Question

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

Copper nitrate hexahydrate $[\mathrm{Cu}{\left({\mathrm{NO}}_3\right)}_2.6{\mathrm{H}}_2\mathrm{O}]$

Answer 1

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 $[\mathrm{Cu}{\left({\mathrm{NO}}_3\right)}_2.6{\mathrm{H}}_2\mathrm{O}]$, 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:

$\underset{(\mathrm{Hydrated}\mathrm{Copper}\mathrm{carbonate})}{2\mathrm{Cu}{\left(\mathrm{NO}\right)}_3.6{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}\stackrel{∆}{\to }\underset{(\mathrm{Anhydrous}\mathrm{Copper}\mathrm{carbonate})}{2\mathrm{Cu}{\left({\mathrm{NO}}_3\right)}_2\left(\mathrm{s}\right)}+\underset{\left(\mathrm{Water}\right)}{6{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}$

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

$\underset{(\mathrm{Copper}\mathrm{nitrate})}{2\mathrm{Cu}{\left({\mathrm{NO}}_3\right)}_2\left(\mathrm{s}\right)}\stackrel{∆}{\to }\underset{(\mathrm{Copper}\mathrm{oxide})}{2\mathrm{CuO}\left(\mathrm{s}\right)}+\underset{\underset{(\mathrm{Reddish}\mathrm{brown}\mathrm{gas})}{(\mathrm{Nitrogen}\mathrm{dioxide}})}{4{\mathrm{NO}}_2\left(\mathrm{g}\right)}+\underset{\left(\mathrm{Oxygen}\right)}{{\mathrm{O}}_2\left(\mathrm{g}\right)}$

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