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The popular plastic polyethylene (polythene) is made from ethene by polymerization. The bond angles in ethene are 120 degrees, due to the three groups of electrons. Ethene is often known as an unsaturated hydrocarbon due to it having a double bond rather than additional hydrogen atoms bonded. The double bond is a region of slightly higher electron density, and so ethene and other alkenes can react with bromine, and halogens to produce bromoalkanes. They can also react with water to produce alcohols but the rate at which this happens is very slow unless a suitable catalst is used. Hydrogen gas (Diatomic hydrogen) can also react to saturate ethene and other unsaturated alkenes. The catalyst for hydrating ethene is typically phosphoric acid adsorbed onto a silica surface, or sulphuric acid. Hydrogenation of ethenes usually occur under high pressure with a finely ground platinum and rhodium catalyst (or the less effective nickel catalyst). Unsaturated fats and vegetable oils are also be saturated using a nickel catalyst.