Question

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

Ammonium dichromate $\left[{\left({\mathrm{NH}}_4\right)}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\right]$

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 orange crystals of Ammonium dichromate $\left[{\left({\mathrm{NH}}_4\right)}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\right]$, following observations are recorded:

• A colourless and odourless gas is evolved on heating Ammonium dichromate. The gas has no effect on litmus indicating its neutral nature.
• The gas evolved does not turn lime water milky.
• Apart from this, it does not rekindles a glowing wooden splinter.
• On heating strongly, decomposition occurs alongside a flash of light and large amount of heat indicating the exothermic nature of the reaction.
• Green residue is left behind in the test-tube.

Exothermic reaction: The reaction in which heat and light are accompanied by a chemical reaction.

• Example: Slaking of lime is an exothermic reaction
• $\underset{(\mathrm{Calcium}\mathrm{oxide})}{\mathrm{CaO}\left(\mathrm{s}\right)}+\underset{\left(\mathrm{Water}\right)}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}\to \underset{(\mathrm{Calcium}\mathrm{hydroxide})}{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)}+\mathrm{Heat}$

Chemical equation:

Iodine shows the process of sublimation on heating as shown below:

$\underset{(\mathrm{Ammonium}\mathrm{dichromate})}{{\left({\mathrm{NH}}_4\right)}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\left(\mathrm{s}\right)}\stackrel{∆}{\to }\underset{(\mathrm{Chromic}\mathrm{oxide})}{{\mathrm{Cr}}_2{\mathrm{O}}_3\left(\mathrm{g}\right)}+\underset{\left(\mathrm{Water}\right)}{4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)}+\underset{\left(\mathrm{Nitrogen}\right)}{{\mathrm{N}}_2\left(\mathrm{g}\right)}$

• Ammonium dichromate is orange coloured.
• Colourless and odourless gas is Nitrogen.
• Green residue is Chromic oxide.