(i) Iron react with steam to form the metal oxide and hydrogen.
3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)
(ii) The reaction of calcium with water is exothermic but the heat evolved is not sufficient for the hydrogen to catch fire.
Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g)
Calcium starts floating because the bubbles of hydrogen gas formed stick to the surface of the metal. Potassium react violently with cold water and its reaction is so violent and exothermic that the evolved hydrogen immediately catches fire.
2K(s) + 2H2O(l) → 2KOH(aq) + H2(g) + heat energy