When should a stopper be placed on a flask during the distillation process?

A stopper should be placed on a flask during the distillation process after the distillation is complete for several reasons. This practice helps to prevent the escape of vapors and ensures that no contamination occurs during the cooling phase of the distilled material. Once the distillation has reached the desired endpoint and the liquid has been collected, a stopper can safely be placed to maintain the integrity of the sample and prevent exposure to external substances or environmental factors that could alter its composition.

Placing a stopper at other times, such as immediately after heating begins or as soon as the first droplet appears, could be problematic. If the flask is sealed too early, pressure could build up due to vapor formation, potentially causing hazardous situations or accidents. Similarly, securing the stopper before the mixture is heated may lead to issues with vapor release and pressure management, which is critical in a distillation setup. Putting a stopper on during any stage before the process has been fully completed can hinder the distillation by preventing effective condensation and retrieval of the distillate.