Hydrogen

Hydrogen and nitrogen gases can react to form ammonia:

This reaction, called the Haber process, is used to manufacture millions of tons of ammonia every year in the United States alone, mostly for use as fertilizer. The Haber process converts nitrogen from the air, which plants cannot use, into a form (ammonia) that they can use. In order to get the biggest yield of ammonia, the reaction has to be carried out at a high pressure (500 times normal atmospheric pressure) and a high temperature (842°F [450°C ]). To make it go faster, a catalyst is also used. More than two-thirds of all the hydrogen produced in the world goes into making ammonia.

A lot of hydrogen is used to make methyl alcohol—about 4 million tons of it a year in the U.S.:

Methyl alcohol is a flammable, poisonous liquid that is used as a solvent and in the manufacture of paints, cements, inks, varnishes, paint strippers, and many other products. It is what burns in the camping fuel, Sterno.

Another major use of hydrogen is in the hydrogenation of unsaturated fats and oils. If the molecules of a fat contain some double bonds between adjacent carbon atoms, as most animal fats do, they are said to be unsaturated. Treating them with hydrogen gas "fills up" or saturates the double bonds: the hydrogen atoms add themselves to the molecules at the double bonds, converting them into single bonds. Saturated (all-single-bond) fats have higher melting points; they're not as soft, they're more stable, and they stand up to heat better in frying. That's why "hydrogenated vegetable oil" on many food labels. Saturated fats raise people's blood cholesterol and increase the risk of heart disease.

In the oxyhydrogen torch, the potentially violent reaction between hydrogen and oxygen is controlled by feeding the gases gradually to each other, thereby turning a potential explosion into a mere combustion. The resulting flame is extremely hot and is used in welding.