Question

In a gas jar containing hydrogen chloride gas, a dry blue litmus paper is dropped. What changes are observed? Now, a blue litmus paper is moistened and dropped into the gas jar. State what is observed and give a reason?

Answer 1

Here in the question, there are two cases, in the first case we have to observe the changes when a dry blue litmus paper is dropped in a gas jar containing hydrogen chloride gas, and in the second case, a blue litmus paper is moistened and dropped into the gas jar.

In the first case:

Dry blue litmus paper will remain unchanged because it will not behave like acid as in the absence of moisture hydrochloric acid (HCl) does not release hydronium [H₃O]⁺ ions.

Chemical reaction involved:

$\mathrm{HCl}\left(\mathrm{l}\right)\stackrel{\mathrm{in}\mathrm{absence}\mathrm{of}\mathrm{water}}{\to }\mathrm{No}\mathrm{reaction}$

In the second case:

It will release hydronium[H₃O]⁺ ion and decreases the pH of the solution which turns blue litmus paper into the red as it shows acidic properties to give a litmus test.

Chemical reaction involved:

$\mathrm{HCl}\left(\mathrm{l}\right)\stackrel{\mathrm{in}\mathrm{presence}\mathrm{of}\mathrm{water}}{\to }{\mathrm{H}}_3{\mathrm{O}}^{+}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)$

Therefore, blue litmus changes to red only in the presence of moisture, and it will remain the same in the absence of moisture.