Why is $\mathrm{Fe}(\mathrm{OH})_3$ colloid positively charged, when prepared by adding $\mathrm{Fe}(\mathrm{OH})_3$ to hot water?
Charge on sol is decided by adsorption of ions present in medium. Adsorption of positively charged $\mathrm{Fe}^{3+}$ ions takes place by the sol of hydrated ferric oxide. Thus, $\mathrm{Fe}(\mathrm{OH})_3$ colloid has positive charge when prepared by adding $\mathrm{Fe}(\mathrm{OH})_3$ to hot water.
Why do physisorption and chemisorption behave differently with rise in temperature?
Behaviour of physisorption and chemisorption on increase in temperature can be explained on the basis of nature of forces present to bind their particles. Physisorption involves weak van der Waals' forces which weakens with increase in temperature. Chemisorption involves formation of chemical bond which requires activation energy hence, it is favoured by rise in temperature.
What happens when dialysis is prolonged?
Traces of electrolyte which stabilises the colloids is removed completely making the colloid unstable. So, coagulation occurs on prolonged dialysis.
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.