Question

_________ have good affinity to wool and silk under acidic medium.

• A. Acid dyes
• B. Sulphur dyes
• C. Direct dyes
• D. Vat dyes

Hint:

Wool and silk are protein fibres, which take anionic dyes from an acid bath. Think of the dye named after the acid medium.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Silk and wool are protein fibres. Their molecules carry amino groups (-NH2) that take up a positive charge in an acid bath.

Step 2: Key Approach:
A dye with a negative (anionic) charge is pulled towards the positive sites on the fibre. So the dye that works in an acidic medium on protein fibres is the one we need.

Step 3: Check Option (A) Acid dyes.
Acid dyes are anionic dyes applied from an acidic bath. They form ionic bonds with the amino groups of wool and silk. Dyeing is done with acetic acid or sulphuric acid. So (A) is CORRECT.

Step 4: Check Option (B) Sulphur dyes.
Sulphur dyes are used mainly on cotton and other cellulosic fibres. They are applied in an alkaline sodium sulphide bath. So (B) is WRONG.

Step 5: Check Option (C) Direct dyes.
Direct dyes go straight on cellulosic fibres such as cotton, usually with salt in a neutral bath. They give poor wash fastness on wool and silk. So (C) is WRONG.

Step 6: Check Option (D) Vat dyes.
Vat dyes are insoluble in water. They are reduced in a strong alkaline bath and suit cotton. Wool and silk can be damaged by strong alkali. So (D) is WRONG.

Final Answer:
Acid dyes have good affinity to wool and silk under acidic medium. This is option (A).