Alum is used by dyers of cloth: (a) As water purifier (b) As mordant (c) For fire proofing fabrics (d) For softening hard water

Alum, known chemically as potassium aluminum sulfate, plays a critical role in the art of fabric dyeing, serving as a mordant. A mordant is a substance used to set dyes on fabrics by forming a coordination complex with the dye, which then attaches to the fabric. This process not only helps in intensifying the color but also ensures that the dye is water-insoluble and won't wash out easily.

During the mordanting process, fabrics are typically treated with alum, which helps to create a bond between the dye molecules and the fabric fibers. This is particularly important for natural dyes derived from plants or insects, which often do not readily adhere to textiles without a mordant. By utilizing alum, dyers can achieve a broad range of vibrant, long-lasting colors on fabrics, which are essential for producing quality textiles.

Aside from its role in dyeing, alum is widely recognized for its ability to purify water. When alum is added to water, it reacts with bicarbonate alkalinity to form a gelatinous precipitate. This reactant, known as aluminum hydroxide, helps to trap and remove the fine suspended particles found in turbid water. This process is known as coagulation or flocculation.

During coagulation, the positively charged ions of alum neutralize the negative charges on sediment particles, which disrupts the charges that keep particles separated. When these particles come closer and aggregate, they form larger particles—flocs—that are more easily removed by sedimentation or filtration. By effectively removing such impurities, alum plays a significant role in providing safe, clean drinking water, a process that is crucial in many parts of the world to prevent waterborne diseases.

Alum's applications in dyeing and water purification have significant relevance in the field of chemistry, particularly for students preparing for competitive exams like the NEET (National Eligibility cum Entrance Test). The chemistry section of the NEET exam tests a student's understanding of chemical processes and their practical applications.

Understanding the chemical properties of substances like alum and their roles in everyday processes like dyeing and water treatment can be crucial for NEET aspirants. Students are expected to grasp concepts such as chemical reactions, compound formation, and practical applications of chemistry in industrial and environmental contexts. A thorough knowledge of how alum functions as a mordant and a water purifying agent can not only aid in exam preparation but also provides a foundation for future studies in environmental science and technology.