From the list provided below choose the ones that match the description given.
Ferrous sulphate, Copper carbonate, Triferric tetroxide, Trilead tetroxide, Sulphur dioxide, Carbon dioxide, Lead nitrate, Zinc nitrate, Oxygen, Nitrogen.

A pale green compound which turns black on strong heating.

Identify the salt P from the following observations:

Salt P has light bluish-green colour. On heating, it produces a black coloured residue. Salt P produces brisk effervescence with dilute $\mathrm{HCl}$ and the gas evolved turns lime water milky, but no action with acidified potassium dichromate solution.

Identify the salt Q from the following observation:

Salt Q is white in colour. On strong heating, it produces buff-yellow residue and liberates a reddish-brown gas. Solution of salt Q produces chalky white insoluble precipitate with an excess of ammonium hydroxide.

State one relevant observation for the following reactions:

• Zinc carbonate is heated strongly.

Identify the type of reaction:

$2{\mathrm{PbO}}_2\to 2\mathrm{PbO}+{\mathrm{O}}_2$