Q4. Discuss the chemistry of Lassaigne’s test (NCERT Exercise 8.21).
The compound is fused with sodium, which converts nitrogen, sulphur and halogens into NaCN, Na2S and NaX (and NaSCN if N and S are both present). The fused mass is boiled with water to give the sodium fusion extract. Nitrogen: FeSO4 then concentrated H2SO4 gives Prussian blue. Sulphur: lead acetate in acetic acid gives black PbS, or nitroprusside gives violet. Halogens: after boiling with HNO3, AgNO3 gives AgCl, AgBr or AgI.
Q5. Why is concentrated sulphuric acid needed in the test for nitrogen?
On heating, it oxidises some of the iron(II) to iron(III). Fe3+ then reacts with the hexacyanidoferrate(II) ion to form Prussian blue, Fe4[Fe(CN)6]3.
Q6. Which test confirms sulphur besides lead acetate, and what colour does it give?
Sodium nitroprusside: S2− + [Fe(CN)5NO]2− → [Fe(CN)5NOS]4−, a violet colour.
Q7. How is phosphorus detected in an organic compound?
The compound is heated with sodium peroxide, which oxidises phosphorus to phosphate. The solution is boiled with nitric acid and ammonium molybdate is added; a yellow colour or precipitate of ammonium phosphomolybdate, (NH4)3PO4·12MoO3, shows phosphorus.
Q8. A sodium fusion extract gives a white precipitate with silver nitrate (after nitric acid) that dissolves in ammonia solution. What is present, and why does the precipitate dissolve?
Chlorine. AgCl is the most soluble of these three silver halides (largest solubility product), so ammonia, which binds Ag+ as [Ag(NH3)2]+, can dissolve it; AgBr is poorly soluble in dilute ammonia but dissolves in concentrated ammonia; AgI does not dissolve appreciably in either.