In the Lassaigne's test for the detection of sulfur,which substance is responsible for the formation of black precipitates?

An organic compound containing phosphorus on oxidation with $Na_2O_2$ gives a compound '$X$'. This '$X$' when boiled with $HNO_3$ followed by treatment with ammonium molybdate reagent gives a yellow precipitate '$Y$'. '$X$' and '$Y$' respectively are:

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

How is the purity of a compound determined?

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

Assertion: Potassium can be used in the Lassaigne test.
Reason: Potassium reacts vigorously.