The entire lower boiling components A distill at it's boiling point until it is removed from the mixture then then the higher boiling components B distill at it's boiling point. Ideal separation are not achieved when separating mixture of compound with boiling points closer together than 10 c. This is because the component B has an appreciable vapor pressure at the boiling point of component A. In a laboratory situation one can plot the volume of distillate vs temperature of the distilling vapor to determine how closely a distillation resembles an ideal separation