Question

Which gas turns moist red litmus blue.

Answer 1

Gas which turns moist red litmus blue is Ammonia (NH₃). Gases which are basic in nature turn moist red litmus paper to blue.

• Litmus paper acts like an indicator of the acidic, basic or neutral nature of any substance. Depending on the color of the litmus paper and nature of substance being tested, the change is color tells us about the nature of substance.

Evolution of ammonia gas occurs as follows:

$$\begin{gathered} \mathrm{NH}4\mathrm{Cl}\left(\mathrm{aq}\right)+\mathrm{NaOH}\left(\mathrm{aq}\right)\to \mathrm{NH}3\left(\mathrm{g}\right)+\mathrm{NaCl}\left(\mathrm{aq}\right)+\mathrm{H}2\mathrm{O}\left(\mathrm{l}\right) \\ (\mathrm{Ammonium}(\mathrm{Sodium}\left(\mathrm{Ammonia}\right)(\mathrm{Sodium}(\mathrm{Water}) \\ \mathrm{chloride})\mathrm{hydroxide})\mathrm{chloride}) \end{gathered}$$

Gas released above turns moist red litmus paper blue because ammonia reacts with water (from moist litmus paper) to form Ammonium hydroxide (NH₄OH).

Ammonium hydroxide splits into the ions as shown below.

$$\begin{gathered} {\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)\to {\mathrm{NH}}_4^{+}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right) \\ (\mathrm{Ammonium}\mathrm{hydroxide})(\mathrm{Ammonium}\mathrm{ions})(\mathrm{Hydroxide}\mathrm{ions}) \end{gathered}$$

The hydroxide ions thus released are responsible for the basic nature of ammonia.

• This indicates that the gas is alkaline in nature as bases turn moist red litmus paper blue.
• Gases which are acidic in nature turn moist blue litmus paper to red. Examples: Carbon dioxide (CO₂), Nitrogen dioxide (NO₂), Chlorine (Cl₂), Sulphur dioxide (SO₂), and Hydrogen sulphide (H₂S).
• Two gases which are neutral toward litmus paper are Hydrogen (H₂) and Oxygen (O₂).