Question

A white crystalline solid on strong heating leaves behind a residue which changes its colour with the temperature and gives out a reddish brown gas

Answer 1

This white crystalline solid is Lead Nitrate[Pb(NO3) 2].On heating, it decomposes and leaves behind a residue lead oxide(PbO) i. e yellow in colour which soon changes its colour and brown fumes of nitrogen dioxide (NO2) gas along with oxygen.
The reaction is:

2Pb(NO3) 2-->2PbO + 4NO2+O2


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Answer 2

Lead nitrate..........