Final answer:
The thermometer in a distillation apparatus should be positioned so that its bulb is level with the side arm of the distillation head, within the path of the vapors. An alcohol thermometer contracts when first placed in hot liquid because the glass expands before the alcohol, but then it rises as the alcohol heats up faster. Proper placement of the thermometer is vital for accurate distillation control.
Step-by-step explanation:
The correct position of the thermometer within the three-way adapter segment of a distillation apparatus is crucial for accurate temperature readings during the distillation process. The bulb of the thermometer should be placed at the location where the vapor is expected to be at its boiling point, ensuring that it measures the temperature of the vapors and not the liquid. It's usually positioned so that the bulb of the thermometer is level with the side arm of the distillation head, where it extends into the path of the rising vapors. This setup ensures that the recorded temperature accurately represents the temperature of the vapor being condensed and collected.
When a cold alcohol thermometer is placed in a hot liquid, the column of alcohol contracts initially because the glass bulb heats and expands before the alcohol inside does. Once the alcohol warms and expands at a greater rate than the glass, the column rises, indicating a temperature increase. This effect is due to the different thermal expansion rates of the glass and the alcohol.
Each component of a distillation apparatus, including the thermometer, plays a specific role in ensuring the efficient transfer of heat and the purity of the distilled substance. The thermometer must be properly placed to achieve accurate readings for the distillation to be controlled and effective.