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ICSE Class 10 Chemistry Question 16 of 63

Study of Compounds — Nitric Acid — Question 11

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Question

Question 6

(a) Nitric acid cannot be concentrated beyond 68% by the distillation of a dilute solution of HNO3. State the reason.

(b) What is passive iron? How is passivity removed?

Answer

(a) An aqueous solution of nitric acid (68% concentration) forms a constant boiling mixture at 121°C. A constant boiling mixture is one which boils without change in composition.

Hence on boiling further, the mixture evolves out the vapours of both acid and water in the same proportion as in the liquid.

Therefore, dilute HNO3 cannot be concentrated beyond 68% by boiling.

(b) Iron becomes passive or inert when treated with pure concentrated nitric acid due to the formation of the extremely thin layer of insoluble iron oxide (passivity) which stops the reaction.

Fe + 6HNO3 ⟶ Fe(NO3)3 + 3H2O + 3NO2

This inert iron is known as passive iron.

Passivity can be removed by rubbing the surface layer with sandpaper or by treating with strong reducing agent.