Q.When an alkali metal dissolves in liquid ammonia the solution can acquire different colours. Explain the reasons for this type of colour change.

Step 1: Dissolution mechanism

When an alkali metal dissolves in liquid ammonia, it ionises:

The metal atom loses its valence electron, which becomes trapped and solvated (surrounded) by ammonia molecules, forming a solvated/ammoniated electron.

Step 2: Dilute solutions — blue colour

In dilute solution, these solvated electrons are essentially free and independent. They absorb energy in the red/infrared region of visible light to move to a higher energy state, and the light that is transmitted/reflected appears blue. This is also why dilute solutions are paramagnetic and conduct electricity well (mobile electrons).

Step 3: Concentrated solutions — bronze/metallic colour …

Explain colour change via the two-electron-environment model: isolated solvated electrons in dilute solution absorb visible light to give blue; at high concentration these e …

• Attributing the blue colour to the metal ion itself rather than to the solvated (ammoniated) electron, which is the actual chromophore.
• Describing only one concentration regime and missing the concentration-dependent colour change (blue → bronze/metallic). …