In the test for halogens,why is nitric acid added to the sodium fusion extract before adding silver nitrate?

The Lassaigne's extract is boiled with dilute $HNO_3$ before testing for halogens because,

Which of the following methods is used to check the purity of an organic compound?

Discuss the chemistry of Lassaigne's test.

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.

In the preparation of sodium fusion extract,the purpose of fusing organic compound with a piece of sodium metal is to