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ICSE Class 10 Chemistry Question 16 of 23

Model Paper 2 — Question 11

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11
Question

Question 5(iii)

What do you see in each of the following reactions:

(a) When excess Ammonia gas is passed through an aqueous solution of Zinc Nitrate.

(b) Copper Sulphate crystals are heated strongly.

(c) Sodium hydroxide is added to Copper Sulphate solution in excess.

Answer

(a) When Ammonia gas is passed through an aqueous solution of Zinc Nitrate:

  • A white, gelatinous precipitate of zinc hydroxide, Zn(OH)2, first appears.
    Zn(NO3)2 + 2NH4OH ⟶ 2NH4NO3 + Zn(OH)2 ↓
  • On passing excess ammonia, the precipitate dissolves and the solution becomes clear and colourless owing to the formation of the soluble tetra-amminezinc(II) complex, [Zn(NH3)4]2+.

(b) The blue crystals of hydrated copper sulphate (CuSO4·5H2O) lose their water of crystallisation, giving off steam and turning into a white powder of anhydrous CuSO4:
CuSO4.5H2O(blue crystal)→HeatCuSO4(white powder)+5H2O\underset{\text{(blue crystal)}}{\text{CuSO}_4.5\text{H}_2\text{O}} \xrightarrow{\text{Heat}} \underset{\text{(white powder)}}{\text{CuSO}_4} + 5\text{H}_2\text{O}

(c) Copper(II) sulphate reacts with NaOH to form a blue precipitate of copper(II) hydroxide (Cu(OH)2), which is insoluble in excess NaOH.

CuSO4 + 2NaOH ⟶ Cu(OH)2 ↓ + Na2SO4