Urea or carbamide, to which the formula 0 = C(NH₉)₂ is generally ascribed, is the form in which by far the largest part of the nitrogen is evolved which is consumed as food by animals. It may be directly prepared from urine by evaporation to one-third of its bulk, and addition of nitric or of oxalic acid; the sparingly soluble nitrate or oxalate is precipitated; the salt is purified by recrystallisation from water, and is then mixed with caustic soda and evaporated to dryness. On treatment with alcohol, the urea alone dissolves, and deposits in crystals from a concentrated solution. It is a white, easily soluble substance, with a saline taste. It unites with acids, forming salts ; but as the carbonyl group, CO, has the property of conferring acidity on neighbouring atoms of hydrogen, the basic qualities of only one of the two amido-groups, -NH₂, can display itself; hence the formula of the hydrochloride is CO(NH₂)₀.HCl, and not C0(NH₂)₂.2HC1, as might be expected. It is therefore a mono-acid base.

Urea can also be produced from inorganic sources, and it was the discovery of its synthesis from potassium cyanide by Wohler in 1827 which caused the abandonment of the old view that compounds containing carbon, with the exception of its oxides, belonged to a special class, and could be produced only by the intervention of " life-power." Its production is as follows: Potassium cyanide is heated to redness with lead oxide; KCN 4- PbO = KCNO + Pb. The cyanate, KCNO, is next dissolved and mixed with a solution of ammonium sulphate, and the mixture is evaporated to dryness. It may be supposed that potassium sulphate and ammonium cyanate are first formed: 2KCNO + (NH₄)₂SO₄ = K₂SO₄ + (NH₄)CNO. But the latter compound is unstable, and undergoes change into its isomeride, urea : (NHJCNO = 0=C(NH₂)₂. On treatment with alcohol, insoluble potassium sulphate remains undissolved, while the soluble urea crystallises from the alcohol on evaporation. Urea is also produced when carbonyl chloride or when ethyl carbonate is treated with aqueous ammonia : 0=CC1₂ + 2NH3 = 0=C(NH₀)₉.HC1 + HC1 ; 0=C(OC₂H₅)₂ + 2NH3 = 0=C(NH₂)₂. + 2C₉H₅OH. Lastly, carbamate of ammonium, when heated in a sealed tube, loses water with formation of urea: H₂N-CO-ONH₄ = 0=C(NH₂)₂ + H₂O.

Biuret,-When urea is heated, a body named biuret is formed, with loss of one molecule of ammonia. We are here reminded of the relation between an acid and an anhydro-acid ; this is evident on inspection of the formulae : -

Urea.

Biuret.

Sulphuric acid.

Anhydrosulphuric acid.