What is the process by which alum purifies dirty water?

Correct option is C

The correct answer is (c) Coagulation

Role of Alum in Water Treatment:

• Alum (Aluminum Sulfate) is commonly used in water treatment to clarify turbid water.
• When added to water, alum undergoes hydrolysis and forms positively charged ions that coagulate (clump together) the suspended impurities.
• These clumps (called flocs) then settle to the bottom or can be filtered out, thus purifying the water.

Additional Information:

Absorption:

• Refers to one substance being taken up internally by another (e.g., water absorbed by a sponge).
• Alum doesn’t work by absorption; it forms flocs with suspended particles.

Adsorption:

• Molecules adhere to a surface (e.g., activated charcoal adsorbing impurities).
• While some adsorption can occur, the primary process with alum is coagulation.

Dialysis:

• A separation process used to remove unwanted small molecules from a solution through a semipermeable membrane.
• Not the mechanism by which alum purifies water.