Hydrogen
a monatomic metallic element, which exists in the state of gas.
Symbol H; at. wt., 1; density, 0.06927; air being 1; weight of a liter of hydrogen, 0.0896 grammes, called a crith. Hydrogen can be obtained by the electrolysis of water, H₂O, the H being liberated at the platinum pole; by the action of metallic sodium and water; also by passing steam over iron filings, 3Fe + 4HOH = Fe3O4 +8H'; by boiling zinc with caustic potash, 2KHO + Zn = K. ZnO2 +2H'; but H is generally made by the action of dilute H2SO4 on zinc. Hydrogen can be purified by passing it through a solution of two parts of K2Cr2O7 in 20 parts of water and one part of H2SO4, then washing it with KHO, and drying it by passing it through concentrated H2SO4.