Question

Which metal nitrate gives colourless gas on thermal decomposing

Answer 1

The nitrates of group I elements, with the exception of lithium nitrate, give off a colourless gas (oxygen) on thermal decomposition.

So, the correct answer can be sodium nitrate, potassium nitrate or caesium nitrate.

Other nitrates given NO2 (brown gas) in addition to oxygen when thermally decomposed. 

Answer 2

Most of the metal nitrates on decomposing gives metal oxide and brown colour NO2 gas.

But exception for certain metal salts like sodium Nitrate and Potassium nitrate which give O2 gas instead of NO2.

sodium Nitrate on decomposing we get sodium nitrite and colourless oxygen gas.

2NaNO3----> 2NaNO2 + O2

chemical test for oxygen gas:
A burning splinter will burn more brightly as it aids or helps combustion.