Question

An element Z reacts with water to form a solution which turns phenolphthalein solution pink. The element Z is most likely to be:
(A) S
(B) Ca
(C) C
(D) Ag

Answer 1

Hint: Phenolphthalein is a commonly used indicator for indicating the pH of a solution and thus is used in acid- base neutralization titrations. Phenolphthalein has different structures in basic and acidic medium and thus is used as an indicator. We know that phenolphthalein is colorless in acidic solution and pink in basic solution.

Complete step by step solution
We know that phenolphthalein is colorless in acidic solution and pink in basic solution.
Now looking at the given elements, we must determine, which of the following in water would form a basic solution. Both sulphur and carbon are non- metals. Their oxides and hydroxides are acidic in nature. Thus, their solution does not turn phenolphthalein pink. Silver is a metal but the $ Ag - O $ bond has a greater covalent character due to less reactivity of the silver metal. It does not produce an alkaline solution and does not turn phenolphthalein to pink.
Calcium however, is an alkaline earth metal and on reaction with water forms ionic compound calcium hydroxide or $ {\text{Ca}}{\left( {{\text{OH}}} \right)_{\text{2}}} $ . It is also known as slaked lime. It is a strong alkali and releases hydroxide ions in the solution making the solution alkaline and thus, turns phenolphthalein pink in colour.
Thus, the correct answer is B.

Note
Phenolphthalein is a weakly acidic organic compound. So its structure has an acidic hydrogen atom in a neutral and acidic solution. In the basic medium, the hydroxide ions released by the base neutralize the proton and the phenolphthalein structure now becomes anionic. This structure formed is known as the quinoid structure and is purple in colour. Thus, the phenolphthalein molecule which is colorless in acidic solution, turns pink in colour in basic solution.