A compound ' X ' is a weak acid and it exhibits colour change at pH close to the equivalence point…

phenolphthalein, $\left({\mathrm{C}}_{20}{\mathrm{H}}_{14}{\mathrm{O}}_4\right)$, an organic compound of the phthalein family that is widely employed as an acid-base indicator. As an indicator of a solution's pH, phenolphthalein is colourless below pH 8.5 and attains a pink to deep red hue above pH 9.0.

It is used for the titration of weak acid and strong base. Phenolphthalein is weak acid give colour in basic medium. It exist in ionized form in basic medium.