Question

Alum is water purifier because it:
A. coagulates the impurity
B. soften hard water
C. gives taste
D. destroy the pathogenic bacteria

Answer 1

Hint- To solve this question first we need to understand the basic functionality of alum. And also, we need to focus on that, dust does not settle in the water in the first place as dust forms a kind of a suspension.

Complete step-by-step answer:
As we know, Alum (aluminum sulfate) is a nontoxic liquid that is commonly used in water treatment plants to clarify drinking water. It’s uses in lakes began in the early 1970’s and is used to reduce the amount of phosphorus in the water. Less quantity of phosphorus leads to lower amounts of algae and the symptoms associated with poor water quality.
And Alum is generally used to control phosphorus release from the lake bottom sediments (internal loading). Research has shown that even when external sources of phosphorus from the surrounding watershed are lowered, the internal cycling can continue to support significant nuisance algal blooms.
What does add alum do?
Alum has a negative charge and tends to disperse in water very fast and very well (Because it's a colloid).
This causes it to join up with all of the offending particles and neutralize them. Now that the particles don't have any repelling charges, they tend to clump together into 'flocs'.
As size increases, the lack of repelling charges cause the alum particles to settle down at the bottom or rise up and float in the water.
After the particles are neutralized, they clump together because of the London Dispersion Force which are part of the Van der Waals forces. Basically, weak intermolecular force arising from quantum-induced instantaneous polarization multipoles in molecules causes even nonpolar particles to attract each other due to the correlated movements of the electrons in interacting molecules. Then they settle down.
Thus, overall, we conclude that the Alum coagulates the suspended or colloidal impurities.
Therefore, option (A) is the correct answer.

Note- Remember that When alum is added to raw water, it reacts with the bicarbonate alkalinities present in the water and forms a gelatinous precipitate. And this attracts other fine particles and raw water suspended material, and settles down at the bottom of the container.