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

(b) as mordant

Step 1: Understanding 'Mordant'

In the context of textile dyeing, a 'mordant' is a substance used to set dyes on fabrics by forming a coordination complex with the dye, which then attaches to the fiber. This process ensures that the dye remains on the fabric and is not washed away. Alum is commonly used as a mordant in the dyeing process.

Step 2: Evaluating Other Uses of Alum

Let's examine the other options to see how alum fits: (a) Water purifier - while alum can be used in water treatment, it is not specifically for dyeing cloth. (c) Fire-proofing - alum is not primarily known for fire-proofing fabrics. (d) Softening hard water - alum doesn't soften water; instead, it is sometimes used to clarify water by removing suspended particles.

Step 3: Selecting the Correct Option

Based on the understanding of how alum functions primarily in these contexts, the option that best fits how dyers of cloth use alum is option (b), as a mordant.

Key Concepts

Mordant

In the world of textile dyeing, understanding mordants is crucial for achieving long-lasting and vibrant colors on fabrics. A **mordant** is essentially a chemical that helps bind dyes to fabric. This happens by forming a bridge between the dye molecules and the fibers of the fabric, creating a more permanent and washable color.

Here's how mordants work:

• They act as a **binding agent**, creating a bond between the dye and the fabric fibers.
• Through chemical reactions, mordants form a stable attachment, known as a coordination complex, which enhances the dye's properties.
• Mordants often alter the final color, allowing dyers to achieve a wide range of hues.

Common mordants include metallic salts such as alum, iron, and copper. Alum, in particular, is favored for its ability to produce clear and vivid colors without significantly altering the aesthetic characteristics of the fabric.

Coordination Complex

In chemistry, a coordination complex is a structure consisting of a central metal atom or ion bonded to surrounding molecules or ions known as ligands. This concept is important in the textile dyeing process as it explains how dyes stay bonded to fabric fibers.

Here's why coordination complexes matter in dyeing:

• The central metal ion (like those found in mordants such as alum) acts as an anchor, holding dye molecules tightly to the fabric.
• This **stable attachment** ensures that the dye resists washing and light exposure, maintaining the fabric's color over time.
• Coordination complexes can modify the absorption of light by the dye, affecting the brilliance and hue of the color.

Understanding coordination complexes helps dyers make informed decisions about **which mordants** and dyes to use to achieve desired textile properties.

Alum

Alum is a popular substance used in various industries, but it plays a particularly significant role in textile dyeing as a mordant. Chemically, alum refers to a group of compounds, but the most commonly used type in dyeing is potassium aluminum sulfate.

Here's how alum is used in dyeing:

• It is dissolved in water to create a mordant bath where the fabric is pre-treated before dyeing.
• Alum helps to fix the dye onto the fibers by forming a coordination complex, ensuring the color is more permanent.
• As a versatile mordant, it enhances the **dye uptake** resulting in richer and more vibrant hues.

Aside from textile dyeing, alum is also used in water purification processes and has applications in other fields like the food industry. However, its key role in dyeing remains due to its efficiency and availability, making it invaluable to fabric dyers aiming for quality and longevity in color.