Question

Dry hydrogen chloride gas does not turn blue litmus paper red whereas hydrochloric acid does. Why?

Answer 1

HCl is naturally a gas. When we make its solution, water is always present as a solvent in less or more amounts. Dry hydrogen chloride does not have any moisture in its composition.

Complete step by step answer:
Let’s see what litmus paper is and how it works in order to find out which compounds will affect its color.
- Litmus paper is a paper which has certain dyes absorbed in it. These dyes are extracted from lichens. Actually, it is a mixture of dyes. So, this paper can be used as an indicator for the pH of the solution. It changes its colours depending upon the nature of the solution.
- Litmus paper turns blue in colour when dipped in a basic solution and turns red when dipped in acidic solution. It shows violet colour when dipped in neutral solution.
- So, hydroxide ions or protons react with the dye present in the litmus paper in aqueous medium to give different dye which shows colour change. So, we can say that the presence of water is necessary as a solvent in order to let litmus paper work.
- If there is no water or aqueous medium, then litmus paper will not work.
- So, when we dip litmus paper in hydrochloric acid, actually the acid has some water in its composition and due to this, the litmus paper can work and gives red colour.
- In case of dry HCl gas, there is no presence of water and so, the dye on litmus paper cannot react with the gas. So, any dry gas will not give any change in the colour of litmus paper.
Thus, we can say that because the dye on litmus paper cannot react with the dry HCl gas in absence of water or aqueous medium, it cannot turn the blue litmus paper red.

Note: Note that if we dip the blue litmus paper in water and then allow it to react with dry HCl gas, then it will turn red in colour. This happens because dry HCl gas gives protons by reaction with water present on the paper and hence reaction occurs.