A compound $\text{'}\mathrm{X}\text{'}$ when treated with phthalic anhydride in presence of concentrated ${\mathrm{H}}_2{\mathrm{SO}}_4$ yields $\text{'}\mathrm{Y}\text{'}$. $\text{'}\mathrm{Y}\text{'}$ is used as an acid/base indicator. $\text{'}\mathrm{X}\text{'}$ and $\text{'}\mathrm{Y}\text{'}$ are respectively

Carbolic acid or phenol reacts with phthalic anhydride in presence of conc. ${\mathrm{H}}_2{\mathrm{SO}}_4$ to form phenolphthalein which is used as an acid base indicator. The type of reaction involved is electrophilic substitution reaction. Phenol is ortho-para directing group. Hence, substitution occurs at para position.

The phenolphthalein is colourless below pH 8.5 and attains a pink to deep red hue above pH 9.0.