Bubble Point: The bubble point of a system at pressure P and whose liquid composition is given by mole fraction x_i is that temperature at which there is onset of vaporization. Mathematically it is the temperature such that

For a binary mixture, this equation can be rearranged to yield

Example 6.2: Calculate the composition of a liquid of an ideal binary mixture of benzene and toluene at 760 mm Hg at a bubble point of 90 °C. The following data apply:

Vapor pressure of benzene = 1.02´10³ mm Hg

Vapor pressure of toluene = 225.1 mm Hg

Solution: At boiling point vapor pressure is equal to the total pressure. Using this definition, boiling point of benzene and toluene can be found to be 80.1 °C, and 130.13 °C, respectively. The bubble point of a mixture can be found from Equation 6.7

x can be found by using Equation 6.8

The results are summarized below:

t/°C	P1/ (mm Hg)	P2/ (mm Hg)	K1	K2	x
90	1.02´10^3	225.1	1.34	0.296	0.672