H2SO4 has a very high boiling point, so you probably will not get it to boil very easily, if it is dilute its boiling point is much lower (water boils @ 100 deg C) as the concentration of sulfuric acid is increased so is the boiling point of solution, no need to boil strong heating will work just fine, HCl has a much lower boiling point. so if there are traces of HCl or chlorides (chloride salts would form HCl in the sulfuric acid, the sulfuric will make sulfates of most metals which were previously chlorides, if these were in solution), the HCl will vapor off as you heat the solution if sulfuric is the majority of solution (once the HCl has concentrated to its azeotrope in solution it can basically be removed, if gold did dissolve it would precipitate back out after the other volatile acids were driven from solution as vapors, to dissolve iron you want the sulfuric to be dilute to get iron into solution, iron will not dissolve in a concentrated sulfuric, there are iron oxides that will not dissolve in acids and can be very difficult, to get into solution (these are also insoluble in acid that dissolve our gold, and can remain as red rouge powders). note also if you dissolve iron into this dilute sulfuric acid solution this would also form ferrous sulfate in solution, copperas would precipitate gold if it was in solution), this dilute sulfuric would not put silver into solution like a concentrated sulfuric could.
