Q.Assertion (A): In the Lassaigne's test for nitrogen in an organic compound, Prussian blue colour is obtained. Reason (R): Prussian blue colour is due to the formation of [Fe(SCN)]²⁺ during the test.

The Lassaigne Test: Why We Fuse Sodium with the Sample

Imagine you have an organic compound — say, a drug, a pesticide, or a dye — and you need to find out if it contains nitrogen, sulfur, or a halogen (chlorine, bromine, iodine). You cannot simply test the original compound directly because these elements are covalently bonded inside the molecule. They are "hidden" — not present as free ions.

The Lassaigne test solves this by breaking the covalent bonds and converting these elements into water-soluble ionic salts. That is the entire point: turn invisible atoms into detectable ions.

The Intuition

Think of the organic molecule as a locked safe. Inside the safe are the atoms you want to detect (N, S, X). The safe is the covalent framework. To get the atoms out, you need to smash the safe open. Sodium metal is the sledgehammer.

When you heat the organic compound with a piece of sodium metal, the sodium reacts violently. It rips apart the carbon‑carbon and carbon‑heteroatom bonds. In the process, the sodium itself gets oxidised (loses an electron) and the atoms from the compound get reduced (gain electrons). The result is a set of simple, stable, ionic compounds:

• Nitrogen (from the compound) combines with sodium and carbon to form sodium cyanide ($\text{NaCN}$).
• Sulfur forms sodium sulfide ($\text{Na}_2\text{S}$).
• Halogens form sodium halides ($\text{NaX}$, where X = Cl, Br, I).

These are all water‑soluble. Once you dissolve the fused mass in water, you have a solution containing $\text{CN}^-$, $\text{S}^{2-}$, and $\text{X}^-$ ions — ready for standard qualitative tests.

The Precise Statement

Lassaigne’s test (or sodium fusion test): A small piece of dry sodium metal is heated with a sample of the organic compound in a fusion tube until the tube is red hot. The fused mass is then extracted with distilled water, boiled, cooled, and filtered. The filtrate (called the Lassaigne’s extract) is used to test for the presence of nitrogen, sulfur, and halogens.

The chemical principle is:

Why This Works (and What Can Go Wrong)

For nitrogen: The cyanide ion ($\text{CN}^-$) is detected by adding ferrous sulfate and ferric chloride — a deep blue precipitate of Prussian blue ($\text{Fe}_4[\text{Fe(CN)}_6]_3$) confirms nitrogen.

For sulfur: The sulfide ion ($\text{S}^{2-}$) gives a purple/violet colour with sodium nitroprusside ($\text{Na}_2[\text{Fe(CN)}_5\text{NO}]$). Alternatively, it forms a black precipitate of lead sulfide ($\text{PbS}$) with lead acetate.

For halogens: The halide ion ($\text{X}^-$) is detected by acidifying the extract with nitric acid and adding silver nitrate — a curdy white precipitate (AgCl), pale yellow (AgBr), or yellow (AgI) indicates the halogen. …