Reflux is the process of boiling reactants while continually cooling the vapor returning it back to the flask as a liquid. It is used to heat a mixture for extended periods and at certain temperatures. The reflux apparatus is shown below. A condenser is attached to the boiling flask, and cooling water is circulated to condense escaping vapors. One should always use a boiling stone or a magnetic stirrer to keep the boiling solution from "bumping."

     If the heating rate has been correctly adjusted, the liquid being heated under reflux will travel only partly up the condenser tube before condensing. Below the condensation point, solvent will be seen running back into the flask; above it, the condenser will appear dry. The boundary between the two zones will be clearly demarcated, and a reflux ring or a ring of liquid will appear there. In heating under reflux, the rate of heating should be adjusted so that the reflux ring is no higher than a third to a half the distance to the top of the condenser. The temperature of a reaction in a refluxing mixture will be approximately the boiling point of the solvent used for the reaction.