[odie5533]

On a quiz question it asked what the boiling point of an equimolar mixture of cyclohexane and toluene was given that the BP of cyclohexane is 84 C and the BP of toluene is 111 C. The choice were "below 84 C", "between 84 C and 111 C", and "above 111 C."

I chose between 84 C and 111 C. If I heated the mixture to 84 C some of it would boil, but the toluene would not be boiling. And after 111 C seems wrong, because the mixture would be already boiling, so it's not really a boiling point. My professor said the correct answer was below 84 C because of the physical chemistry of the mixture. I can understand that the mixture might have stronger bonds holding it together if it was homogeneous, and that since it was a mixture of the two the bonds between molecules wouldn't sit the same thus requiring less energy to boil. But my problem with that answer is that only part of the mixture boils below 84 C. The toluene is still going to be there at that temperature, and will be until 111 C. Is the boiling point of a mixture defined as the point when vapors first are released (84 C), when they are last released (111 C), or the range over which the entire mixture will vaporize (84 - 111 C)?

[camp]

I hate to tell you this but its all relative to the species present.  Look up liquid-vapor phase equalibria and azeotropes.  You will get a much better understanding after you study those.

[Yggdrasil]

The definition of boiling point is the temperature at which the vapor pressure of the solution equals the ambient pressure.  For a mixture of toluene and cyclohexane, this will occur at below 84^o and you will see bubbles of vapor forming in your solution (i.e. boiling).  The composition of the vapor (i.e. the fact that it contains very little toluene) is irrelevant to the concept of boiling.