An auxochrome is which of the following?

• A. An ionizing group present in dyes
• B. A colorless solvent
• C. A mordant
• D. A base

Answer: (A)

Auxochromes are substituents on a chromophore that can donate or withdraw electrons and often ionize, extending conjugation and changing the pigment’s light absorption. In dyes, these groups (for example amino or hydroxyl groups) attach to the dye molecule and become charged in solution, which stabilizes the excited state and shifts absorption toward longer wavelengths, intensifying the color. That’s why an ionizing group present in dyes best describes an auxochrome. A colorless solvent, a mordant, or a general base do not capture this defining role of a substituent that modulates the dye’s absorption spectrum.