Question

Indigo is an example of:

• A. \( \text{Azoic dyes} \)
• B. \( \text{Vat dyes} \)
• C. \( \text{Reactive dyes} \)
• D. \( \text{Acid dyes} \)

Hint:

The "magical" moment when a green-looking fabric is pulled out of an indigo vat and turns blue in the air is the process of Oxidation.

Answer 1

The Correct Option is B

Concept: Dyes are categorized by their application method and chemical class. Vat dyes are ancient dyes known for being insoluble in water.

Step 1: Define Vat Dyes.
Indigo is the most famous example of a Vat dye. These dyes are naturally insoluble. To dye fabric, they must be "reduced" in a large "vat" using a chemical like sodium hydrosulphite to become soluble (leuco form). Once the fabric is removed from the vat and exposed to air, the dye "oxidizes" and turns back into its insoluble, permanent blue color inside the fiber.

Step 2: Contrast with other dyes.

• Acid dyes: Used for wool and silk (protein fibers).
• Reactive dyes: Form covalent bonds with cotton.
• Azoic dyes: Formed "inside" the fiber using two separate components.