The temperature at which bubbles of vapour first appear on heating a liquid. For single component mixtures the bubble point and the dew point are the same. For a mixture, the vapour will have a different composition from the liquid. Together with the dew point at different compositions these provide useful data when designing distillation systems. At the bubble point:
where Ki is the distribution or K-factor and is the ratio of the mole fraction in the vapour phase, y, to the mole fraction in the liquid phase, x, at equilibrium.