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 $FeSO_4$ and dilute sulphuric acid to a part of the 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.

(A) If the organic compound contains both $N$ and $S$,then during fusion it may form either sodium thiocyanate $(NaSCN)$ or a mixture of sodium cyanide $(NaCN)$ and sodium sulphide $(Na_2S)$ depending upon the amount of sodium metal used. If the sodium metal used is less,only $NaSCN$ is produced.
This then reacts with $Fe^{3+}$ ions (produced by oxidation of $Fe^{2+}$ ions during the preparation of Lassaigne's extract) to give red colouration due to the formation of ferric thiocyanate.
$Fe^{2+} \xrightarrow{\text{Aerial oxidation}} Fe^{3+}$; $Fe^{3+} + 3SCN^- \longrightarrow [Fe(SCN)_3]$ (Blood red colour)
If excess sodium is used,$NaCN$ and $Na_2S$ are formed. $NaCN$ reacts with $FeSO_4$ to give Prussian blue colour due to the formation of ferric ferrocyanide,$Fe_4[Fe(CN)_6]_3$.
$6NaCN + FeSO_4 \longrightarrow Na_4[Fe(CN)_6] + Na_2SO_4$
$3Na_4[Fe(CN)_6] + 4Fe^{3+} \longrightarrow Fe_4[Fe(CN)_6]_3 + 12Na^+$
From the above discussion,it follows that Manish and Rajni used excess sodium,forming $NaCN$ which gave Prussian blue colour,while Ramesh used less sodium,forming $NaSCN$ which gave red colouration.