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I am aware that in theory the position of the thermometer needs to be right at the transition point of the vapor into the condenser. In practice,I don´t reach the exact boiling temp. at this height but rather 2-3 °C less. When I put it down further it goes up to the correct one. In some distillation setups ther is quite a huge distance between liquid and condenser (20 cm) and the vapor condenses already on the thermometer.
What does speak against a lower position? It is still going to read the temperature of the currently boiling fraction? But we were all taught that one must not do this.

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The temperature drop is obviously due high heat loses on the way up. It is usually minimized by proper thermal isolation of the upward path. The favorite way in student labs is wrapping the glass by $\ce{Al}$ foil (IR reflection), cotton wool or PU foam as thermal isolation and another foil to wrap it, all tied by a thin rope or adhesive tape.

But, you are not really interested in the temperature near boiling liquid. You are interested in temperature of vapor before leaving to the condenser.

If e.g. water in the vessel is already boiling, but the thermometer shows just $\pu{90 ^\circ C}$, it means vapor is condensing on its well cooled way to the thermometer. At this point, the ascending path is acting like a trivial distillation column and you are measuring the temperature at the "column head", which has not been reached by the vapor fully yet, and part of the gas is still air (or more volatile liquid).


The liquid has temperature of its boiling point.

The gas at a given point has initially temperature gained by convection of moist air. At places with the local temperature below the local dew point, vapor condensation occurs and the local composition of the gas reflects the local liquid saturated vapor pressure at given temperature.

At surface colder than the boiling point, there is ongoing vapor condensation and surface warming until local vapor partial pressure meets the vapor pressure at surface temperature. Air or more volatile liquid vapor is present to top the total pressure to be equal to external pressure.

If there is too little of more volatile liquid, boiling does not keep pace with the heating and the temperature of liquid starts to raise to keep the needed partial pressure of vapor. Temperature of liquid will progressively raise, as it boiling point progressively raise, while temperature in the head stagnates, until liquid reaches the new boiling point. Soon after, the head temperature steeply raise toward the new boiling point.

Whatever you know about the column distillation, is applicable on the simple distillation as well, considering it is a very, very poor distillation column.

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  • $\begingroup$ So without insulation the correct temperature is at a lower point. My question now is how does the liquid differ from the vapor and why. Shouldn´t it be 100 °C in both? and if there was a higher boiling second phase, the temp. still would only reach 100 °C until the water is gone? $\endgroup$
    – Lukas4235
    Dec 5, 2023 at 10:36

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