Zsigmondy introduced a term called gold number to describe the protective power of different colloids. This is defined as the minimum amount of the protective colloid in milligrams required to just prevent the coagulation of 10 mL of a given gold sol when 1 mL of 10 % solution of sodium chloride is added to it. Example: Gold number of gelatine is 0 . 005 - 0 . 01 , egg albumin 0 . 15 - 0 . 25 .

What are different types of adsorption? Give any four differences between characteristics of these different types.

There are two types of Adsorption – 1. Physical Adsorption or Physiosorption and 2. Chemical Adsorption or Chemisorption. Physical Adsorption Chemical adsorption 1. It arises because of vander Waals' forces. 1. It is caused by chemical bond formation. 2. It is not specific in nature. 2. It is highly specific in nature. 3. It is reversible in nature. 3. It is irreversible. 4. It depends on the nature of gas. Easily liquefiable gases are adsorbed readily. 4. . It also depends on the nature of gas.Gases which can react with the adsorbent show chemisorption. 5. Enthalpy of adsorption is low 20 - 40 kJ mo l - 1 5. Enthalpy of adsorption is high 80 - 240 kJ mol - 1 .

State Hardy - Schulze rule.

It has been observed that, generally greater the valence of the coagulating ion added, the greater is its power to cause coagulation. This is known as Hardy- schulze rule. In the coagulation of a negative sol, the coagulating power of cations is in the order A l 3 + > Ba 2 + > Na + .

Give two examples for heterogeneous catalysis.

Oxidation of sulphur dioxide Into sulphur trioxide In the presence of Pt . 2 SO 2 g → Pt s 2 SO 3 g The reactant Is In the gaseous state while the catalyst Pt Is In the solid state. Combination between dinitrogen and dihydrogen to form ammonia In the presence of finely divided Iron In Haber’s process. N 2 g + 3 H 2 g → Fe s 2 NH 3 g