Question

In the distillation apparatus, it is important that the bulb of the thermometer is placed below the sidearm of the still head. Briefly explain why the thermometer placement is so important.

Answer 1

Before answering this question, it would be wise to review essential basics especially related to distillation

BASICS OF DISTILLATION

It is the method to separate substances (liquid from dissolved solids or low-boiling liquid from the high-boiling liquid) based on their differences in boiling point. The substance having low-boiling point turns into vapors. These vapors enter the side-arm leading to the condenser which is kept at a much lower temperature through the continuous in- and out-flow of water outside it. The heat is controlled based on the temperature reading so that less volatile substance remains in the mixture until a more volatile substance turns to vapors, enters the sidearm, and gets separated through condensation. All this happens at the boiling point of a substance being separated. If the temperature is higher, the vapors of two or more substances may enter the sidearm.

ANSWER TO THE QUESTIONS ASKED

Refer to the attached picture of the distillation apparatus and the explanation below.

The vapors must enter the sidearm for them to be condensed. The temperature needs to be controlled such that vapors of only one substance enter the side-arm. If the bulb of the thermometer were placed lower, the temperature at the sidearm will be lower than the boiling point of the substance being separated because the heat source is at the bottom and the temperature decreases as one go to the top. It would become difficult to calibrate the apparatus in the sense that the temperature needed would be higher than the boiling point of a substance so that when the vapors reach the sidearm, the temperature does not drop below their boiling point.

Similarly, if the bulb of the thermometer were kept higher, it would also need calibration as the temperature recorded would be lower than that of the sidearm. The wrong calibration may lead to the vaporizing of two or more substances, especially in the case of fractional distillation in which the difference in boiling points is less than 25⁰C.

In addition, the calibrations that will be required to be done in the case of differing thermometer placements will be dependent on conditions and the size of the apparatus.

So, to avoid the above confusion and make the process more scientific as the boiling point has a direct connection with vapors and vapors must enter the sidearm, the bulb of the thermometer should be just below the sidearm of the still head. This way, the heat from the bottom can be controlled such that the temperature does not rise appreciably above the boiling point of the liquid until it is separated.