(A) Iron reacts with dil. HCl to produce ferrous chloride and hydrogen.
Fe+2HCl→FeCl2+H2
(B) Zinc reacts with dil sulphuric acid to produce zinc sulphate and hydrogen (lab method).
Zn+ dil. H2SO4→ZnSO4+H2.
However conc. sulphuric acid cannot be used because SO2 and H2O is evolved.
H2SO4+H2→2H2O+SO2
(C) Zinc reacts with NaOH to form sodium zincate and hydrogen.
Zn+2NaOH→Na2ZnO2sodium zincate+H2
(D) During the electrolysis of aqueous NaCl in Nelson's cell, hydrogen gas is obtained. Water dissociates to small extent, H2O⇌H++OH−.
Hydrogen ions are reduced to hydrogen.
2H++2e−→H2.