Question

Alum helps in purifying water by:
(A) Forming Si complex with clay particles
(B) Sulphate part which combines with the dirt and remove it
(C) Aluminium which coagulates the mud particles
(D) Making the mud water soluble

Answer 1

Hint: Alum is commonly used to make the suspended particles present in the water, such as algae, protozoa, viruses, bacteria, and some metal ions, clump together into larger particles and settle down.

Complete step by step solution:

• The dust particles present in water are all similar and ionized, therefore, they have a similar charge which is mainly negative, causing them to repel each other and stay separate. As they're so small, the gravitational force acting on them isn't very strong either. Also, the dust particles have a lot of kinetic energy already. These are some reasons why the particles remain suspended for so long.
• So, when we add alum, Alum has a positive charge and tends to disperse in water very fast and very well. This causes it to attract and join up with all of the dust particles and neutralize them. Now, the particles don't have any repelling charges, so they clump together into “Flocs.” With an increase in the size as well as the lack of repelling charges causes, the alum particles settle down at the bottom.
• After the particles are completely neutralized, they clump together because of the London dispersion force present between them, which are part of the Van der Waals force. Basically, weak intermolecular force arising from instantaneous polarization multipoles in molecules causes even nonpolar particles to attract each other due to the correlated movements of the electrons in interacting molecules.

So, from the above statements we can conclude that the correct answer is (c).

Note: The water purified by alum isn't too safe to drink because the dissolved salts and bacteria present in the water may remain unaffected in this process, it only helps in the coagulation of the dust particles present in the water.