Suppose you have four test tubes labelled as ‘A’, .‘B’, ‘C’ and T) ‘A’ contains plain water, ‘B’ contains solution of an alkali, ‘C* contains solution of an acid, and ‘D’ contains solution of sodium chloride. Which one of these solutions will turn phenolphthalein solution pink ?

Phenolphthalein is a widely used acid-base indicator that exhibits distinct color changes based on the pH of a solution. It remains colorless in acidic and neutral environments. Specifically, it is colorless when the pH is below approximately 8.2. When the pH exceeds this threshold, indicating a basic or alkaline environment, the molecule undergoes a structural change to form a pink or fuchsia-colored dianion. In the given scenario, Solution ‘A’ (plain water) and Solution ‘D’ (sodium chloride) are neutral, while Solution ‘C’ is acidic; therefore, phenolphthalein will remain colorless in all three. Only Solution ‘B’, which contains an alkali (a base), will provide the necessary alkaline environment (pH > 8.2) to turn the phenolphthalein solution pink.