(i) The process of hardening of leather is known as tanning. Tannin, which is obtained from a mixture of polyhydroxy benzoic acids and contains negatively charged colloidal particles. Animal skin is colloidal in nature and has positively charged particles. When they are soaked in tannin, their mutual coagulation takes place and, thus leather becomes hard after tanning.
(ii) Lyophilic sols are more stable than lyophobic sols. This is due to the fact that lyophilic colloids are extensively solvated, i.e. colloidal particles are covered by a sheath of the liquid in which they are dispersed.