Question

Give reasons for the following observations :

(i) Physisorption decreases with increase in temperature.

(ii) Addition of alum purifies the water.

(iii) Brownian movement provides stability to the colloidal solution.

Answer 1

(i) Physisorption is a reversible process as represented below :
$\underset{\left(Adsorbate\right)}{Gas}+\underset{\left(Adsorbent\right)}{Solid}\rightleftharpoons \text{Gas |Solid +Heat}$
Thus, process is exothermic. According to Le Chatelier's principle, on increasing temperature, the equilibrium shifts in the backward direction and thus, the adsorption will decrease.

(ii) $A{l}^{3+}$ ions given by the alum bring about coagulation of negatively charged colloidal mud particles which settle down at the bottom of the vessel.

(iii) Brownian movement i.e., zig-zag movement of the colloidal particles is due to the hitting of these particles by the molecules of the dispersion medium with different forces from different directions and the particles do not settle. This provides stability to the colloidal solution.