question_answer 1)

  When a metal of group I was dissolved in liquid ammonia, the following observations were obtained : (i) Blue solution was obtained initially. (ii) On concentrating the solution, the blue colour changed to bronze colour. How do you account for the blue colour of the solution? Give the name of the product formed on keeping the solution for some time.

Answer:

  The alkali metals dissolve in liquid ammonia. The colour of the solution is blue. The alkali metal atoms ionise in solution. \[M+(x+y)N{{H}_{3}}\to {{M}^{+}}(N{{H}_{3}})+{{e}^{-}}{{(N{{H}_{3}})}_{y}}\] The solution thus contains ammoniated cations and ammoniated electrons. When light falls on the solution, the ammoniated electrons are excited by absorbing energy corresponding to red light and the light transmitted is blue. This imparts blue colour to the solution. When solution is concentrated, the colour changes to bronze. This is due to the formation of metal ion clusters which brings metallic lustre to solution.