Question

On reaction of sodium hydroxide with Iron (III) ion gives a coloured precipitate. Identify the colour.
A.Blue
B.Pink
C.Red
D.Brown

Answer 1

Sodium and ammonium hydroxide have been used for the qualitative analysis of different cations, where the different cations give differently coloured precipitate with the hydroxides. The precipitate formed is ferric hydroxide.

Complete Stepwise Solution:
Sodium hydroxide reacts with ferric ions to form ferric hydroxide. The ferric ions is the reaction medium, paired up with hydroxide anions to precipitate in the form of ferric hydroxide which is a reddish brown precipitate that is insoluble in excess of sodium hydroxide.
Ammonia solution or ammonium hydroxide also reacts with the ferric ions to form the reddish brown precipitate of ferric hydroxide which is again insoluble in excess of ammonia.

Hence, the correct answer is option D.

Notes: These two reagents also form different coloured precipitates with transition metals salts. Copper is precipitated by both the reagents in the form of hydroxides. But when ammonia is added then the molecules act as the ligand and attach themselves to the secondary valence of the copper ion. Hence a copper ammonia complex is formed. So initially the blue salt solution of copper turns pale blue due to the formation of cupric hydroxide which then turns deep blue due to the formation of the cupric ammine complex. While the action of sodium hydroxide does not have the same reaction with the cupric ions. They can only form the precipitate of cupric hydroxide. Similar reactions are also seen in case of other transition metal ions like nickel, cobalt, etc.