Question Video: Explaining Why the Thermometer Is Placed at the Top of the Distillation Column Chemistry

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.