Suppose you have four test tubes labelled as 'A', 'B', 'C' and 'D'. '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 common acid-base indicator. Its color changes depending on the pH of the solution: - In acidic solutions (pH ~8.2), phenolphthalein turns pink or magenta. Let's analyze the solutions: - Solution 'A': Plain water is generally neutral (pH ≈ 7). Phenolphthalein will be colorless. - Solution 'B': Solution of an alkali is basic. Phenolphthalein will turn pink. - Solution 'C': Solution of an acid is acidic. Phenolphthalein will be colorless. - Solution 'D': Solution of sodium chloride (NaCl) is the salt of a strong acid (HCl) and a strong base (NaOH). A solution of such a salt is neutral (pH ≈ 7). Phenolphthalein will be colorless. Therefore, only the solution of an alkali (Solution 'B') will turn phenolphthalein solution pink.