Question

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

Lead nitrate $\left[\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2\right]$

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 white powder of Lead nitrate $\left[\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2\right]$, the following observations are recorded:

• A crackling sound is heard when Lead nitrate is heated in a test tube which shows melting.
• On strong heating, a reddish-brown gas is evolved which turns blue litmus red indicating its acidic nature.
• Another gas is evolved which rekindles a glowing wood splinter.
• A yellow residue is left behind in the test tube which on further heating shows fusion with the glass due to high temperature.

Chemical equation:

Decomposition reaction takes place and Lead nitrate decomposes on strong heating to give Nitrogen dioxide $\left({\mathrm{NO}}_2\right)$ gas as shown below.

$\underset{(\mathrm{Lead}\mathrm{nitrate})}{2\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2\left(\mathrm{s}\right)}\stackrel{∆}{\to }\underset{(\mathrm{Lead}\mathrm{monoxide})}{2\mathrm{PbO}\left(\mathrm{s}\right)}+\underset{\underset{(\mathrm{Reddish}\mathrm{brown}\mathrm{gas})}{(\mathrm{Nitrogen}\mathrm{dioxide}})}{4{\mathrm{NO}}_2\left(\mathrm{g}\right)}+\underset{\left(\mathrm{Oxygen}\right)}{{\mathrm{O}}_2\left(\mathrm{g}\right)}$

• Reddish-brown gas is nitrogen dioxide.
• Gas that rekindles a glowing wood splinter is oxygen.
• The yellow residue which fuses with glass is Lead monoxide.