Lead nitrate powder taken in a test tube and heated emits brown smoke, is it smoke?

(a) O2

(b) NO2

(c) N2

(d) PbO

Explanation: When lead nitrate is heated in a test tube, it decomposes and releases brown fumes. This brown gas is nitrogen dioxide.

`2Pb(NO_3)_2 \to 2PbO + 4NO2_{(g)} + O2_{(g)}`

This is the thermal decomposition reaction of lead nitrate, broken down into lead oxide, nitrogen dioxide, and oxygen gas. Nitrogen dioxide gas (smoke) is brown in colour.

Correct Answer - (b)