(a) Define the boiling it of a liquid. (b) Describe with the a d. labelled diagram, an experiment to determire the boiling point of a small quantity of a li...

(a) The boiling point of a liquid is the temperature at which its saturated vapour pressure becomes equal to the external atmospheric pressure, so that the liquid changes to vapour throughout its bulk.

(b) Determination of the boiling point of a small quantity of a liquid

The apparatus is arranged as shown below.

Procedure

1. Introduce a little mercury into a J-tube. Remove the air trapped in the shorter, closed limb.
2. Put a small quantity of the liquid under test into the shorter limb, above the mercury.
3. Place the J-tube in a beaker containing water. Heat the water bath gently and stir it continuously so that its temperature is uniform.
4. Observe the mercury levels in the two limbs. When the mercury levels are at the same horizontal level, read and record the thermometer temperature.
5. Allow the water bath to cool, still stirring it. Again record the temperature at which the mercury levels are equal.
6. The mean of the heating and cooling readings is the boiling point of the liquid.

When the mercury levels are equal, the pressure of the liquid vapour in the closed limb is equal to atmospheric pressure in the open limb. Hence, at this temperature, the saturated vapour pressure of the liquid equals atmospheric pressure and the temperature is its boiling point.

(c) Factors affecting the boiling point of a liquid include:

1. External pressure: an increase in external pressure raises the boiling point, while a decrease in pressure lowers it.
2. Impurities: dissolved non-volatile impurities generally raise the boiling point of a liquid.

(d) At the boiling point, the heat supplied to pure water is absorbed as latent heat of vaporisation. This energy is used to overcome the intermolecular forces of attraction between water molecules and to separate the molecules further apart against atmospheric pressure as steam forms. Since the supplied heat does not increase the average kinetic energy of the molecules, the temperature of the water remains constant while it changes to steam.