Q.Nitrogen detection in an organic compound is carried out by Lassaigne's test. The blue colour formed is due to the formation of

The first step in analysing any organic compound is finding out WHICH elements it contains -- qualitative analysis, always done before the quantitative estimation covered separately. Beyond the near-universal trio of carbon, hydrogen and oxygen, a compound may also carry nitrogen, sulphur, halogens, phosphorus, or (in organometallics) a metal.

Carbon and hydrogen are confirmed together by the copper oxide test: the substance, ground with roughly three times its weight of dry copper oxide, is heated strongly in a tube whose outlet dips into lime water. Carbon oxidises to $CO_2$, turning the lime water milky; hydrogen oxidises to water, which condenses on the tube's cool walls and turns anhydrous $CuSO_4$ blue when collected on it.

Nitrogen, sulphur, halogens and phosphorus are all detected via Lassaigne's sodium fusion test, which converts covalently-bonded N/S/halogen into water-soluble sodium salts by fusing the compound with metallic sodium, then dissolving, boiling and filtering to give the Lassaigne's extract used for every test that follows.

• Nitrogen: present N converts to NaCN; treated with ferrous sulphate, ferric ion and conc. HCl (the HCl dissolving interfering ferrous hydroxide), this gives a Prussian blue precipitate, $Fe_4[Fe(CN)_6]_3$. If BOTH N and S are present together, the fusion instead gives NaCNS, and testing with $FeCl_3$ gives a blood-red colouration ($Fe(CNS)_3$) instead of blue -- so a red result signals N+S together, not N alone; this matters for a compound like hydrazine, which has NO carbon at all and so cannot form NaCN under any circumstances, meaning Lassaigne's test genuinely FAILS (gives no blue colour) for it. …