Preparation of Dihydrogen :

A. Laboratory Method : 

i. Dihydrogen is prepared in laboratory by the action of dilute hydrochloric acid on zinc granules

ii. Dihydrogen can be prepared by the action of aqueous solution of sodium hydroxide on zinc

                                  

B. Industrial methods 

i. By electrolysis of pure water: Pure water is a poor conductor of electricity. 

     Therefore a dilute aqueous solution of acid or alkali is used to prepare dihydrogen by electrolysis

     For example, electrolysis of 

     dilute aqueous solution of sulphuric acid yields two volumes of hydrogen at cathode

     and one volume of oxygen at anode.

ii. From carbon on hydrocarbon :

    Reaction of steam on hydrocarbon at 1270 Kelvin in presence of nickel catalyst gives water gas 

    It is also produced by coal

Carbon monoxide is a water gas converted into carbon dioxide by reacting with the steam  in presence of iron chromate as a catalyst this reaction is  called "Water-gas shift" reaction