Question

Explain with reason: (i) The colloidal solution of Fe(OH)$_3$ obtained from FeCl$_3$ is positively charged.
(ii) Alum is used for the purification of water.
(iii) Rivers form a delta while joining a sea/ocean.

Hint:

Remember: The charge on colloids depends on the ions adsorbed, and coagulation by electrolytes is the key principle in both water purification and delta formation.

Answer 1

(i) The colloidal solution of Fe(OH)$_3$ obtained from FeCl$_3$ is positively charged:
When FeCl$_3$ is hydrolyzed, Fe(OH)$_3$ sol is formed. The Fe$^{3+}$ ions from FeCl$_3$ get adsorbed on the surface of Fe(OH)$_3$ particles.
This leads to the development of a positive charge on the colloidal particles. Hence, the sol of Fe(OH)$_3$ is positively charged. (ii) Alum is used for the purification of water:
Alum (K$_2$SO$_4 \cdot$ Al$_2$(SO$_4$)$_3 \cdot$ 24H$_2$O) produces Al$^{3+}$ ions in water. These ions neutralize the negative charges on colloidal impurities present in water.
As a result, the colloidal particles coagulate, settle down, and can be easily removed. Hence, alum acts as a coagulant in water purification. (iii) Rivers form a delta while joining a sea/ocean:
Rivers carry colloidal particles like clay and silt. When river water meets sea water (which contains electrolytes like NaCl, MgCl$_2$), the electrolytes coagulate the colloidal particles.
These coagulated particles settle at the river mouth, gradually forming delta regions. Conclusion:
The above examples demonstrate the role of adsorption and coagulation in colloidal chemistry and natural phenomena.