Question

Test the gas evolved first with dry and then with wet litmus paper. In which of the two cases, does the litmus paper show change in color?

Answer 1

Change in color:-

• The litmus paper will show the change in color with wet litmus paper.
• The color of the litmus paper is changed by the hydrogen ions.
• Dry $\mathrm{HCl}$ gas does not contain ${\mathrm{H}}^{+}$ ions. It is only in the aqueous solution that an acid dissociates to give ions.
• Since in this case, neither $\mathrm{HCl}$ is in the aqueous form nor the litmus paper is wet, therefore, the color of the litmus paper does not change.
• In the case of wet litmus paper, the $\mathrm{HCl}$gas gets dissociated into ${\mathrm{H}}^{+}$ions due to the presence of moisture on the litmus paper. Hence, the color of the litmus paper changes.