Question

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

Washing soda crystals $({\mathrm{Na}}_2{\mathrm{CO}}_3.10{\mathrm{H}}_2\mathrm{O})$

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 Washing soda white coloured crystals, the 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.
• No other gas is evolved on heating.
• On strong heating, a white powdery substance is obtained which shows fusion with glass due to high temperature.

Chemical equation:

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

$\underset{(\mathrm{Hydrated}\mathrm{Sodium}\mathrm{carbonate})}{{\mathrm{Na}}_2{\mathrm{CO}}_3.10{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}\stackrel{∆}{\to }\underset{(\mathrm{Anhydrous}\mathrm{Sodium}\mathrm{carbonate})}{{\mathrm{Na}}_2{\mathrm{CO}}_3.{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}+\underset{\left(\mathrm{Water}\right)}{9{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}$