Hydrogen has a chemical formula of H₂ and a molecular weight of 2.01588. It is extremely flammable under normal temperature and pressure. Colorless, transparent, odorless, tasteless and insoluble in water gas. Hydrogen is the least dense gas known in the world. The density of hydrogen is only 1/14 that of air, that is, at 1 standard atmosphere and 0°C, the density of hydrogen is 0.089g/L. Therefore, hydrogen can be used as the filling gas for airships and hydrogen balloons (due to its flammability and low safety, airships are now mostly filled with helium). Hydrogen is the substance with the smallest relative molecular mass and has strong reducibility. It is often used as a reducing agent to participate in chemical reactions.

Hydrogen (H2) was first produced artificially in the early 16th century, when the method used at that time was to put the metal in a strong acid. From 1766 to 1781, Henry Cavendish discovered the element of hydrogen. The combustion of hydrogen produces water (2H₂+O₂ ignition = 2H₂O). Based on this property, Lavoisier named the element “hydrogenium” (“water-forming substance”). Meaning, "hydro" means "water", "gen" means "generate", and "ium" is the general suffix of the element). In the 1850s, when the British doctor B. Hobson wrote the "New Edition of Natural History" (1855), he translated "hydrogen" into "light gas", meaning the lightest gas.

In the industry, hydrogen is generally produced from natural gas or water gas instead of the high-energy-consuming electrolysis of water. The hydrogen produced is used in cracking reactions and ammonia production in the petrochemical industry. Hydrogen molecules can enter the crystal lattice of many metals, causing "hydrogen embrittlement", making hydrogen storage tanks and pipelines need to use special materials (such as Monel), and the design is more complicated.