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Related Topic: Nitrogen and its Compounds

Related Subtopic: Test for nitrates (nitrate ions, NO3-)



Questions 4.6.2

  1. From the observations, outline how you would test for a salt which is suspected to be a nitrate.
  2. Explain the steps involved in the test, and include chemical equations for the steps.
  3. Suggest a name for the complex compound, FeSO4.NO.
  4. State an important caution to be taken in this experiment.
  5. Identify two properties of concentrated sulphuric (VI) acid shown by its settling at the bottom of the test tube.


Answers to Questions 4.6.2


  1. I. Prepare fresh solutions of iron (II) sulphate and potassium nitrate (or sodium nitrate).
    II. Mix the two solutions, about 1 cm3 of each, in a test tube.
    III. Hold the test tube in a slanting position and add concentrated sulphuric (VI) acid gently along the wall of the test tube.
    A brown ring at the boundary between the acid and aqueous mixture confirms the presence of a nitrate.

  2. The aqueous mixture contains potassium nitrate (KNO3) and iron (II) sulphate (FeSO4). At the boundary with the acid, sulphuric (VI) acid reacts with the nitrate salt to produce nitric acid (HNO3).

    H2SO4(l) + 2KNO3(aq) = K2SO4(aq) + 2HNO3(aq)

    Nitric acid oxidizes iron (II) sulphate to iron (III) sulphate and it is itself reduced to nitrogen (II) oxide (NO).

    This oxide combines with the unreacted iron (II) sulphate to form the brown ring, FeSO4.NO.

    FeSO4(aq) + NO(g) = FeSO4.NO(aq)

  3. Iron (II) sulphate nitrogen (II) oxide complex.
  4. Add concentrated sulphuric (VI) acid gently to avoid explosive reaction with water in the aqueous mixture. Or, add acid to the aqueous mixture and not the other way round.
  5. Concentrated sulphuric (VI) acid is denser than water. It is somewhat oily and therefore does not readily mix with water (till shaken).

    6. NB: The brown ring is unstable and quickly decomposes when shaken slightly. It is a complex compound whose formula is FeSO4.NO.