Questions & Answers

In Lassaigne’s test, sodium metal is used because:
a.) It is very reactive.
b.) Its melting point is low.
c.) Its compounds are soluble in water.
d.) All of the above.

Answer Verified Verified
Hint: To answer this question, we must first understand what Lassaigne’s test is and why is it performed. We should also try to recall the utility of Sodium in it which will ultimately help us decide which property of it is being utilized.

Complete step by step solution:
Lassaigne’s test is a general test performed to detect the presence of halogens, nitrogen and sulphur in an organic compound. Since, these elements are almost always covalently bonded to the organic compounds; they must be first converted to their ionic form to be detected. This is done by fusing the organic compound with sodium metal. The ionic compounds formed during the fusion are extracted in aqueous solution and are detected by means of simple chemical tests.
For Halogens: Halogens present in an organic compound forms sodium halide on fusion with sodium metal. Sodium halide extracted in water can be easily identified by adding silver nitrate solution after acidifying with dilute nitric acid. The different halogens give different colored precipitate.
For Nitrogen: Nitrogen on fusion with sodium metal gives sodium cyanide (NaCN) soluble in water. This is converted into sodium ferrocyanide by the addition of sufficient quantities of ferrous sulphate. A Prussian blue precipitate confirms the presence of nitrogen.
For Sulphur: Sulphur on fusion with Sodium metal will convert it into sodium sulphide. Sulphide ions are readily identified using sodium nitroprusside. A violet colored precipitate is obtained to confirm Sulphur.

We can therefore see that all the properties of sodium listed above are utilized in this extraction method.
Hence, the correct answer is Option (D) all of the above.

Note: Sodium is used in Lassaigne's test because of electropositive nature and in order to convert covalent form to ionic form. It is quite reactive in nature.
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