Question

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): Direct dyes exhibit poor wash fastness. Reason (R): Direct dyes do not react with the fibre structure but rely on dye to fibre associations such as hydrogen bonding and dipole moments to help with colour fastness after dyeing. In the light of the above statements, choose the most appropriate answer from the options given below:

• A. \( \text{Both (A) and (R) are correct and (R) is the correct explanation of (A)} \)
• B. \( \text{Both (A) and (R) are correct but (R) is not the correct explanation of (A)} \)
• C. \( \text{(A) is correct but (R) is not correct} \)
• D. \( \text{(A) is not correct but (R) is correct} \)

Hint:

To improve the wash fastness of direct dyes, "after-treatments" with fixing agents are often used to make the dye molecules larger and more trapped inside the fiber.

Answer 1

The Correct Option is A

Concept: The strength of the bond between a dye molecule and a fiber determines its "wash fastness"—the ability of the color to stay in the fabric during laundering.

Step 1: Analyze Assertion (A).
Direct dyes are water-soluble dyes used primarily for cotton. They are known for having poor wash fastness because the dye molecules are relatively easy to wash out of the fiber when exposed to water and soap.

Step 2: Analyze Reason (R).
Direct dyes are "direct" because they have a natural affinity for cellulose. However, unlike reactive dyes, they do not form a "covalent bond" (a permanent chemical link). Instead, they stay inside the fiber through weaker intermolecular forces like hydrogen bonds and Van der Waals forces. Because these bonds are weak, water can easily break them.

Step 3: Determine the link.
The poor wash fastness (A) is the direct result of the dye relying on weak physical associations rather than strong chemical reactions (R). Thus, (R) explains (A).