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a    a flammable colourless gas that is the lightest and most abundant element in the universe. It occurs mainly in water and in most organic compounds and is used in the production of ammonia and other chemicals, in the hydrogenation of fats and oils, and in welding. Symbol: H; atomic no.: 1; atomic wt.: 1.00794; valency: 1; density: 0.08988 kg/m3; melting pt.: --259.34°C; boiling pt.: --252.87°C  
   See also       deuterium       tritium  
b    (as modifier)  
hydrogen bomb     
     (C18: from French hydrogène, from hydro- + -gen; so called because its combustion produces water)  

heavy hydrogen  
      n      another name for       deuterium  
hydrogen bomb  
      n   a type of bomb in which energy is released by fusion of hydrogen nuclei to give helium nuclei. The energy required to initiate the fusion is provided by the detonation of an atomic bomb, which is surrounded by a hydrogen-containing substance such as lithium deuteride,   (Also called)    H-bomb      See also       fusion bomb  
hydrogen bond  
      n   a weak chemical bond between an electronegative atom, such as fluorine, oxygen, or nitrogen, and a hydrogen atom bound to another electronegative atom. Hydrogen bonds are responsible for the properties of water and many biological molecules  
hydrogen bromide  
1    a colourless pungent gas used in organic synthesis. Formula: HBr  
2    an aqueous solution of hydrogen bromide; hydrobromic acid  
hydrogen carbonate  
      n      another name for       bicarbonate  
hydrogen chloride  
1    a colourless pungent corrosive gas obtained by the action of sulphuric acid on sodium chloride: used in making vinyl chloride and other organic chemicals. Formula: HCl  
2    an aqueous solution of hydrogen chloride; hydrochloric acid  
hydrogen cyanide  
      n   a colourless poisonous liquid with a faint odour of bitter almonds, usually made by a catalysed reaction between ammonia, oxygen, and methane. It forms prussic acid in aqueous solution and is used for making plastics and dyes and as a war gas. Formula: HCN,   (Also called)    hydrocyanic acid  
hydrogen embrittlement  
      n     (Engineering)   the weakening of metal by the sorption of hydrogen during a pickling process, such as that used in plating  
hydrogen fluoride  
1    a colourless poisonous corrosive gas or liquid made by reaction between calcium fluoride and sulphuric acid: used as a fluorinating agent and catalyst. Formula: HF  
2    an aqueous solution of hydrogen fluoride; hydrofluoric acid  
hydrogen iodide  
1    a colourless poisonous corrosive gas obtained by a catalysed reaction between hydrogen and iodine vapour: used in making iodides. Formula: HI  
2    an aqueous solution of this gas; hydriodic acid  
hydrogen ion  
1    an ionized hydrogen atom, occurring in plasmas and in aqueous solutions of acids, in which it is solvated by one or more water molecules; proton. Formula: H+  
2    an ionized hydrogen molecule; hydrogen molecular ion. Formula: H2+  
hydrogen peroxide  
      n   a colourless oily unstable liquid, usually used in aqueous solution. It is a strong oxidizing agent used as a bleach for textiles, wood pulp, hair, etc., and as an oxidizer in rocket fuels. Formula: H2O2  
hydrogen sulphate  
      n      another name for       bisulphate  
hydrogen sulphide  
      n   a colourless poisonous soluble flammable gas with an odour of rotten eggs: used as a reagent in chemical analysis. Formula: H2S,   (Also called)    sulphuretted hydrogen  
hydrogen sulphite  
      n      another name for       bisulphite  
hydrogen tartrate  
      n      another name for       bitartrate  
potassium hydrogen tartrate  
      n   a colourless or white soluble crystalline salt used in baking powders, soldering fluxes, and laxatives. Formula: KHC4H4O6,   (Also called (not in technical usage))    potassium bitartrate, cream of tartar  
sulphuretted hydrogen  
      n      another name for       hydrogen sulphide  
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