The decomposition of acid azides to isocyanates and nitrogen is known as the Curtius rearrangement. The reaction is a preparative method for isocyanates, ureas, amides, and amines. When coupled with a hydrolytic step, the Curtius rearrangement becomes a practical procedure for replacing a carboxyl group by an amino group. The overall process of converting an acid through its azide to an amine is commonly referred to as the Curtius reaction. Acid azides are commonly prepared by treating hydrazides in cold, aqueous solution with nitrous acid. Azides can be rearranged in inert solvents, from which isocyanates can be isolated in the presence of water or alcohol., which will react with the intermediate to form urethans or ureas.