Lassaigne's test is used for the detection of which of the following?

In $Kjeldahl’s$ method,$CuSO_4$ acts as

For testing halogens in an organic compound with $AgNO_{3}$ solution,sodium extract (Lassaigne's test) is acidified with dilute $HNO_{3}$. What will happen if a student acidifies the extract with dilute $H_{2}SO_{4}$ in place of dilute $HNO_{3}$?

In the detection test of nitrogen,the Prussian blue colour is due to which compound?

Given below are two statements :
Statement $I$ : In Lassaigne's Test,when both nitrogen and sulphur are present in an organic compound,sodium thiocyanate is formed.
Statement $II$ : If both nitrogen and sulphur are present in an organic compound,then the excess of sodium used in sodium fusion will decompose the sodium thiocyanate formed to give $NaCN$ and $Na_{2}S$.
In the light of the above statements,choose the most appropriate answer from the options given below.

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