Why a freshly prepared solution of `FeSO_4` is used in Lassaigne's test for nitrogen ?

Why is a freshly prepared solution of FeSO_(4) used for the detection of nitrate and nitrite ?

Why is a freshly prepared solution of FeSO_(4) used for the detection of nitrate and nitrite?

Which colour is formed in the Lassaigne's test for nitrogen?

Hydrazine does not give Lassaigne's test for nitrogen. Why ?

Will NaCN give a positive Lassaigne's test for nitrogen ?

Justify the following statements. i. Water mixed with formic acid cannot be separated by fractional distillation. ii. o -nitrophenol is steam volatile while p-nitrophenol is not. iii. Freshly prepared FeSO_4 solution is insisted during the test for nitrogen in Lassaignes test. iv. Copper spiral is used in the apparatus for Dumas method.