Prolonged heating is avoided during the preparation of ferrous ammonium sulphate to :

Solution

Chemistry ma’am · 4 steps

Iron here is Fe²⁺, and heat with air pushes it to Fe³⁺, so keep the warming short.

1. What is made. Ferrous ammonium sulphate (Mohr's salt) is FeSO₄.(NH₄)₂SO₄.6H₂O. You dissolve equimolar FeSO₄.7H₂O and (NH₄)₂SO₄ in water with a little dilute H₂SO₄. Then you warm it briefly and cool it so the salt crystallises.
2. Weak point. Iron in this salt is in the +2 state. Dissolved oxygen from the air easily oxidises Fe²⁺ to Fe³⁺, and heat speeds this up.

   4Fe²⁺ + O₂ + 4H⁺ → 4Fe³⁺ + 2H₂O

3. Why it matters. If oxidation happens, ferric salt contaminates the crystals and they turn yellowish instead of pale green. So heating is kept short, only until the solids dissolve.
4. So. Prolonged heating is avoided to prevent oxidation of Fe²⁺ to Fe³⁺, option (c).

The idea to keep. Whenever a salt contains Fe²⁺, think oxidation first: heat and air push it to Fe³⁺. Dilute H₂SO₄ handles hydrolysis; short heating handles oxidation.

Answer from the NTA final answer key. Solution by PrepWiser.