The Dense White Fumes are sulfur trioxide solid particles, SO3(s)
They prove the removal of Nitric Acid, HNO3 by Fractional Distillation.
The PbSO4 precipitate cannot form if any HNO3 is left over from the Aqua Regia in the previous procedure. Aqua Regia is a mixture of HNO3 and HCl.
So, H2SO4 is added for two reasons:
1. It provides the SO4-2 ions to precipitate PbSO4 for the lead test.
2. It will remove the HNO3 by Fractional Distillation. In Fractional Distillation each liquid is distilled at its own boiling point.
Since HNO3 boils at 130o C, and H2SO4 boils at 338o C, the HNO3 will have distilled off when the H2SO4 boils.
When H2SO4 boils, it decomposes into sulfur trioxide, the The Dense White Fumes:
H2SO4 -------> SO3 (s) + H2O
Hence: The Dense White Fumes prove that the HNO3 is gone so that the Test for Lead CAN work IF lead is present.
WARNING: The Dense White Fumes do NOT prove lead present. They prove that HNO3 is gone so that the Test for Lead CAN work IF lead is present.
WARNING: Do NOT confuse steam, H2O(g) with SO3(s) , Dense White Fumes. The fumes are SOLID particles and do not evaporate. They should rise two meters high. Have your instructor varify your fumes to be sure.