The green colour solution of an Fe2 salt changes to a brown solution of an Fe3+ salt by

Analyze the question: Fe²⁺ (green) changes to Fe³⁺ (brown). We need the name of this process.

Concept: Oxidation and reduction can be defined in terms of electron transfer.

• Oxidation = loss of electrons (increase in oxidation number)
• Reduction = gain of electrons (decrease in oxidation number)

Solution:

Fe²⁺ → Fe³⁺ + e⁻

• The iron loses one electron.
• Its oxidation number increases from +2 to +3.
• Loss of electrons means the iron(II) ion is oxidized.

This is why a green iron(II) solution, such as FeSO₄, turns brown or yellow-brown when exposed to air or when treated with oxidizing agents such as acidified KMnO₄, H₂O₂ or chlorine water.

The other options do not fit:

• Reduction (D) would be the reverse, Fe³⁺ → Fe²⁺.
• Conversion, elimination and substitution are not the standard terms for electron transfer in redox chemistry.

The correct answer is C. oxidation.

Exam tip: Remember OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain. Also remember the colours: Fe²⁺ is green and Fe³⁺ is brown or yellow-brown. These are often tested, including in the brown ring test and in hydroxide precipitates (green Fe(OH)₂ and reddish-brown Fe(OH)₃).