On heating ammonium dichromate the gas evolved is:
A) oxygen
B) ammonia
C) nitrous oxide
D) nitrogen

Hint: Ammonium dichromate occurs as a bright orange red coloured crystalline solid. It is easily ignited/burnt generating a voluminous green coloured residue. When it is heated in a closed box/container, the box may get ruptured owing to the disintegration of the material. Ammonium dichromate is also a strong oxidizing agent.

Complete answer:
> The reaction i.e. heating of ammonium dichromate (given in question) is the basis of a common demonstration called the "ammonium dichromate volcano". The decomposing ammonium dichromate yields orange sparks and also generates green coloured chromium (III) oxide crystals into the air, thus creating an effect similar to a miniature volcano.
> On heating Ammonium dichromate $(NH_4)_2Cr_2O_7$, decomposition takes place resulting into the formation of solid chromium(III) oxide $Cr_2O_3$, nitrogen gas, and water vapour (gaseous due to the temperature of the reaction). This can be better understood from the following chemical equation:

$\left( {NH_4} \right)_2Cr_2O_7(s) \to Cr_2O_3(s) + N_2(g) + 4H_2O(g)$

Therefore, the correct option is (D) i.e. on heating ammonium dichromate the gas evolved is nitrogen.

Note: Chromium salts create irritation to the skin as well as respiratory tracts. These salts are also carcinogenic. Ammonium dichromate comprises chromium in the hexavalent form $(Cr^{6+})$. The experiment demonstrated above can even be performed at lab-scale by heating a pile of ammonium dichromate with the help of a bunsen burner thus, producing a "volcano" effect.