Heating the solution will form vapors, since water is a part of the acids,and the reaction, water vapors of first, as it is more volatile, the nitric will concentrate with the HCl in solution, then as it concentrates the nitric acid will begin to vapor off in the fumes and will begin to come off as vapors of Nox gases, free HCl will then concentrate more and free HCl will hydrogen chloride gas will vapor off, these gases will not be pure as there is some overlap of the gases that escape, but this is the order of volatility, a tiny bit of sulfuric acid is used for two reasons it will not boil off and will stay in solution with the heated salts preventing them from being baked dry, if evaporation carried too far by mistake, it will also be useful to form lead sulfate which will precipitate later with dilution.
As gold salts concentrate the solution can change from a yellow to solution to more of an orange solution, and of coarse if base metals are involved the color of the solution will reflect those metals, copper is a metal that gives strong color to solutions.
Free acid vapors off fairly easy at the proper temperature and concentration of solution.
Because the free nitric usually will not come off at the first evaporation, although most of the free nitric acid will, some may remain after the first evaporation, another reason is the nitric acid can form nitrate salts with metals, example copper nitrate, and during evaporation these nitrate salts can crystallize remaining with the gold when adding Acid to re-wet these nitrate salts will take hydrogen from the acid, reforming HNO3 again, this can be seen when acid is added to the concentrated solution a plume of Nox gas will usually evolve, the added acid (usually HCl) contains water which will also dilute these salts of gold chloride, this volume of added acid in normally very small so the work of evaporation a second time does not take that long, again we evaporate the nitric formed in solution, repeating the process a third time will normally eliminate the nitric from solution, leaving us with a gold chloride solution, and depending on on our previous work only a little other metal chlorides in solution.
But salts of metals do not lose their acid anion easily at the temperatures we evaporate the free acids from solution, these metal salts like gold chloride or copper nitrate can be crystallized, you could evaporate the copper nitrate salt to a dry state and it will not eliminate the nitrate anion, copper nitrate has to be heated at very high temperature before it will release this nitrate Anion as NO2 gas and form copper oxide.
So if we have nitrate salts with our gold chloride salts re-wetting with acid keeps the gold dissolved when we go and try to precipitate the gold from this solution, and can also give us trouble determining gold is in solution with our stannous chloride testing procedures.
With poor mans aqua regia there can be more trouble to evaporate the nitric or getting rid of the nitrate salts from solution, especially if more base metals like copper are involved, which is usually the case, as in the evaporation process copper nitrate can be formed, the copper nitrate can be baked dry with the gold salts, (evaporation does not eliminate them), re-wetting the copper nitrate salts with acid, easily forms nitric acid again in solution, so an excess of nitrate salts when using poor mans aqua regia will give more trouble to eliminate the oxidizer, (nitrates and nitric), than you would have when using nitric acid with real aqua regia (and also where we normally eliminate most of the base metals as possible before dissolving our gold).
ut the aqua regia solution into a big evaporating dish. It will contain some sediment, mostly silver chloride; this does no harm at this point, so do not filter it yet.
