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Question

State your observations when following salts are heated strongly in dry state. Write chemical equations in support of your answer.

Copper nitrate hexahydrate [Cu(NO3)2.6H2O]


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Solution

Dry heat test: A sample of the substance is taken in a test tube and heated first gently, then strongly in a flame, and observations are recorded. It is a method of qualitative analysis.

  • Qualitative analysis: The analysis which aims at detecting the presence of any substance in a given sample.

Observations:

On heating Copper nitrate hexahydrate [Cu(NO3)2.6H2O], following observations are recorded:

  • Vapours are noticed coming from the test tube which when condensed turn into a clear liquid.
  • The liquid does not affect litmus. This indicated that the colourless liquid is water as water is neutral.
  • This water results from the loss of water of crystallisation.
  • On strong heating, a reddish brown gas is evolved which turn blue litmus red indicating its acidic nature.
  • Another gas is evolved which rekindles a glowing wood splinter.
  • A powdery black residue is left behind in the test-tube.

Water of crystallization: The fixed number of water molecules present in one formula unit of a salt.

  • These molecules of water are not chemically bonded to the compound and are lost easily.
  • They provide shape to the crystals of the molecules.
  • Example: Copper nitrate has 6 molecules of water of crystallisation.

Chemical equation:

Six molecules of water of crystallisation are lost on heating according to the chemical equation as shown below:

2Cu(NO)3.6H2O(s)(HydratedCoppercarbonate)2Cu(NO3)2(s)(AnhydrousCoppercarbonate)+6H2O(l)(Water)

After losing water of crystallisation, decomposition reaction takes place and Copper nitrate decomposes on strong heating to give Nitrogen dioxide gas as shown below.

2Cu(NO3)2(s)(Coppernitrate)2CuO(s)(Copperoxide)+4NO2(g)(Nitrogendioxide(Reddishbrowngas))+O2(g)(Oxygen)

  • Reddish brown gas is Nitrogen dioxide.
  • Gas which rekindles a glowing wood splinter is Oxygen.
  • The powdery black residue is Copper oxide.

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