Lassaigne's extract of an organic compound when heated with sodium nitroprusside gives a Violet coloured  ‘X’.   ‘X’  is

This is a test to identify sulphur present in organic compound.Sulphur in sodium fusion extract or Lassaigne's extract exists as Na2S.Na2S + Na2FeCN5NO →  Na4FeCN5NOS                Sod.nitroprusside              Violet colour