Sulfuric acid has a boiling point of 337 °C (639 °F), while hydrofluoric acid has a boiling point of 19.5 °C (67.1 °F). This means that hydrofluoric acid will completely evaporate at a much lower temperature than sulfuric acid.
In fact, hydrofluoric acid is one of the few acids that can evaporate at room temperature. This is due to its high vapor pressure, which is a measure of the tendency of a liquid to evaporate. The vapor pressure of hydrofluoric acid is 3.2 kPa at 25 °C, while the vapor pressure of sulfuric acid is only 0.01 kPa at the same temperature.
This means that hydrofluoric acid will evaporate much more readily than sulfuric acid, even at room temperature. Therefore, it is not accurate to state that sulfuric acid combined with hydrofluoric acid completely evaporates at high temperatures. While sulfuric acid does eventually evaporate at high temperatures, hydrofluoric acid will have already completely evaporated at a much lower temperature.
