Chemistry - interpreting Fractional Distillaion.

[SA32]

I did fractional distillation to separate two solvents in my chem lab last week and I was just wondering if someone could check my thinking.

So for the first little while after the distillation began, no solvent collected in the receiving flask because the temperature was still making its way up to the more volatile component's boiling point. When the temperature steadied out (at the more volatile component's boiling point) and became more or less constant, the receiving flask started collecting solvent. When the temperature began to increase rapidly, I replaced the receiving flask with an empty flask to collect the second component. Soon the temperature steadied out and became relatively constant, and the flask began to collect the second solvent.

I was wondering why we see this steady, constant temperature, followed by a sharp increase, and then another period of constant temperature. Is it because during these periods of constant temperature, the components are in the process of changing from a liquid to a vapour, and temperature remains constant during a phase change?

Thanks!

 

[chemisttree]

No, the reason for the steady temperature is that only one component is making it to the thermometer bulb (and the condenser). When the temperature is rising you have a mixture of the lower and higher boiling solvents coming over and the ratio of the two is changing. When the higher boiling solvent pushes out the last remains of the lower boiling fraction the temperature settles out again, reflecting the boiling point of the higher boiling component.
Just a note, when the temperature is changing, you should collect that fraction separately from the lower boiling one and the higher boiling one since it is a mixture. You won't collect much if you are doing things properly and your second 'pure' fraction won't be contaminated with traces of the lower boiling component.