CBSE Class 10 Science 2020 · Paper 31/2/1 · Question 16(b)

(i) Labelled diagram: solid sodium chloride and concentrated sulphuric acid in a test tube closed by a cork; a delivery tube passes through the cork and carries the hydrogen chloride gas out, through a guard tube of anhydrous calcium chloride that dries it. $\text{NaCl}(s)+{\text{H}}_2{\text{SO}}_4(\text{conc.})⟶{\text{NaHSO}}_4+\text{HCl}(g)$ (ii) The colour changes only in the second case: wet blue litmus paper turns red, while dry litmus paper shows no change. (iii) Dry HCl gas shows no acidic character, because without water it does not form ions. In water (HCl solution) it gives ${\text{H}}^{+}$ ions, present as hydronium ions, ${\text{H}}_3{\text{O}}^{+}$: $\text{HCl}+{\text{H}}_2\text{O}⟶{\text{H}}_3{\text{O}}^{+}+{\text{Cl}}^{-}$. These ions give HCl solution its acidic character.