Question

A gas is produced when concentrated ${\mathrm{H}}_2{\mathrm{SO}}_4$ is added to a solid sodium chloride taken in a test tube and the tube is heated. The gas coming through the delivery tube is passed over a dry litmus paper and then over a moist litmus paper. What would you observe? Explain your answer.

Answer 1

The reaction of concentrated ${\mathrm{H}}_2{\mathrm{SO}}_4$ with Sodium Chloride (NaCl)

Sodium chloride $\left(\mathrm{NaCl}\right)$ reacts with concentrated Sulfuric acid $\left({\mathrm{H}}_2{\mathrm{SO}}_4\right)$ to form Sodium hydrogen sulfate $\left({\mathrm{NaHSO}}_4\right)$ and Hydrochloric acid gas $\left(\mathrm{HCl}\right)$.

$\mathrm{NaCl}\left(\mathrm{s}\right)+{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)\to {\mathrm{NaHSO}}_4\left(\mathrm{aq}\right)+\mathrm{HCl}\left(\mathrm{g}\right)$

1. We know that the litmus paper is used as an indicator for the basic and acidic solution.
2. If the solution is basic, it turns red to blue color.
3. If the solution is acidic, it turns blue to red.
4. For a neutral solution, the color does not change.
5. On dry litmus paper, Hydrochloric acid $\left(\mathrm{HCl}\right)$ gas is unable to produce ${\mathrm{H}}^{+}$ ions therefore nothing happens to the litmus paper.
6. On wet litmus paper, $\mathrm{HCl}$ gas mixes with the water to produce ${\mathrm{H}}^{+}$ ions and hence, turns litmus paper red.

Therefore, in this reaction $\mathrm{HCl}$ gas doesn't affect dry litmus paper but changes litmus paper to red.