Question

What is responsible for the blue color of the solution of alkali metal in liquid ammonia? Give a chemical equation also.

Answer 1

Liquid ammonia will dissolve all of the alkali metals and other electropositive metals such as $Ca$, $Sr$, $Ba$, $Eu$, and $Yb$ (also $Mg$ using an electrolytic process), giving characteristic blue solutions. The blue colour of the solution is due to the ammoniated electron which absorbs energy in the visible region of light and thus imparts blue colour to the solution.

Complete answer:
In the reaction of alkali metal and liquid ammonia it forms sodium amide and liberates hydrogen gas.
$N{a^ + } + {e^ - } + N{H_2} \to NaN{H_2} + \dfrac{1}{2}{H_2}$
Alkali metals dissolve in liquid ammonia giving deep blue solutions, which conduct electricity. The blue colour of the solution is due to ammoniated electrons. The ammoniated electrons absorb energy corresponding to a red region of visible light. Therefore, the transmitted light is blue in colour.
Most metals are insoluble in virtually all solvents, but the alkali metals (and the heavier alkaline earth metals) dissolve readily in liquid ammonia to form solvated metal cations and solvated electrons, which give the solution a deep blue color.

Additional information:
Solvated electrons produced by dissolution of reducing metals in ammonia and amines are the anions of salts called electrodes. Such salts can be isolated by the addition of macrocyclic ligands such as crown ether and cryptands. These ligands bind strongly the cations and prevent their re-reduction by the electron.

Note:
The dilute solutions are paramagnetic in nature because they contain free ammoniated electrons. Liquid ammonia becomes paramagnetic due to loss of electrons. As the concentration increases, the ammoniated metal gets bound by free electrons and thus the solution becomes a bronze color.