Question

a few drops of phenolphthalein indicator were added to an unknown solution 'A'. it acquired pink colour. now another unknown solution 'B' was added to it drop by drop and the solution become colourless. predict the nature of 'A' and 'B'.

Answer 1

Phenolphthalein is a Synthetic Acid-Base indicators. Which reacts with acid to be colourless and when it reacts with Base it then be of Pink color.

Hence, the given unknown solution A may be BASIC in nature which results to change the color of Phenolphthalein to pink.
While the solution B may be of acidic nature. Since, Original color of Phenolphthalein is also colourless.