How is the purity of a compound determined?

In the Lassaigne's test for nitrogen in an organic compound,the Prussian blue colour is obtained due to the formation of:

Which of the following compounds will give a positive Lassaigne's test for both nitrogen and halogen?

Match column $-I$ and column $-II$ with the correct relation:

Column $-I$	Column $-II$
$(i).$ Prussian blue	$(a).$ Presence of phosphorus
$(ii).$ Yellow precipitate	$(b).$ Presence of sulphur
$(iii).$ Purple solution	$(c).$ Presence of nitrogen
$(iv).$ Black precipitate	$(d).$ Presence of chlorine

Appearance of blood red colour,on treatment of the sodium fusion extract of an organic compound with $FeSO_4$ in presence of concentrated $H_2SO_4$ indicates the presence of element/s

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 $HNO_{3}$?