Q. 39

Three students, Manish, Ramesh and Rajni were determining the extra elements present in an organic compound given by their teacher. They prepared the Lassaigne’s extract (L.E.) independently by the fusion of the compound with sodium metal. Then they added solid FeSO4 and dilute sulphuric acid to a part of Lassaigne’s extract. Manish and Rajni obtained Prussian blue colour but Ramesh got red colour. Ramesh repeated the test with the same Lassaigne’s extract, but again got red colour only. They were surprised and went to their teacher and told him about their observation. Teacher asked them to think over the reason for this. Can you help them by giving the reason for this observation. Also, write the chemical equations to explain the formation of compounds of different colours.

Answer :

As we know that theLassaigne’s extract is prepared to detect the presence of N, S, Halogens and P.

As when the Lassaigne’s extract is prepared, it allows all the elements present in the particular compound to come into the elemental state, so that they can easily be detected.


Now, the reaction involved is for Nitrogen estimation:


Na + C + N NaCN


NaCN + FeSO4 + dilute sulphuric Fe4[FeCN6]3 (Prussian blue colour solution)


The reaction involved is for sulphur estimation:


Na + C + N + S NaCNS


3NaCNS + FeSO4 + dilute sulphuric Fe(CNS)3 (Blood red colour) + 3Na+


Hence, the Ramesh has N as well as S present in his organic compound so, he was getting the Blood red colour of Fe(CNS)3 and Manish and Rajni were having only Nitrogen in their compound, hence they got Prussian blue colour.


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