Question

An aqueous solution turns red litmus solution blue. Excess addition of which of the following solutions would reverse the change?
A) Baking Powder
B) Lime
C) Ammonium hydroxide
D) Hydrochloric Acid

Answer 1

Hint:To solve this question, we must first have some knowledge about litmus papers and Red litmus paper specifically. Then we need to assess the properties of red litmus paper and go though each of the provided options to check whether it satisfies the property of Red Litmus or not, and then only we can conclude the correct answer.

Complete answer:
Before we move forward with the solution of this given question, let us first understand some basic concepts:
Litmus Paper: is a filter paper we treat with a natural-soluble dye that the lichens provide us. The piece of paper that gives the result that is referred to as litmus paper can come in use as a $pH$ indicating tool.
Under the acidic conditions, the blue paper turns red where the $pH$ level is below \[4.5\] . While the red paper does not change its color under the alkaline conditions where the $pH$ level is above \[8.3\] .
Red litmus consists of a weak diprotic acid. Upon exposure to base, a reaction takes place between the hydrogen ions and the base. And, it produces a change in color that turns into blue color.
And, blue litmus paper already consists of the blue conjugate base which when reacts with an acid, as a result of which it changes to red color.
Step 1: So, from the above explanation we can conclude that:
Red litmus turns blue, when treated with a base.
Blue litmus turns red, when treated with an acid.
Step 2: Since, Red litmus is turned to blue by the addition of base like sodium hydroxide and for reversing this reaction, turning blue litmus to red requires addition of an acid like hydrochloric acid.
So, clearly we can conclude that the correct answer is Option D.

Note:The litmus paper gives a general indication of acidity or alkalinity to the user because it correlates to the shade of red or blue color that the paper turns to. For testing the $pH$ of a substance, we have to drip a strip of the paper in the solution or use a dropper or a pipette for dropping a small amount of the solution over the paper.