Alum is used as

Alum, specifically potassium aluminum sulfate, is widely utilized as a purifier for water due to its properties as a chemical coagulant and flocculant [t1][t4]. When added to turbid or polluted water, alum undergoes hydrolysis to form a gelatinous precipitate of aluminum hydroxide [t5]. This precipitate neutralizes the negative electrical charges of suspended fine particles like silt, clay, and bacteria, causing them to aggregate into larger clumps called 'floc' [t3][t4]. These flocs are heavy enough to settle at the bottom through gravity, effectively removing turbidity and impurities from the water column [t3][t9]. While alum also possesses astringent and antiseptic properties used in personal care (like styptic pencils for minor cuts) and acts as a mordant in the textile industry, its most vital and widespread large-scale application remains municipal and emergency water purification [t4][t5].

Sources

1. [1] https://www.britannica.com/science/alum