Alkaline metals are the most reactive of all metals. They react vigorously with water to produce hydrogen gas and a metal hydroxide. For example, when sodium metal is dropped into water, it reacts to form sodium hydroxide and hydrogen gas:

$$2Na(s) + 2H_2O(l) → 2NaOH(aq) + H_2(g)$$

This reaction is so vigorous that it can cause the water to boil and splatter.

The high reactivity of alkaline metals is due to their low ionization energy. This means that it is easy for these metals to lose electrons and form positive ions. When an alkaline metal reacts with water, the metal atoms lose electrons to the water molecules, which become hydroxide ions. The hydrogen atoms in the water molecules gain electrons and become hydrogen gas.

The high reactivity of alkaline metals also makes them very dangerous. These metals can react explosively with air and other substances, so they must be handled with care.