(physics, astronomy) a spectral line of atomic hydrogen having a wavelength of about 21cm; used extensively in radio astronomy

a hydrogen electrode

(chemistry) the sulfur analogue of hydrogen peroxide, H₂S₂ / (erroneously) hydrogen sulfide

(chemistry, physics) an electrode in which hydrogen gas is adsorbed on platinum

(informal) dihydrogen monoxide (water (H₂O), but not hydrogen peroxide (H₂O₂) or the hydroxyl radical (-OH))

(chemistry) potassium bicarbonate

(inorganic chemistry, chiefly Britain) Alternative spelling of hydrogen sulfide

(inorganic chemistry) The compound sodium hydrogen arsenate.

(chemistry) the hydride of selenium, H₂Se; it is a toxic, inflammable gas, having properties similar to those of hydrogen sulfide

(inorganic chemistry) An inorganic compound with the formula HCl. Forms hydrochloric acid when dissolved in water.