Question

What happens when gelatin is added to gold sol $?$

Answer 1

Hint :Colloids can be defined as a heterogeneous system in which one substance is dispersed as fine particles in another substance. The substance that gets dispersed is called the dispersed phase and the substance on which it gets dispersed is called dispersion medium. Sol is a colloid which has solid as the dispersion phase and liquid as the dispersion medium.

Complete Step By Step Answer:
Based on the interaction between the dispersed phase and dispersion medium, colloidal sols are divided into lyophilic and lyophobic colloids.
Lyophilic colloids are solvent loving colloids. They are formed when the dispersed phase is directly mixed with a suitable liquid which is the dispersion medium. They are quite stable and cannot be easily coagulated. Examples of dispersed phases are gum, gelatine, starch etc.
Lyophobic colloids are solvent hating colloids. Special methods have to be adopted to prepare the sol. They are not stable and can be easily coagulated by heating, shaking or adding electrolytes. Examples of dispersed phases are metals and their sulphides.
Gelatin is a lyophilic sol and gold is a lyophobic sol. So, when gelatin is added to gold sol it forms a protective layer over the gold sol. Now, it cannot come in contact with any electrolyte. Thus, it is prevented from coagulation. Here, gelatin acts as a stabilising agent.

Note :
Lyophilic sols are more stable than lyophobic sols because of the strong interaction between the solvent and the dispersed phase.
Lyophilic sols are reversible sols because when the solvent gets separated from the dispersed phase, we can get back the sol by just mixing it again with the same solvent. Lyophobic sols are irreversible sols since we cannot retain back the sol by just adding solvent.