(a)2 K + H2O = K2O + H2;
(b)Hg + H2O = H2 + HgO;
(c)4 Na + O2 = 2 Na2O;
(d)Ca + H2SO4 = CaSO4 + H2;
(e)2 Ag + 2 HCl ←H2 + 2 AgCl.
