Chemistry Industrial Chemistry PMDC Conceptual Practice
PMDC Verified Question 12 of 41
Which statement correctly describes the solubility transformation of vat dyes during textile application?
A
Insoluble pigments are reduced to a soluble leuco form, absorbed, and then re-oxidized inside the fiber.
B
Water-soluble salts are applied directly and permanently fixed to the fiber without redox reactions.
C
Insoluble solid particles sublime as a vapor and diffuse into synthetic fibers at elevated temperatures.
D
Cationic dye molecules form coordinate covalent complexes directly with inorganic mordant salts.
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option A: Insoluble pigments are reduced to a soluble leuco form, absorbed, and then re-oxidized inside the fiber.
Concept:

Vat dyes are water-insoluble pigments that require a reversible two-step reduction-oxidation cycle for fabric fixation.

Formula / Reaction:

$$\text{Insoluble Dye (Colored)} \xrightarrow{\text{Reduction (}\text{Na}_2\text{S}_2\text{O}_4\text{/NaOH)}} \text{Soluble Leuco Base} \xrightarrow{\text{Oxidation (Air/}\text{O}_2\text{)}} \text{Insoluble Dye (Trapped)}$$

Solution:

  • Reduction: Sodium dithionite (\(\text{Na}_2\text{S}_2\text{O}_4\)) in alkali reduces insoluble carbonyl groups to soluble enolate (leuco) forms that penetrate cellulose fibers.


  • Oxidation: Exposure to air or oxidants regenerates the original insoluble colored pigment locked within the fiber matrix.


Why other options are incorrect:

  • Option B: Describes direct or reactive dyes, not vat dyes.


  • Option C: Describes disperse dyes applied via high-temperature thermofixation.


  • Option D: Describes mordant dyeing processes.

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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