Why is it necessary to use acetic acid and not sulphuric acid for acidification of sodium extract for testing sulphur by lead acetate test?

If on adding $FeCl_3$ solution to acidified Lassaigne solution of an organic compound a blood red colouration is produced,it indicates the presence of

Why is the test for nitrogen,sulphur,or halogens not carried out by the direct addition of reagents to an organic compound?

Give the answers for the Lassaigne test:
$(i)$ If nitrogen and sulphur are both present,what observation is observed?
$(ii)$ If bromine is present,what is the observation?
$(iii)$ In the Lassaigne extract,if black precipitates are not obtained upon adding $CH_3COOH$ and lead acetate,what does this indicate?
$(iv)$ What is the reason for adding $FeSO_4$ to the Lassaigne extract?
$(v)$ Why is the Prussian blue colour observed?
$(vi)$ What does the violet colour with sodium nitroprusside in the Lassaigne extract indicate?

Lassaigne's test for nitrogen is negative for which compound?

In Lassaigne's test for the detection of halogens,the sodium fusion extract is first boiled with concentrated nitric acid. This is