Question

Assertion: $N{H_2} - N{H_2}$ gives Prussian blue colour during Lassaigne test (element detection test);
Reason: $NaCN$ gives Prussian blue colour of $F{e_4}{\left[ {Fe\left( {C{N_6}} \right)} \right]_3}$ with $FeS{O_4},FeC{l_3}.$
A. Both Assertion and Reason are correct and Reason is the correct explanation for
Assertion.
B. Both Assertion and Reason are correct and Reason is not the correct explanation for
Assertion.
C. Assertion is correct but Reason is incorrect.
D. Assertion is incorrect and Reason is correct.

Answer 1

Lassaigne’s test is also called as sodium fusion Lassaigne's test is used to qualitatively analyse the presence of various elements (like halogen, nitrogen and sulphur). in organic compounds.

Complete Step by step answer: In this test, the carbon and nitrogen which is present in the organic compound first will get fused with sodium metal to convert it into sodium cyanide. This sodium cyanide is soluble in water. This sodium cyanide will be converted to sodium ferrocyanide simply by the addition of a desirable amount of ferrous sulphate. These sodium ferrocyanides contain ferric ions which are then made to react with ferrocyanide. Then the resultant precipitate will form a colour Prussian blue. This Prussian blue indicates the presence of nitrogen atoms.
The reaction involving in this are given below;
$Na + C + N \to NaCN$ this is the sodium fusion extract
$6NaCN + FeS{O_4} \to N{a_4}\left[ {Fe{{\left( {CN} \right)}_6}} \right] + N{a_2}S{O_4}$ when this sodium extract reacts with ferrous sulphate then formation of Sodium ferrocyanide.
$N{a_4}\left[ {Fe{{\left( {CN} \right)}_6}} \right] + F{e^{3 + }} \to F{e_4}{\left[ {Fe{{\left( {CN} \right)}_6}} \right]_3}$ there will be a formation of ferric ferrocyanide with Prussian colour is obtained.
So, the final result is correct.
While hydrazine $N{H_2} - N{H_2}$ does not give Prussian colour as carbon is essential for the Lassaigne test, as $NaCN$ is formed in the first step. But there is no carbon in hydrazine. So the assertion is incorrect.
Thus, Assertion is incorrect and Reason is correct

Hence, the correct option is D.

Note: While solving such questions sometimes what happens both options are correct and seems that the 2nd option is the correct explanation too, but students make mistakes by not understanding the concept in depth and mark answers as A.For the qualitative analysis of halogen. The sodium extract which is formed will made to react with nitric acid and then it will be treated with silver nitrate.