Q.Write any three characteristics of dyes.

Dyes — Classification and Structure

A dye is a coloured organic compound that binds firmly to a substrate (cloth, paper, hair) and does not wash out easily. Colour arises because the molecule absorbs certain wavelengths of visible light; the light-absorbing unsaturated group responsible is called a chromophore (e.g. $-N=N-$ azo, $>C=O$ carbonyl, $-NO_2$ nitro, quinonoid rings). Groups like $-OH$, $-NH_2$, $-NR_2$ and $-SO_3H$ are auxochromes — they deepen/intensify the colour and, being acidic or basic, help the dye fix onto the fabric.

Dyes are classified by application (acid, basic, direct, vat, mordant, disperse) or by structure (azo, triphenylmethane, anthraquinone, indigoid, phthalein).

Azo dyes contain the $-N=N-$ link and are made by coupling a diazonium salt with a phenol or amine. For example, benzene diazonium chloride couples with $\beta$-naphthol to give a bright orange azo dye:

$C_6H_5N_2^+Cl^- + \text{2-naphthol} \rightarrow C_6H_5-N=N-C_{10}H_6OH$ …