CBSE Class 10 Science 2018 · Paper 31/1/C · Question 7(a)

The litmus paper changes colour only in the second case: the wet blue litmus paper turns red, while the dry blue litmus paper shows no change. Reason: the gas evolved is hydrogen chloride. Dry $\text{HCl}$ gas does not give ${\text{H}}^{+}$ ions; in the water on the wet paper it ionises and gives ${\text{H}}^{+}$ ions (as ${\text{H}}_3{\text{O}}^{+}$), which turn blue litmus red. Inference: the evolved gas ($\text{HCl}$) is acidic in nature, but it shows its acidic behaviour, forming ${\text{H}}^{+}$ ions, only in the presence of water. Equations: $\text{NaCl}+{\text{H}}_2{\text{SO}}_4\to {\text{NaHSO}}_4+\text{HCl}$ (on stronger heating, $2\text{NaCl}+{\text{H}}_2{\text{SO}}_4\to {\text{Na}}_2{\text{SO}}_4+2\text{HCl}$); $\text{HCl}+{\text{H}}_2\text{O}\to {\text{H}}_3{\text{O}}^{+}+{\text{Cl}}^{-}$.