Explanation
The given organic compound is mixed with dry copper oxide ($$CuO$$) and heated in a hard glass tube. The products of the reaction are passed over (white) anhydrous copper sulphate and then bubbled through lime water. If copper sulphate turns blue due to the formation of $$CuS{ O }_{ 4 }.5{ H }_{ 2 }O$$ (by water vapor) then the compound contains hydrogen. If lime water is turned milky by $$C{ O }_{ 2 }$$, then the compound contains carbon.
The detection of various elements present in an organic compound is called qualitative analysis. Carbon and hydrogen are present in almost all the organic compounds. Carbon and hydrogen are detected by heating the organic compound with cupric oxide ($$CuO$$) strongly, where carbon is oxidized to carbon dioxide and hydrogen to water. Carbon dioxide is tested by lime water test, whereas water is tested by anhydrous copper sulphate test.
Lassaigne's test is a general test for the detection of halogens, nitrogen and sulphur in an organic compound. These elements are covalently bonded to the organic compounds. In order to detect them, these have to be converted into their ionic forms. This is done by fusing the organic compound with sodium metal. The ionic compounds formed during the fusion are extracted in aqueous solution and can be detected by simple chemical tests. The extract is called sodium fusion extract or Lassaigne's extract.
Lassaigne's test is the most reliable test for the detection of nitrogen, halogens and sulphur in an organic compound. The compound is heated with sodium metal to convert the elements present in the organic compound into the water-soluble salts of sodium.
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