This reaction is also known as hydration. The alkene is heated with steam and passed over a hot phosphoric acid catalyst. An alcohol is formed. This reaction is widely used to produce ethanol from ethene. As with hydrogen halides, there are two possible products. In the mechanism the C=C double bond attacks an H+ ion from the acid catalyst to form a carbocation, a lone pair on the oxygen of a water molecule then attacks the positive carbon, and the protonated alcohol loses an H+ ion, which regenerates the catalyst.