The UV spectrum of p-nitrophenol in aqueous solution is shown in Fig. P18.59 (spectrum A). When a few drops of concentrated NaOH are added, the solution turns yellow and the spectrum changes (spectrum B). On addition of a few drops of concentrated acid, the color disappears and spectrum A is restored. Explain these observations.

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absorbance 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 200 spectrum A 280 spectrum B 360 440 wavelength (A), nm 520 600 Figure P18.59 UV spectra for Problem 18.59. Spectrum A was taken in the presence of acid; spec- trum B was taken in the presence of NaOH.