Why does the white precipitate of silver halide become coloured in the presence of dye eosin?
White coloured precipitate of silver halide becomes coloured in the presence of dye eosin because dye eosin (coloured) gets adsorbed on the surface of silver halide precipitate.
What is the role of activated charcoal in gas mask used in coal mines?
Role of activated charcoal in gas mask can be explained on the basis of adsorption. Activated charcoal adsorbs various poisonous gases on its surface present in coal mines.
How does a delta form at the meeting place of sea and river water?
Formation of delta at the meeting place of sea and river water is due to coagulation. River water (colloid of sea water + clay) has many dissolved electrolytes. The place where river meets sea is the site for coagulation. Deposition of coagulated clay results in delta formation.
Give an example where physisorption changes to chemisorption with rise in temperature. Explain the reason for change.
Adsorption of $\mathrm{H}_2$ on finely divided nickel (physisorption) involves weak van der Waals' forces. When temperature is increased, hydrogen molecules dissociate into hydrogen atoms, form chemical bonds with the metal atoms at the surface (chemisorption).
Why is desorption important for a substance to act as good catalyst?
Desorption is important for a substance to act as a good catalyst so that after the reaction, the products formed on the surface separate out (desorbed) to create free surface again for other reactant molecules to approach the surface and react.
If desorption does not occur then other reactants are left with no space on the catalysts surface for adsorption and reaction will stop.