[banzaiben]

In a laboratory where we were asked to distill 4-methylcyclohexene out of 4-methylcyclohexan-1-ol (after adding phosphoric acid to create the alkene), we were required to monitor the vapor temperature in a distillation head in order to determine when the distillation was nearing completion. 

My TA briefly mentioned that the reason we were all seeing a vapor temperature around 80^oC was that the mixed solvents were boiling in such a way that made a lower vapor temperature and that our temperatures could not be expected to be between 100-105^oC (the boiling point of 4-methylcyclohexene). 

My only question is: what is the name of this principle or where can I read more on it? I'm familiar with the notion that impurities in compounds will depress melting points and broaden their ranges but we've  never discussed this in the context of boiling points and every search I've done to follow up has been fruitless.  Does anyone have any leads?

[fledarmus]

What are the mixed solvents? Is this aqueous phosporic acid? Are you collecting a mixture of water and your 4-methylcyclohexene?

If so, you might want to consider http://en.wikipedia.org/wiki/Steam_distillation as a possible source for explanation...