Question

In a dye identification test, a dyed fabric is immersed in an aqueous alkaline solution of a reducing agent. The colour of the dyed fabric changes and some dye bleeds out in the solution. The correct combination of the fabric and dye is:

• A. Wool and acid dye
• B. Polyester and disperse dye
• C. Cotton and vat dye
• D. Acrylic and basic dye

Hint:

Vat dyes become soluble only in alkaline reducing conditions, forming a leuco compound—this property helps identify them quickly.

Answer 1

The Correct Option is C

The key clue in this question is that the dyed fabric is treated with an alkaline reducing agent, and the dye becomes soluble and begins to bleed out. This behaviour is characteristic of vat dyes. Vat dyes, such as Indigo and anthraquinone dyes, are insoluble in water in their oxidized form but become soluble when reduced in a strongly alkaline medium containing sodium hydrosulphite or similar reducing agents. During this process, the dye converts from its insoluble “vat” form to a soluble leuco form, which can dissolve and even bleed out. Cotton is the primary fibre dyed with vat dyes because vat dyes provide excellent wash fastness, light fastness, and chemical resistance once re-oxidized on the fibre. Polyester with disperse dyes (option B) does not react with alkaline reducing agents because disperse dyes are nonionic and remain unaffected. Wool with acid dye (option A) would not bleed in strong alkaline conditions; instead, wool would get damaged due to alkali attack. Acrylic dyed with basic dye (option D) is also unaffected by reducing alkali solutions, as basic dyes are cationic and bond strongly to acrylic. Therefore, the only correct fabric–dye combination that matches the described reaction is cotton dyed with vat dye.