Question

State what do you observe when moist blue litmus paper is placed in a gas jar of Hydrogen chloride.

Answer 1

• The moist blue litmus paper will turn red.
• Hydrogen Chloride gas reacts with water vapours present on blue litmus paper and forms Hydrochloric acid ($\mathbf{HCl}$) which form ${\mathrm{H}}^{+}$ ion and ${\mathrm{Cl}}^{-}$ion on dissociation. ${\mathrm{H}}^{+}$ ion changes blue litmus to red.
• The reaction is shown below.
  $\mathrm{HCl}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to {\mathrm{H}}_3{\mathrm{O}}^{+}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)$
• ${\mathrm{H}}_3{\mathrm{O}}^{+}$ has been produced and ${\mathrm{H}}_3{\mathrm{O}}^{+}$ is acidic in nature.
• Therefore, moist blue litmus paper turns into red when placed in a gas jar of Hydrogen chloride.