Hydrogen compounds of nitrogen
Ammonia NH₃ is a polar molecule shaped like a triangular pyramid, with an unshared electron pair on nitrogen. It is a colourless, sharp-smelling gas about 1.7 times lighter than air (Mᵣ = 17); about 700 volumes dissolve in 1 volume of water. In the laboratory it is made by adding alkali to an ammonium salt: 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2NH₃↑ + 2H₂O (with heating, teacher only). In industry N₂ + 3H₂ ⇌ 2NH₃ + Q: the pressure is raised, the temperature is moderate (400–500 °C) and the catalyst is iron. In water it gives a weakly alkaline medium: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ (often written NH₄OH, although no such separate substance really exists). With acids it forms salts: NH₃ + HCl → NH₄Cl, the NH₄⁺ ion forming by the donor–acceptor mechanism. Ammonia burns: 4NH₃ + 3O₂ → 2N₂ + 6H₂O, and over a catalyst 4NH₃ + 5O₂ → 4NO + 6H₂O. Ammonium salts dissolve well in water, decompose on heating, and release ammonia with alkali: NH₄⁺ + OH⁻ → NH₃↑ + H₂O — the qualitative test for the ammonium ion. Ammonia is used for fertilizers, nitric acid and refrigeration. It burns eyes and airways: never smell it directly, and never mix it with chlorine bleach (a poisonous gas forms).
“Safety table”: make a table “Ammonia: useful / dangerous / what I do not do / first aid”. Put the rule about never mixing household cleaners in a separate frame. Ammonia solution is not opened or smelled at home; the “fountain” experiment is shown only by the teacher.