Distilling flasks are glass flasks used to separate mixtures by distillation. A distillation flask has three parts: a spherical base, a cylindrical neck and sidearm that is connected to the flask neck. Distillation is the process of separating components of a mixture based on differences in the components' boiling points.
Distilling flasks are typically made from Pyrex, a glass material that can withstand high heat. This makes them ideal for the distillation process, which requires the contents of the flask to be heated. As the components are heated, each changes in turn from a liquid to a gas. The molecules then move through the sidearm of the flask into a condenser.