Hydrogen is an odorless, tasteless, highly flammable diatomic gas at normal pressure and temperature. Hydrogen is produced from water by electrolysis, when two electrons combine with two hydrogen ions to create hydrogen gas (H2). The reverse process of hydrogen recombining with oxygen to become water releases a great deal of energy and is therefore the basis of hydrogen car technology, using hydrogen as an energy-storage medium.
Hydrogen is highly flammable. It combines violently with oxygen to form water (H2O). The Hindenburg airship disaster is an example of its flammability; however, the fire was due mostly to the burning of the balloon skin, as most of the hydrogen gas used to provide the lifting force for the airship escaped upward once a tear at the top had opened up.
Hydrogen is highly buoyant, more so than helium. Helium is used to fill balloons, however, because it is not flammable. Because of hydrogen's buoyancy, its presence in the Earth's early atmosphere dissipated with time, as it rose to high altitudes where the solar wind slowly blew it into space.
Hydrogen does not normally conduct electricity. Jim Swenson of Argonne National Lab has pointed out that scientists believe hydrogen at the center of gas giants like Jupiter may be under enough pressure to turn metallic. In that case, hydrogen would be quite conductive.
Hydrogen freezes at about 14 degrees Kelvin and boils at 20 degrees Kelvin. The triple point--the temperature and pressure at which the three phases coexist--is at 13.8 degrees Kelvin and 0.070 atmospheres. The critical point--the temperature and pressure above which multiple phases coexist--is 33 degrees Kelvin and 12.8 atmospheres.
Hydrogen and Metals
Hydrogen diffuses rapidly into many solid metals, even at normal temperatures. Hydrogen can weaken metals and make them more prone to fracture. It also reduces their ductility, that is, their ability to be drawn into thinner strands, like wire, without cracking.
Characteristics of Ionic and Covalent Compounds
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What Are the Characteristics & Uses of Hydrogen?
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Characteristics of Hydrogen Bonding
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The Physical & Chemical Properties of Hydrogen
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What Is Hydrogen?
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