Question

Write equations for the reactions of calcium and potassium with water.

Answer 1

The reaction of calcium with water is exothermic, but the heat that evolved is not sufficient for the hydrogen to catch fire. 

\[\ce{Ca_{(s)} + 2H2O_{(l)} -> Ca(OH)2_{(aq)} + H2_{(g)}}\]

Calcium starts floating because bubbles of hydrogen gas form and stick to the surface of the metal.
Potassium reacts violently with cold water, and its reaction is so violent and exothermic that the evolved hydrogen immediately catches fire.

\[\ce{2K_{(s)} + 2H2O_{(l)} -> 2KOH_{(aq)} + H2_{(g)}}\]