In water treatment and daily life, alum is commonly used for which main purpose?

Introduction / Context:
Alum is a familiar name in school chemistry and in household use. It appears as colourless crystals and is often added during water treatment. This question checks whether you understand the primary practical use of alum and can distinguish it from other possible but incorrect roles such as working as a medicine, fertilizer, or disinfectant.

Given Data / Assumptions:

• The substance in question is alum, usually meaning potash alum, KAl(SO4)2·12H2O.
• We are concerned with common uses in everyday life and basic water treatment.
• Options list analgesic, water purification, fertilizer, disinfectant, and plant growth hormone source.
• We assume a basic school level understanding of coagulation and flocculation in water treatment.

Concept / Approach:
Alum is widely used as a coagulant in water purification plants and sometimes in household water storage. When added to muddy or turbid water, alum causes fine suspended particles to clump together into larger flocs which then settle to the bottom, making the water clearer. It is not primarily a pain reliever, fertilizer, or hormone source. Although alum has some mild antiseptic and astringent properties, its main textbook use is in water purification.

Step-by-Step Solution:
Step 1: Recall that in water treatment units, alum is added to raw water before sedimentation and filtration. Step 2: Alum reacts with natural alkalinity in water to form a gelatinous precipitate of aluminium hydroxide. Step 3: This precipitate traps and carries down fine suspended particles, making the water clearer. This process is called coagulation and flocculation. Step 4: Analgesics are medicines used to relieve pain and include drugs like paracetamol; alum is not classified as an analgesic. Step 5: Fertilizers supply nutrients like nitrogen, phosphorus, and potassium to plants. Alum is not a common fertilizer. Step 6: Disinfectants kill microorganisms; alum mainly clarifies water but does not by itself fully disinfect it.

Verification / Alternative check:
If you have seen people add small pieces of alum to a bucket or tank of muddy water, you may have observed the suspended clay and dirt settling down faster. Textbooks on environmental science and public health engineering clearly state that alum is used as a coagulant in water purification. Water treatment plants routinely dose alum before settling basins to improve clarity, confirming that its key role is purification rather than disinfection or pain relief.

Why Other Options Are Wrong:
As an analgesic for pain relief: Alum is not used as a general pain killer; analgesics are pharmaceutical drugs, not mineral salts like alum.
As a nitrogen fertilizer in agriculture: Alum does not supply major plant nutrients and is not used as a standard fertilizer.
As a household surface disinfectant: While alum may have mild antiseptic effect, it is not the typical disinfectant used for cleaning surfaces; products like bleach and phenolic cleaners are used instead.
As a source of plant growth hormones: Plant hormones include auxins, gibberellins, and cytokinins, not alum salts.

Common Pitfalls:
Students sometimes confuse clarification with disinfection and think that any agent used in water treatment must be a disinfectant. Another confusion arises from the mild antiseptic and astringent use of alum in some traditional remedies, which may lead to selecting it as an analgesic. Always remember that the central textbook role of alum is as a coagulant in the purification of drinking water by removing suspended impurities.

Final Answer:
Alum is commonly used in the purification of drinking water as a coagulant to remove suspended impurities.