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At normal temperature and pressure, hydrogen is a gas that is extremely flammable, colorless, transparent, odorless, odorless, and difficult to dissolve in water. Hydrogen is the smallest known Density gas in the world. The Density of hydrogen is only 1/14 of that of air. That is, at 0 ° C, at a standard atmospheric pressure, the Density of hydrogen is 0.089g / L. Therefore, hydrogen can be used as a filling gas for airships and hydrogen balloons (because hydrogen is flammable and not very safe, airships are now filled with helium). Hydrogen is the substance with the smallest molecular weight and is mainly used as a reducing agent