Write equations for the reactions of calcium and potassium with water
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) + Heat
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