Final answer:
During distillation, the boiling point of the liquid in the distilling flask increases as the more volatile substances boil off, leading to an increase in the concentration and boiling point of the remaining less volatile substances.
Step-by-step explanation:
During distillation, the boiling point of the liquid remaining in the distilling flask generally increases. This happens because as the more volatile component boils off, the concentration of the less volatile substances increases, hence their collective boiling point goes up. Distillation is used to separate mixtures based on differences in volatility and boiling points.
For instance, during the fractional distillation of crude oil, the different hydrocarbon fractions are separated as the temperature of the vapor rises and condenses at various levels in the distillation column. This is due to differences in the boiling points of the components, with lighter, more volatile fractions condensing at higher points, and heavier, less volatile fractions condensing lower down. The same principle applies to the distillation of a solution of table salt and water; as the water evaporates, it leaves behind the salt.