Question

When sodium hydroxide solution is added to a freshly prepared solution of ferrous sulphate, a green precipitate of ferrous hydroxide is formed which on standing turns brown because:

• A. ferric salts are unstable
• B. the mixture becomes dirty
• C. ferrous hydroxide becomes ferric oxide
• D. green ferrous hydroxide oxidises to brown ferric hydroxide

Answer 1

The correct option is D green ferrous hydroxide oxidises to brown ferric hydroxide

$2NaOH+{FeSO}_4\to {Na}_2{SO}_4+Fe{\left(OH\right)}_2$

(Green ppt.)

Green precipitate of $Fe{\left(OH\right)}_2$ on standing oxidises to $Fe{\left(OH\right)}_3$ which is brown coloured.

$+2$ $+3$

$Fe{\left(OH\right)}_2$ $\to$ $Fe{\left(OH\right)}_3$ (oxidation)

Hence, the right answer is green ferrous hydroxide oxidizes to brown ferric hydroxide.