Answer:(a) When sodium chloride is added to ferric hydroxide sol, the sol gets coagulated. This is because the Cl– ions from NaCl neutralize the +ve charge on Fe(OH)3 sol.      Fe2O3.xH2O/Fe3+(b) Smoke coming out of the chimenys contain a lot of unburnt carbon particles smoke is the colloidal solution of the solid particles such as carbon arsenic dust particles etc.These particles are injurious to health and have to be precipitated since carbon particlesare charged particles and cannot be precipitated, thus metal plate of oppositely charged particles is used for their precipitation.This is done by the cottrell smoke precipitator which neutralizes the charge on the carbon. (c) Physical adsorption is multilayered because forces between adsorbate and adsorbent molecule are of Van der Waal’s type.Chemisorption is non-layers because forces of attraction are of chemical bond type.