Question

Dry \[{\text{HCl}}\] gas changes the colour of the dry litmus paper.
A) True
B) False

Answer 1

Hint:Dry litmus paper shows colour change in presence of \[{H^ + }\] ions. Dry \[{\text{HCl}}\] gas given is dry and one needs to be careful about the colour change as there is a need for the ions to react and choose the correct choice accordingly.

Complete step by step answer:
1) The dry \[{\text{HCl}}\] gas is neutral in nature, does not show acidic property due to the absence of hydronium ions as it does not dissociate and liberate \[{H^ + }\] and \[C{l^{^ - }}\] ions.
2) So, when it contacts with dry litmus paper the colour of the paper remains unchanged. As the gas dissociates in moist or aqueous conditions and liberates \[{H^ + }\] ions, but in the given case neither the litmus paper is wet nor the gas is moist.
3) Dry \[{\text{HCl}}\] gas does not dissociate due to the strong formation of the covalent bond between hydrogen and chloride ions.
4) When \[{\text{HCl}}\] reacts with litmus paper is in moist condition then the colour of litmus paper changes to a red colour. Because when moist gas reacts with litmus paper, it becomes acidic, tuning the paper red.
5) The \[{H^ + }\]ions change \[{\text{pH}}\]of the solution, which makes solution acidic means reduces \[{\text{pH}}\] of the solution, which is responsible for paper colour change. \[{\text{pH}}\] is the concentration of hydrogen ions in a substance. Dilute \[{\text{HCl}}\] is less acidic hence will have a higher pH than concentrated Hydrochloric acid.
Therefore, the statement Dry \[{\text{HCl}}\] gas changes the colour of the dry litmus paper is false which shows option B as the correct choice.

Note:
Moist litmus paper used to test acidity or alkalinity of a solution. Litmus indicator turns the solution red in acidic condition, blue in alkaline condition, and purple colour in neutral condition. However, Litmus paper does not provide exact information about the strength of an acidic or alkaline solution.