Free Dyes MCQs with Answers

1. A dye differs from a pigment in that a dye

• A. is always black
• B. is insoluble and sits on the surface as fine particles
• C. is soluble in the medium and becomes attached to the fibre
• D. contains no colour at all

Explanation: A dye dissolves and bonds to the fibre by ionic, hydrogen or covalent bonds, so the colour becomes part of the material and resists washing, whereas a pigment is insoluble and must be held in place by a binder as in paint. This is why a dyed cloth keeps its colour through the wash while a painted surface can flake. Fastness to washing and to light is the main measure of dye quality.

2. The part of a dye molecule responsible for its colour is called the

• A. auxochrome
• B. chromophore
• C. monomer
• D. catalyst

Explanation: A chromophore is an unsaturated group such as the azo linkage or a nitro group whose delocalised electrons absorb visible light, and the wavelength absorbed determines the colour seen. An auxochrome such as the hydroxyl or amino group deepens the colour and, more importantly, helps the dye bind to the fibre. A molecule needs both to be a useful dye.

3. Azo dyes contain the group

• A. N=N joining two aromatic rings
• B. C=O
• C. OH only
• D. COOH

Explanation: The azo group links two aromatic systems into one extended conjugated network, and the resulting absorption in the visible region makes these compounds strongly coloured, which is why azo dyes are the largest commercial class. They are made by coupling a diazonium salt with a phenol or an aromatic amine. Varying the rings and their substituents produces the whole range of shades.

4. A mordant is used in dyeing to

• A. bleach the fabric before dyeing
• B. dissolve the dye in water
• C. remove excess dye after the process
• D. fix a dye to a fibre it would not otherwise bind to, usually by forming a metal complex

Explanation: A mordant, commonly a salt of aluminium, chromium or iron, is absorbed by the fibre first and then forms a coordination complex with the dye, anchoring it to the cloth. The metal used affects the final shade, so one dye with different mordants gives different colours. Direct dyes need no mordant because they bind to the fibre unaided.