Study of Ammonia

Ammonia is a colourless gas with a sharp, choking smell, made of one nitrogen atom joined to three hydrogen atoms (NH3). It is lighter than air and extremely soluble in water. When it dissolves it forms ammonium hydroxide (NH4OH), a weak alkali, which is why ammonia is the only common gas that turns moist red litmus blue. This single fact - that it is the only alkaline gas - is the basis of the test for ammonia in the board exam.

Ammonia is prepared by heating a mixture of ammonium chloride and slaked lime (calcium hydroxide): 2NH4Cl + Ca(OH)2 -> CaCl2 + 2H2O + 2NH3. The tube is slanted downwards at the mouth so that the water formed does not run back and crack the hot glass. The damp gas is dried by passing it up a tower packed with lumps of quicklime (CaO). It cannot be dried with conc. sulphuric acid, calcium chloride or phosphorus pentoxide, because all of these are acidic or react with the basic gas. It is collected by downward displacement of air, never over water, because it is far too soluble.

Ammonia is one of the most soluble gases known - one volume of water dissolves over 700 volumes of the gas. This huge solubility is shown by the fountain experiment: a flask full of dry ammonia is fitted with a jet, and a drop of water is let in. The gas rushes into the water, the pressure inside the flask falls suddenly, and the outside air pressure pushes water up through the jet as a fountain. If the water contains red litmus, the fountain turns blue, proving the gas was both very soluble and alkaline.

As a base, ammonia neutralises acids. With hydrogen chloride gas it gives dense white fumes of ammonium chloride: NH3 + HCl -> NH4Cl. Its solution (NH4OH) precipitates metal hydroxides - for example it gives a pale-blue precipitate with copper sulphate, which dissolves in excess ammonia to a beautiful deep-blue solution of tetraamminecopper(II) sulphate. Ammonia is made on a huge scale by the Haber process, where nitrogen and hydrogen combine (1:3 by volume) over a finely divided iron catalyst, and is used to make nitric acid and fertilisers.