Q.Lassaigne's test for the detection of nitrogen fails in

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. …