(a) Iron (II) oxide (FeO), as well as iron (III) oxide (Fe2O3), are oxides of iron. When iron reacts with steam, both oxides are formed and the product is in a combined form (FeO.Fe₂O₃ or Fe₃O₄).

3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)

(b) Calcium and potassium are two highly reactive metals in the reactivity series.

Potassium reacts very violently with water, and hydrogen is released. The reaction is so exothermic that the evolved hydrogen catches fire immediately.

2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g) + heat energy

The reaction of calcium with water is not as violent as the above reaction. The heat evolved is not sufficient for the hydrogen to catch fire. Also, calcium starts floating because the bubbles of hydrogen gas formed stick to the surface of calcium.

Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)