MODEL RELEASED. Rotary evaporator. Chemist using a rotary evaporator. This device allows a solvent to be evaporated from a solution at a low temperature. The solution is in the round bottomed flask at right. The flask is sitting in a hot water bath. A vacuum pump (centre right) lowers the pressure in the flask, allowing the solution to boil at a lower temperature than normal. To prevent violent boiling the flask is continuously rotated. The solvent evaporates and rises through the tube. As it rises it cools and condenses back into liquid and is collected in the flask at lower centre.