Question

Three different substances were taken and tested with litmus paper. The results are given below. Based on the results, answer the question.

Substance/ Litmus	X	Y	Z
Red Litmus	Turns blue	No change	No change
Blue Litmus	No change	Turns red	No change

What could be substance Y?
(A) Acid
(B) Base
(C) Water
(D) Salt

Answer 1

Hint: Litmus is an acid- base indicator, i.e. it is used to detect whether a given substance is acidic or basic in nature. Litmus papers are available in either red or blue colors. Both the papers show the same colour for the same solution.

Complete answer
We know that litmus paper is used to check the nature of the given substance. Litmus paper shows red colour when an acid is added and blue colour when a base is added.
If a drop of acid is dropped on a red litmus paper, it remains red in colour while if we add a drop of a base, the colour of the paper changes from red to blue. If we add a drop of acid on blue litmus paper, the blue paper turns red and if a drop of a base is added, the colour of the blue litmus paper remains unchanged. If we add a drop of neutral solution on either red or blue litmus paper, the colour of the indicator remains unchanged.
In the given question, when red litmus is subjected to the substance Y, there is no change in colour but when it is subjected to blue litmus, it turns red. Thus, the substance Y is an acid.
The correct answer is A.

Note:
Red litmus paper is composed of a weak diprotic acid. On exposure to a basic solution, the acid is protonated to react with the base. The conjugate base so formed has a blue colour explaining the change in colour observed in alkaline solution.