Problem 79 - A Complete Resource Book in Chemistry for JEE Main

Question

The alum used for purifying water is (a) potash alum (b) chrome alum (c) ferric alum (d) ammonium alum

Step-by-Step Solution

Answer

The alum used for purifying water is potash alum (option a).

Step 1: Understanding Alums and Their Use in Water Purification

Alums are double sulfate salts with capabilities for flocculation—a process used to remove impurities in water by causing them to clump together. Not all types of alum are used for water purification, so it is critical to know which specific alum is effective in this application.

Step 2: Identifying Suitable Alum for Water Purification

Potash alum, whose chemical formula is ext{K}_2 ext{SO}_4 ext{·Al}_2 ext{(SO}_4 ext{)}_3 ext{·24H}_2 ext{O}, is commonly used for water treatment due to its ability to aid in coagulation, which is essential in removing unwanted particles. Potash alum is effective and widely applied in purifying water.

Step 3: Analyzing Options Given in the Exercise

From the options provided, the requested alum for water purification needs to be identified. Potash alum is the alum used for purifying water, as it increases the size of impurities, allowing them to be filtered out. The other options, such as chrome alum, ferric alum, and ammonium alum, are not primarily used for water purification.

Key Concepts

Potash Alum

Potash alum is a vital component in the water purification process. It is a type of alum known for its role in making water clearer and safer. Chemically, it is represented by the formula \( \text{K}_2\text{SO}_4\text{·Al}_2\text{(SO}_4\text{)}_3\text{·24H}_2\text{O} \).This compound is prized for its ability to cause small particles suspended in water to come together.

These particles can include dirt, bacteria, and other impurities that can make water unsafe or unpleasant to drink.

Potash alum is commonly used in municipal water treatment facilities and even in personal water purification processes.

• It helps in improving the taste and appearance of water.
• It assists in reducing the turbidity of the water, making it clearer.
• It is non-toxic when used appropriately, thus safe for water purification purposes.

Whether in large-scale applications or household use, potash alum remains a trusted choice for improving water quality.

Flocculation

Flocculation is a key process in water purification that involves the aggregation or clumping of particles. It is crucial in the removal of suspended solids from water, making it clearer and free of contaminants.

The process begins when chemicals, like potash alum, are added to the water. These added chemicals have positive charges that neutralize the negative charges on particles within the water itself.

Once the charges are neutralized, the particles can come together to form larger clumps known as flocs.

Flocs are heavy enough to settle at the bottom of the container, where they can be easily filtered out or removed. Here are some advantages of flocculation:

• Enhances the efficiency of sedimentation and filtration.
• Reduces the amount of organic matter and microorganisms.
• Improves the water's color and overall quality.

Flocculation, when properly executed, significantly aids in delivering safe and clean water.

Coagulation

Coagulation is another essential step in the water purification process, closely associated with flocculation. It refers to the initial destabilization of particles, where tiny contaminants become destabilized and begin to clump together.

This process turns suspended particles into more manageable aggregates by counteracting the natural forces keeping them in suspension.

Coagulation typically involves the addition of coagulants like potash alum, which play a dual role. The coagulant’s positive charge binds with the negatively charged particles, neutralizing them.

Upon neutralization, these particles are no longer repelled from one another and instead form small clusters that facilitate the flocculation process. Advantages of coagulation include:

• Immediate reduction in water turbidity.
• Effectively aids in the removal of pathogens and color.
• Prepares water for more effective filtration.

Coupled with flocculation, coagulation is central to ensuring water is ready for consumption.