Vapour pressure of liquid and boiling

Vapour pressure:

Vapour pressure is defined as the partial pressure exerted by the vapors above the liquid surface in equilibrium with liquid at a given temperature.

fig 3.3

In illustrations 1a and 1b, the liquid and vapor states of a given substance are represented. The individual molecules or particles are represented at a particular instant of their random motion. When equilibrium is reached (illustration 1c), the number of molecules evaporating is equal to the number of molecules condensing. The pressure read on the manometer M is the vapor pressure of the liquid at that temperature.

Vapor pressure of a liquid depends on the following factors:

a) Nature of the liquid

b) Temperature of the liquid.

Nature of the liquid

Liquids that have weak intermolecular forces are more volatile and have a higher vapor pressure.

For e.g., vapor pressure of ethyl alcohol is greater than that of the water.

Temperature of the liquid

Vapor pressure increases with increase in temperature. This is because with an increase in temperature, rate of evaporation also increases.

Boiling:

Vapour pressure measures the tendency for the molecules to escape from liquid to the gas phase. At lower temperatures the vapour pressure of a liquid is much lower than the pressure on the surface of the liquid. When the temperature of the liquid is gradually increased, its vapour pressure also increases. Ultimately a stage is reached when the vapour pressure of the liquid equals the pressure of the air above it. At this point, molecules and vapours formed within the liquid can easily rise through the liquid in the form of bubbles and escape into the air. This phenomenon is known as boiling and the temperature at which this occurs is known as boiling point.

Boiling point is the temperature at which the vapour pressure of a liquid becomes equal to the atmospheric pressure. The boiling point, therefore, depends upon the atmospheric pressure and it changes with the change in the pressure above the liquid. As the atmospheric pressure increases, it is necessary to heat the liquid to a higher temperature to make its vapour pressure equal to atmospheric pressure. At high altitudes like Ooty, having lower atmospheric pressure, water boils at a much lower temperature than in the plains like Bangalore where atmospheric pressure is higher.

Generally, boiling points for most of the liquids are specified as normal boiling points. The normal boiling point of a liquid is the temperature at which the vapour pressure of the liquid becomes equal to one atmospheric pressure (standard pressure).

It may be noted that once the boiling starts, the temperature of the liquid remains constant, until the whole of the liquid has vaporized, even though heating is continued.

Problem

10. Why does the temperature of the boiling liquid remains constant even though heating is continued ?

Solution

When a liquid is heated, the heat supplied is consumed to pull the liquid molecules apart against strong attractive forces, which hold them together in the liquid form. The heat energy goes to compensate the loss of energy due to escaping molecules, which have to overcome the attractive forces between the molecules of the liquid. As a result, there is no change in the average kinetic energy of the molecules remaining in the boiling liquid. Since the average kinetic energy is proportional to the temperature (1/2mV² T), therefore, the temperature of the liquid does not change till the whole of liquid has been converted into the vapour state.

Effect of change in external pressure on the boiling point

A liquid may be made to boil at any desired temperature by changing the external pressure.

For example, a liquid may be made to boil at higher temperature by increasing the pressure on its surface, as is done in a pressure cooker. It may be made to boil at a lower temperature than the normal boiling point by reducing the external pressure on it. It is observed that some substances decompose at their normal boiling points and therefore, they cannot be heated up to their normal boiling points. They are made to boil at reduced temperatures. This principle is used in purifying less stable liquids by distillation under reduced pressure. This process is called vacuum distillation..

Difference between evaporation and boiling

Although boiling and evaporation are similar processes, yet they differ in few aspects. The main points of difference between evaporation and boiling are given below: