In a microscale distillation set-up, where should the tip of the thermometer be placed to accurately measure the temperature of the vapor being distilled?

• A. Inside the receiving flask
• B. Fully immersed in the boiling liquid in the distillation flask
• C. Just below the sidearm of the distillation head, where vapor enters the condenser
• D. At the very top of the distillation head, far from the vapor path

Answer: (C)

Verified step by step guidance

1. Understand the purpose of the thermometer in a distillation setup: it is to measure the temperature of the vapor that is about to condense, which reflects the boiling point of the substance being distilled.
2. Recognize that placing the thermometer inside the receiving flask will measure the temperature of the condensed liquid, not the vapor, which is not useful for determining boiling point.
3. Note that immersing the thermometer fully in the boiling liquid in the distillation flask measures the liquid temperature, but this may not accurately represent the vapor temperature, especially if the liquid is a mixture.
4. Identify that the best position for the thermometer tip is just below the sidearm of the distillation head, where the vapor passes before entering the condenser; this placement ensures the thermometer measures the temperature of the vapor phase.
5. Avoid placing the thermometer at the very top of the distillation head, far from the vapor path, because it will not be in contact with the vapor and thus will not give an accurate temperature reading.