mixtures. Reactions Urea is basic. As such it is protonates readily. It is also a Lewis base forming complexes of the type . Molten urea decomposes into ammonia gas and isocyanic acid: Via isocyanic acid, heating urea converts to a range of condensation products including biuret , triuret , guanidine , and melamine: Urea reacts with malonic esters to make barbituric acids. Stability In the presence of heat, urea breaks down to form isocyanate. In aqueous solution, urea slowly equilibrates with ammonium cyanate. This hydrolysis cogenerates isocyanic acid, which can carbamylate proteins, in particular the N-terminal amino group and the side chain amino of lysine, and to a lesser extent the side chains of arginine and cysteine. Each carbamylation event adds 43 daltons to the mass of the protein, which can be observed in protein mass spectrometery. For this reason, pure urea solutions should be freshly prepared and used, as aged solutions may develop a significant concentration of cyanate (20 mM in 8 M urea). Dissolving urea in ultrapure water followed by removing ions (i.e. cyanate) with a mixed-bed ion-exchange resin and storing that solution at 4 °C is a recommended preparation procedure. However, cyanate will build back up to significant levels within a few days. Alternatively, adding 25–50 mM ammonium chloride to a concentrated urea solution decreases formation of cyanate because of the common ion effect. Related compounds Ureas describes a class of chemical compounds that share the same functional group, a carbonyl group attached to two organic amine residues: , where groups are hydrogen (–H), organyl or other groups. Examples include carbamide peroxide, allantoin, and hydantoin. Ureas are closely related to biurets and related in structure to amides, carbamates, carbodiimides, and thiocarbamides. Uses Agriculture More than 90% of world industrial production of urea is destined for use as a nitrogen-release fertilizer. Urea has the highest nitrogen content of all solid nitrogenous fertilizers in common use. Therefore, it has a low transportation cost per unit of nitrogen nutrient. The most common impurity of synthetic urea is biuret, which impairs plant growth. Urea breaks down in the soil to give ammonium ions (). The ammonium is taken up by the plant through its roots. In some soils, the ammonium is oxidized by bacteria to give nitrate (), which is also a nitrogen-rich plant nutrient. The loss of nitrogenous compounds to the atmosphere and runoff is wasteful and environmentally damaging so