Alum’s capacity to purify water is due to

The correct option is impurities’ coagulation.

Here is a step-by-step explanation of why this option is correct:

1. Nature of Impurities in Water:
Most suspended impurities in natural water sources, such as clay, silt, and organic matter, are colloidal in nature. These colloidal particles carry a negative electrical charge. Because they all share the same negative charge, they repel each other and remain suspended in the water, preventing them from settling down naturally.

2. Role of Alum (Chemical Coagulant):
Alum is chemically known as hydrated potassium aluminum sulfate, represented by the chemical formula:
${\mathrm{KAl}}_2{\left({\mathrm{SO}}_4\right)}_4·24H_{2}O$
When alum is added to water, it dissociates and releases highly charged trivalent aluminum cations (${\mathrm{Al}}^{3+}$).

3. The Coagulation Process:
The positively charged aluminum ions (${\mathrm{Al}}^{3+}$) attract and neutralize the negative charges on the colloidal impurity particles. Once the electrical charges are neutralized, the repulsive forces between the impurities disappear. The particles then come together and aggregate into larger, heavier clumps called flocs. This process of neutralizing charges and clustering particles together is known as coagulation.

4. Sedimentation and Purification:
The newly formed heavy flocs can no longer remain suspended in the water. Under the influence of gravity, they quickly settle down to the bottom of the container. This makes it easy to separate the clear, purified water from the settled impurities through simple decantation or filtration.