Here is a fun video demonstrating the change in reactivity of the alkali metals with water.
What are the main reactions taking place in this video? First, the alkali metal, M, undergoes an exothermic reaction with the water to form an alkali metal hydroxide and hydrogen gas.
2 M(s) + H2O → 2 MOH(aq) + H2 (g)
But the real exciting (i.e., explosive) part comes from the familiar exothermic reaction of hydrogen gas with the oxygen in the air to produce water.
2 H2 (g) + O2 (g) → 2 H2O(g)