Question

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

Ammonium chloride $\left({\mathrm{NH}}_4\mathrm{Cl}\right)$

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 crystalline Ammonium chloride $\left({\mathrm{NH}}_4\mathrm{Cl}\right)$, following observations are recorded:

• A colourless gas with pungent odour is observed.
• The gas evolved also turns blue litmus red and red litmus blue indicating a mixture of gases.
• White coloured powder is found to be deposited on the cooler parts of test-tube.

Chemical equation:

Ammonium chloride decomposes according to the chemical equation as shown below.

$\underset{(\mathrm{Ammonium}\mathrm{chloride})}{{\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{s}\right)}\underset{\mathrm{Cooling}}{\overset{\mathrm{Heating}}{\rightleftharpoons }}\underset{\left(\mathrm{Ammonia}\right)}{{\mathrm{NH}}_3\left(\mathrm{g}\right)}+\underset{(\mathrm{Hydrogen}\mathrm{chloride})}{\mathrm{HCl}\left(\mathrm{g}\right)}$

• Colourless gases evolved are Ammonia $\left({\mathrm{NH}}_3\right)$ gas which turns red litmus blue due to the basic nature and Hydrogen chloride $\left(\mathrm{HCl}\right)$ gas which turns blue litmus red being acidic in nature.
• White coloured deposition can be seen on the cooler parts of the test-tube because of the reversible nature of the reaction i.e.; gases evolved recombine to form solid Ammonium chloride.