Answer:
(NH4)2SO4 gives colourless and odourless gas, which is ammonia (NH3), on thermal decomposition. This is because ammonium sulfate decomposes upon heating to produce ammonia gas and water:
(NH4)2SO4 → 2NH3 + H2O + SO2
The other compounds listed in the question do not produce colourless and odourless gases on thermal decomposition.
- Pb(NO3)2 decomposes to form nitrogen dioxide gas, which has a reddish-brown color and a pungent odor.
- ZnSO4.7H2O decomposes to form water vapor and sulfur dioxide gas, which has a pungent odor and a choking sensation.
- (NH4)2Cr2O7 decomposes explosively to form nitrogen gas, water vapor and chromium oxide, which has a green color.
Therefore, the correct answer to the question is (NH4)2SO4.