Question

The $ Fe{\left( {OH} \right)_2} $ is precipitated from $ Fe\left( {II} \right) $ solutions as a white solid but turns dark green and then brown due to the formation of:
(A) $ Fe{\left( {OH} \right)_2} $ and $ Fe{\left( {OH} \right)_3} $
(B) Only $ Fe{\left( {OH} \right)_3} $
(C) $ F{e_2}{O_3}.{\left( {{H_2}O} \right)_n} $
(D) $ F{e_2}{O_3}.2{H_2}O $

Answer 1

Hint: During a chemical reaction, sometimes the color of the compound changes due to the change in physical and chemical properties of the compound. Sometimes when two different species or substances combine and create a new different chemical compound due to absorption and radiation of light.

Complete step by step answer
First, we will understand the basics of a chemical reaction. A chemical reaction is a reaction in which two different substances are combined to produce a different substance. In a chemical reaction, the substances which are combined to form another substance are termed as ‘reactants’, and the substance formed as the result of the reaction is termed as the ‘product’.
A chemical reaction is not visible to us, but we can predict the changes that occurred in the reaction. Now we are back to our question, which $ Fe{\left( {OH} \right)_2} $ is precipitated from $ Fe\left( {II} \right) $ solutions as a white solid but turns dark green and then brown. So, here due to the formation of a new substance, the solution is first obtained as white solid then turns dark green and then brown. We will understand this with the help of the following reactions. So, the reactions are given as follows:
 $ 4Fe{\left( {OH} \right)_2} + {O_2} + 4{H^ + } \to 4Fe{\left( {OH} \right)_3} + 2{H_2}O $
 $ 2Fe{\left( {OH} \right)_3} + \left( {n - 3} \right){H_2}O \to F{e_2}{O_3}.{\left( {{H_2}O} \right)_n} $
According to the above reactions, we can conclude that white-colored ferrous hydroxide slowly turns dark green and brown due to the formation of ferric oxide. The color change is due to the absorption and radiations of lights with different characteristics.
Therefore, the correct option is (C).

Note:
 $ Fe{\left( {OH} \right)_2} $ is precipitated from $ Fe\left( {II} \right) $ solutions. This precipitate is also known as the ‘green rust’. The crystal lattice of iron (II) ions are easily replaced by iron (III) ions which are produced by progressive oxidation. The color changes quickly due to the presence of oxygen