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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
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B
the mixture becomes dirty
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C
ferrous hydroxide becomes ferric oxide
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D
green ferrous hydroxide oxidises to brown ferric hydroxide
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Solution

The correct option is D green ferrous hydroxide oxidises to brown ferric hydroxide
2NaOH+FeSO4Na2SO4+Fe(OH)2
(Green ppt.)
Green precipitate of Fe(OH)2 on standing oxidises to Fe(OH)3 which is brown coloured.

+2 +3
Fe(OH)2 Fe(OH)3 (oxidation)
Hence, the right answer is green ferrous hydroxide oxidizes to brown ferric hydroxide.

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