In household drinking water treatment, alum is commonly used for which main purpose in purifying the water?

Introduction / Context:
Alum is a familiar name in basic household water purification and school level science experiments. Many learners know that alum is added to dirty water, after which the water looks clearer. However, the exact role of alum in purifying drinking water is often confused with disinfection or softening. This question tests understanding of the specific physical and chemical process by which alum helps to clean water for safe use.

Given Data / Assumptions:
- The context is household or small scale drinking water treatment.
- Alum is added to turbid or muddy water during purification.
- We assume no other advanced treatment like chlorination is included in this specific step.
- The question asks for the main purpose alum serves in this process.

Concept / Approach:
Alum is commonly used as a coagulant in water treatment. When alum is added to water, it reacts to form a gelatinous precipitate of aluminium hydroxide. This precipitate traps fine suspended particles, clay, and other impurities in a process known as coagulation and flocculation. The heavy flocs then settle to the bottom of the container, leaving the upper layer of water much clearer. Alum does not primarily kill germs like a disinfectant, and it does not remove dissolved salts in the way that water softeners or advanced treatment plants do. Its main role is to help suspended impurities come together and settle out of the water.

Step-by-Step Solution:
Step 1: Recall that turbid water looks muddy because of fine suspended particles that do not settle easily on their own. Step 2: Remember that alum is added so that very small particles can join together into larger clumps called flocs. Step 3: Understand that these flocs become heavier and slowly settle to the bottom, a process called sedimentation. Step 4: Recognise that the clearer water at the top can then be decanted or filtered for further treatment and use. Step 5: Match this role with the option that describes coagulation and settling of suspended impurities.

Verification / Alternative check:
Simple school demonstrations show that if alum is added to muddy water, after some time the water becomes clearer while a layer of sludge forms at the bottom. However, if this water is tested for germs, it may still contain bacteria and viruses, which shows that alum alone does not fully disinfect water. Municipal water treatment plants also list alum under their coagulation and flocculation step, followed later by filtration and disinfection. This confirms that the main function of alum is to remove suspended particles by helping them settle out.

Why Other Options Are Wrong:
- Alum does not primarily kill harmful bacteria directly; disinfectants like chlorine or boiling are needed for that purpose.
- Alum does not remove dissolved salts to make hard water soft; special ion exchange or other methods are required for softening.
- Alum is not used to add fluoride for tooth decay prevention; fluoride is added in controlled amounts using different compounds in fluoridation programmes.

Common Pitfalls:
Many students think that any substance used in water treatment must kill germs and therefore confuse alum with disinfectants. Others assume that clear water is always safe water and do not distinguish between removal of visible particles and removal of microscopic pathogens or dissolved chemicals. It is important to remember that alum mainly handles suspended impurities through coagulation and sedimentation, and safe drinking water often requires additional steps like filtration and disinfection.

Final Answer:
Alum is used in drinking water purification mainly To coagulate suspended impurities so they settle at the bottom.