(i) Dry $\mathrm{HCl}$ gas does not contain any hydrogen ions in it, so it does not show acidic behaviour. Dry $\mathrm{HCl}$ gas does not change the colour of dry blue litmus paper because it has no hydrogen ions $\left[{\mathrm{H}}^{+}\left(\mathrm{aq}\right)\right]$ in it. However, when $\mathrm{HCl}$ gas dissolves in water, it forms hydrogen ions and hence shows acidic behaviour.
(ii) The aqueous solutions of glucose and alcohol do not show acidic character because their hydrogen does not separate out as hydrogen ions $[{\mathrm{H}}^{+}\left(\mathrm{aq}\right).$ ions] on dissolving in water. Hence, they do not conduct electricity.
(iii) An acid is substance which dissociates on dissolving in water to produce hydrogen ions $[{\mathrm{H}}^{+}\left(\mathrm{aq}\right).$ ions]. Hydrogen ions do not exist as ${\mathrm{H}}^{+}$ions in a solution, they attach themselves to the polar water molecules to form hydronium ions, ${\mathrm{H}}_3{\mathrm{O}}^{+}$.
${\mathrm{H}}^{+}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_3{\mathrm{O}}^{+}$
When a solution of an acid is diluted, the number of $\left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right]$ions per unit volume decreases. Thus, $\mathrm{pH}$ of the solution increases.