Ammonium carbonate decomposes upon heating in aq. solution to produce carbon dioxide and ammonia, the latter gas can be identifed from its characteristic odour and also by using a moist red litmus paper. Red litmus turns to blue when it comes in contact with ammonia. The balanced chemical reaction for the decomposition is:
`(NH4)2CO3 (s) rarr CO2 (g) + H2O (l) + 2NH3 (g)`
Sodium carbonate is thermally stable upto a very high temperature. Therefore, this experiment can be utilised to distinguish between aqueous solutions of sodium carbonate and ammonium carbonate.
Heat both the test tubes containing abovementioned solutions and place a piece of moist red litmus paper at the mouth of both. The litmus paper which turns blue indicates the presence of ammonium carbonate in the corresponding test tube.