Video Transcript
When performing a distillation, why
is the thermometer placed at the top of the column instead of in the solution?
Here we have two similar
distillation setups. There’s a distillation flask that
contains the sample, a fractionating column, an adapter, a condenser, another
adapter, a second flask to collect the liquid that’s been distilled, and finally a
heat source. The only difference between these
two setups is that on the one on the left, the thermometer is placed at the top of
the column and on the one on the right, the thermometer is placed in the
solution. To figure out why it would be
preferable to place the thermometer at the top of the column instead of in the
solution, let’s think about what the thermometer would be measuring for each
case.
If the thermometer is placed in the
solution, it’s going to be measuring the temperature of the solution. Since the temperature of a solution
doesn’t change while it’s boiling, this will ultimately be constant during the
experiment once the solution comes to a boil. If we have the thermometer placed
at the top of the column, there’s no way that it could be measuring the temperature
of the solution. It’s simply too far away. Instead, it will be able to measure
the temperature of the vapor after the liquid begins boiling, becomes a gas, and
makes its way up through the column.
So what do we want to measure
during a distillation experiment? The temperature of the vapor or the
temperature of the solution? As we’ve already said, the solution
temperature will be constant during the experiment. But if we have the thermometer
located at the top of the column, the temperature reading will change throughout the
course of the experiment. The temperature will increase once
vapor reaches the top of the column and we begin collecting liquid. If we then see a decrease in
temperature, that means the component that we were collecting has finished
distilling and its vapor is no longer reaching the thermometer.
And if we see an increase in
temperature, that indicates that a new component of the mixture that has a higher
boiling point has started distilling. So by measuring the temperature of
the vapor instead of the solution, we’re able to get much more information
throughout the course of the experiment. So we place the thermometer at the
top of the column instead of in the solution when we’re performing a distillation
experiment so that the temperature of the vapor is measured.