Question

Identify the effect of dry Hydrogen chloride gas on litmus paper:

Answer 1

Litmus paper:

• Litmus paper is a paper which has a particular mixture of dyes absorbed in it.
• Litmus paper is used as an indicator of the pH of the solution because it will change colour 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 an acidic solution.
• The Hydroxide ions or protons react with the dye present in the litmus paper in the aqueous medium to give a different dye which shows the colour change.
• Hence, the presence of water is necessary as a solvent in order to let litmus paperwork, If there is no water or aqueous medium, then the litmus paper won't work.

Effect of dry Hydrogen chloride$\left(\mathrm{HCl}\right)$ gas on litmus paper:

Aqueous Hydrogen chloride gas(Hydrochloric acid):

• In $\mathrm{HCl}$ solution, water is present as a solvent in less or more amounts.
• When the litmus paper is dipped in Hydrochloric acid, since the acid has some water in its composition the litmus paper gives red colour (This is because the dry $\mathrm{HCl}$ gas gives protons by reacting with water present on the paper and hence the reaction occurs).

Dry hydrogen chloride gas:

• In the case of dry $\mathrm{HCl}$ gas, there is no presence of water, so the dye on litmus paper cannot react with the gas.
• Therefore, dry gas will not show any change in the colour of litmus paper.

Therefore, the effect of dry Hydrogen chloride $\left(\mathrm{HCl}\right)$ gas will not show any change in the colour of litmus paper.