Alkali Metals in Aqueous Solution and Ammonia
In aqueous solution, alkali metals such as lithium, sodium, and potassium react vigorously with water to liberate hydrogen gas. This is because alkali metals have a strong tendency to lose their outermost electron, forming metal cations and hydroxide ions in the process. The reaction of alkali metals with water is highly exothermic and produces a lot of heat.

Stability in Ammonia
On the other hand, alkali metals are stable in liquid ammonia. When alkali metals are dissolved in liquid ammonia, they form solvated electrons which are responsible for the stability of the metal-ammonia solutions. The solvated electrons are delocalized throughout the solution, providing a stabilizing effect. This stabilization prevents the violent reaction observed in water, allowing alkali metals to be dissolved and stored in liquid ammonia without the same explosive behavior.

Explanation
The difference in reactivity of alkali metals in aqueous solution and ammonia can be attributed to the different solvent environments. Water is a polar solvent that stabilizes ions through hydration, leading to the rapid and exothermic reaction with alkali metals. In contrast, liquid ammonia is a nonpolar solvent that can solvate electrons, providing a stabilizing effect for alkali metals.
In conclusion, alkali metals are liberated as hydrogen gas in aqueous solution due to their high reactivity with water. However, they are stable in liquid ammonia due to the formation of solvated electrons that prevent the violent reactions seen in water. This difference in behavior highlights the importance of solvent choice in controlling the reactivity of alkali metals.