Question

The auxochrome part of a dye:
A. is the colour producing group of the dye chemical
B. influences the intensity of the dye
C. acts as a mordant
D. none of the above

Answer 1

Hint: Think about the meanings of the components of the word auxochrome and also how an auxochrome is present along with the chromophore to produce colour.

Complete step by step solution:
The word auxochrome is derived from the Greek components ‘auxano-’ which means increase or amplify, and ‘-chrome’ which means colour. This means that the auxochrome group amplifies the original colour of the dye.
The auxochrome by itself does not produce any colour, but when the group is present in conjugation with a chromophore, it augments the colour that is produced by the chromophore.
Some examples of auxochromes are the hydroxyl group ($-OH$), amino group ($-N{{H}_{2}}$), aldehydic group ($-CHO$), and the methyl mercaptan group ($-SC{{H}_{3}}$). When these groups are present along with chromophoric groups like azo group ($R-N=N-R'$), keto group ($R-(CO)-R'$), nitro group ($-N{{O}_{2}}$), nitroso group ($-NO$), etc.
Now, let us look at an example for further clarity:
Benzene on its own does not have any colour, but when a nitro group is attached to the benzene ring to form nitrobenzene it obtains a pale yellow colour. So, we can say that the nitro group acts as a chromophore. Now, if we consider the compound p-hydroxynitrobenzene we can see that it exhibits a deep yellow colour. So here the hydroxyl group acts as an auxochrome.

Hence, the correct answer to this question is ‘B. influences the intensity of the dye’

Note: A mordant is a substance that helps a dye bind to the surface of any material such that it is not washed away by the action of acids, bases, or water. Some dyes act as mordants themselves but some dyes need added mordants for binding. These mordants are independent of auxochromes.