Problem 9 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

Alum is used by dyers of cloth as a mordant.

Step 1: Understand the Function of Alum

Alum is a chemical compound that is used for various purposes due to its properties. It is important to understand what alum does to a material, such as cloth. Specifically, alum helps in the dyeing process by fixing dyes on the fibers of cloth.

Step 2: Define Mordant in Dyeing

A mordant is a substance used in dyeing that helps fix the dye onto fibers. This is needed because most dyes do not form permanent bonds with fabrics without assistance. Alum acts as a mordant to bind dyes to textiles, ensuring that the colors remain vivid and do not wash out easily.

Step 3: Compare the Options

Now look at the given options from the exercise: (a) as a water purifier, (b) as mordant, (c) for fireproofing fabrics, (d) for softening hard water. Alum's role in the dyeing process aligns with option (b), as it is known to be used as a mordant. The other options describe different uses of alum not specific to dyers of cloth.

Key Concepts

Properties of Alum

Alum, often seen as a humble compound in the realm of chemistry, actually possesses a variety of unique properties that make it indispensable, especially in the textile industry.
One of the most notable properties of alum is its astringent nature. This means that it can cause body tissues to shrink or contract, which directly contributes to its effectiveness in fixing dyes to fabrics.
Another significant property is its ability to act as a coagulant. This refers to its use in turning small particles into larger aggregates. This specific property of alum makes it excellent for water purification but is equally important in dyeing, where it helps aggregate dye molecules to ensure a uniform application on textiles.
Finally, alum is traditionally known for its stability under normal storage conditions, making it a reliable choice for dyers who store and use the compound intermittently. This stability, coupled with its availability and cost-effectiveness, solidifies alum's standing as a preferred mordant in textile dyeing.

Fixing Dyes on Fabrics

The process of fixing dyes on fabrics is crucial to ensure the longevity and vibrancy of colored textiles. Simply put, dyeing itself is not enough as the dye molecules need to permanently attach to the fabric fibers.
Alum, as a mordant, plays a key role in this fixation process. When applied, alum helps create a bridge between the dye molecules and the fibers, forming a strong bond. This bond is essential because without it, colors can easily fade with washing or exposure to light.
This process can be described as follows:

• Preparation: The fabric first undergoes thorough cleaning to remove any impurities that could interfere with dyeing.
• Application of Alum: The fabric is then treated with a solution of alum, ensuring it is evenly applied.
• Dye Application: The fabric is dipped into the dye bath. With alum already present, the dye molecules effectively bond to the fibers.
• Final Fixation: This usually involves a steaming or heating step to further solidify the dye bonds.

Thus, fixing dyes on fabrics with alum not only enhances the appearance of the textiles but also ensures they are durable and retain their colors over time.

Chemical Compounds in Textile Industry

The textile industry heavily relies on various chemical compounds to achieve characteristics like color, durability, and texture. Understanding these chemicals helps explain how textiles are processed and improved for consumer use.
In dyeing, mordants like alum are just one of many chemical agents. These chemicals can be categorized based on their roles:

• Mordants: These help dyes bond to fabric, ensuring colors are vivid and long-lasting.
• Bleaches: Used to lighten fabric color or remove impurities before dyeing.
• Finishing Agents: Applied after dyeing to improve fabric texture, increase water resistance, or add softness.
• Fixatives: They enhance colorfastness, ensuring the dye does not wash out easily.
• Binders: These are crucial in fabric printing, helping pigments adhere properly.

Each chemical plays a specific role in refining fabrics into their final, desirable forms. Combining these compounds in varied processes allows the textile industry to produce diverse fabric types, tailored to different uses and preferences.