Common hydrogen has a molecular weight of 2,01594 g. As a gas it has a density of 0.071 g/l at 0ºC and 1 atm. Its relative density, compared with that of the air, is 0.0695. Hydrogen is the most flammable of all the known substances. Hydrogen is slightly more soluble in organic solvents than in water. Many metals absorb hydrogen. Hydrogen absorption by steel can result in brittle steel, which leads to fails in the chemical process equipment.
At normal temperature hydrogen is a not very reactive substance, unless it has been activated somehow; for instance, by an appropriate catalyser. At high temperatures it’s highly reactive.
Although in general it’s diatomic, molecular hydrogen dissociates into free atoms at high temperatures. Atomic hydrogen is a powerful reductive agent, even at ambient temperature. It reacts with the oxides and chlorides of many metals, like silver, copper, lead and mercury, to produce free metals. It reduces some salts to their metallic state, like nitrates, nitrites and sodium and potassium cyanide. It reacts with a number of elements, metals and non-metals, to produce hydrides, like NaH, KH, H2S and PH3. Atomic hydrogen produces hydrogen peroxide, H2O2, with oxygen.
Atomic hydrogen reacts with organic compounds to form a complex mixture of products; with etilene, C2H4, for instance, the products are ethane, C2H6, and butane, C4H10. The heat released when the hydrogen atoms recombine to form the hydrogen molecules is used to obtain high temperatures in atomic hydrogen welding.
Hydrogen reacts with oxygen to form water and this reaction is extraordinarily slow at ambient temperature; but if it’s accelerated by a catalyser, like platinum, or an electric spark, it’s made with explosive violence.