Boiling and Melting/Freezing point

Boiling Point

Meaning of Boiling Point

Boiling happens at a particular temperature and pressure, the temperature of the liquid remains steady during boiling. Boiling occurs throughout the entire volume of the liquid and wind has no effect on boiling.

Boiling point of a liquid is that temperature at which its saturated vapour pressure is equal to the atmospheric pressure.

Saturated vapour is a vapour that is in contact with its own liquid within a confined space. The pressure exerted by the saturated vapour is known as saturated vapour pressure.

Saturated vapour pressure can be defined as the pressure exerted by the vapour of the liquid which is in dynamic equilibrium with its own liquid.

Effect of Temperature on S.V.P

Increase in the temperature will increase the S.V.P of the liquid.

Determination of the Boiling Point of a Liquid

Determination of the Boiling Point of a Liquid

The J-shaped tube is filled with mercury and a drop of liquid is introduced into the space on top of the mercury in the shorter arm. The liquid evaporates and the pressure of the vapour depresses the mercury level slightly. The tube is heated in a beaker of water as the temperature increases, the liquid evaporates and more drops introduced. The s.v.p of the liquid increases. At a stage the mercury levels in both arms of the tubes become equal and the temperature is read at this point. This temperature is the boiling point of the liquid because at this temperature s.v.p of the liquid is equal to the atmospheric pressure.

Precautions:

1. A pure sample of the liquid must be used.
2. The mixture should be well stirred to ensure even distribution of heat.

Effect of Pressure and Dissolved Substances on Boiling and Freezing Point

Effect of Pressure on Boiling Point

1. Reduced pressure lowers the boiling point

If a flask of boiling water is allowed to cool until the water stops boiling and cold water is now poured over the bottom of the flask, boiling resumes and cease when the pouring of water stops. The poured cold water causes condensation of the vapour inside the flask, which consequently reduced the pressure on the surface of the water.  This reduced pressure lowers the boiling point of the liquid and make it boils again.  Hence reduced pressure always lowers the boiling point of a liquid.  Dissolved impurities raises the boiling point of liquid.

2. Increased pressure raises the boiling point

The fact that increased pressure raises the boiling point is put into a useful application in the pressure cooker.  The increased pressure of the trapped gas above the liquid raises the boiling of the liquid inside the cooker.  This provides a high cooking temperature needed to conserve fuel and save time. Pressure cookers are advantageous especially in mountainous area where atmospheric pressure is low.

Effect of Pressure on Freezing Point

Increase pressure lowers the melting point of all substances that expand on solidifying(ice block), but for substances that contracts on solidifying(paraffin), their melting point is raised by increased pressure.

Effect of Dissolved Impurities on Boiling Point

Dissolved impurities raises the boiling point of pure liquid.

Effect of Dissolved Impurities on Melting/Freezing Point

Dissolved impurities lowers the melting point of pure solid or the freezing point of pure liquid.

EVALUATION – Post answers below

1. Define the boiling point of a liquid.
2. Describe an experiment to determine the boiling point of a small quantity of a liquid.