Question

An aqueous solution turns the red litmus solution blue. Excess addition of which solution would reverse the change-ammonium hydroxide solution or hydrochloric acid?

Answer 1

Hint : This question requires the knowledge of acidic and basic natures of compounds and the characteristics of acidic and basic solutions. The litmus paper test is used to determine whether the given solution is acidic or basic in nature. Acids change blue litmus paper to red, while bases change red litmus paper to blue.

Complete Step By Step Answer:
Litmus paper is a filter paper that has been coated with a natural-soluble dye obtained from lichen. Litmus paper is a type of paper that can be used to determine the pH of a solution. Although litmus paper or red and blue test strips can determine if a solution is acidic or basic, they cannot determine the strength of the solution. On universal, or alkacid, indicator paper, each number on the pH scale is represented by a different colour. When neutral solutions are employed, the paper turns green.
The basic basic ingredients used to manufacture litmus paper are wood cellulose, lichens, and adjunct chemicals. Litmus paper, as the name implies, is mostly made of paper. Litmus paper must be free of contaminants that might affect the pH of the system being evaluated.
In the question, it is clearly indicated that the aqueous solution is basic in nature, since it turns red litmus paper to blue. So, addition of excess acidic solution can reverse the change. Thus, hydrochloric acid should be used since it is acidic in nature, while ammonium hydroxide is basic in nature.

Note :
When exposed to an acid, a purple or neutral litmus paper turns violet to red, and when exposed to an alkaline (or basic) environment, it turns blue. They can detect acids and bases by turning blue or red, depending on whether the item is a base or an acid.