Question

Why is ammonia gas not collected over water?

Answer 1

Collection of ammonia:

• In the laboratory preparation of ammonia, ammonia is collected by downward displacement of air.
• Ammonia gas can not be collected over water because this gas is highly soluble in water and due to its high solubility. We are unable to collect it over water.
• Ammonia on reaction with water gives hydronium ion and ammonium ion.
• Chemical equation: $\underset{\mathrm{Ammonia}}{{\mathrm{NH}}_3\left(\mathrm{g}\right)}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}\to \underset{\mathrm{Ammonia}\mathrm{ion}}{{\mathrm{NH}}_4^{+}\left(\mathrm{aq}\right)}+\underset{\mathrm{Hydroxide}\mathrm{ion}}{{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)}$
• So, ammonia is collected over mercury.

So, this is the reason why we can not collect it over water.