Why is gelatin mixed with gold sol?
A ) Gold sol is a lyophobic sol, gelatin acts as a stabilizing agent.
B ) Gold sol is a lyophilic sol, gelatin acts as a stabilizing agent.
C ) Gelatin produces negative charge on gold particles in gold sol.
D ) Gelatin helps gold sol to get its critical micelle concentration.

Hint: As long as colloidal particles have a charge, they do not undergo coagulation. If an electrolyte is added to the colloidal solution, coagulation occurs. The colloidal particles absorb the ions from the electrolyte, and the charge on colloidal particles is neutralized.

Complete answer:
> Lyophilic sols have strong affinity for solvent whereas lyophobic sols have little affinity for solvent. If a colloidal particle comes in contact with an electrolyte, it absorbs ions having charge opposite to that of colloidal particles. Due to this, charge of colloidal particles is neutralized and coagulation occurs.
> The stability of lyophilic sols is greater than that of lyophobic sols. This is due to extensive solvation of lyophilic sol. The solvent (usually water) molecules form a sheath around each colloidal particle.
> When a lyophilic sol is added to a lyophobic sol, a layer of lyophilic sol Is formed around each colloidal particle of lyophobic sol. This helps in stabilizing the lyophobic colloid from the effects of electrolytes. Such lyophilic sols, that are added to protect lyophobic sols are called protective colloids.
> Thus, the stability of a lyophobic sol can be increased by adding a protective colloid to it. This property of protecting a colloid is unique to lyophilic colloids.
> Gelatin is a lyophilic sol. Gold sol is a lyophobic sol. When gelatin is added to gold sol, it acts as a stabilizing agent as it protects the gold sol by forming a layer of gelatin particles around each gold particle. Due to this, the gold colloidal particles cannot come in contact with electrolytes. Due to this, the charge on gold colloidal particles remains intact and they do not undergo coagulation.

Hence, the correct answer is the option A ) Gold sol is a lyophobic sol, gelatin acts as a stabilizing agent.

Additional Information The surfactant concentration above which, micelle are formed is known as critical micelle concentration.

Note: Gelatin does not produce a negative charge on gold colloidal particles. It only prevents the contact between gold colloidal particles and electrolytes. Gelatin forms a layer of gelatin particles around each gold particle. Due to this, the ions of the electrolyte cannon come in contact with gold particles.