Indigo is applied to cotton as a vat dye. Which statement describes this process?
Correct answer A. It is reduced to a soluble leuco form, absorbed by the fibre, then oxidised back in air
Explanation
Indigo is insoluble in water. In the vat it is reduced in alkali to the soluble, almost colourless leuco form, which soaks into the fibre. Exposure to air oxidises it back to insoluble blue indigo inside the fibre.
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