attraction between the molecules. As a result, a large number of molecules can leave the surface of the liquid, which will consequently have higher vapour pressure. Thus, the vapour pressure of a liquid increases with increase in temperature. The variation of vapour pressure of liquids with temperature is of the type shown in fig, with the highest and lowest limits corresponding to critical point and triple point, respectively.

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