Question

During a fractional distillation, a student observes that the temperature of their distillate was maintained at one temperature, then suddenly dropped, before raising to a second higher temperature. What could be the cause of this drop in temperature?

Answer 1

During a fractional distillation, a temperature drop followed by a rise indicates the sequential condensation of components with different boiling points. This is due to the depletion of a fraction and the commencement of distillation of the next, higher-boiling fraction.

Step-by-step explanation:

The observation of the temperature during a fractional distillation maintaining at one temperature, then suddenly dropping before rising to a second higher temperature, can be explained by the changing composition of the mixture being distilled. During fractional distillation, components of a mixture are separated based on their boiling temperatures.

Initially, the component with the lowest boiling point will vaporize and condense, reflected in the steady temperature of the distillate. As this component is depleted, the temperature may drop momentarily before the next component with a higher boiling point begins to vaporize and condense, causing the temperature to rise again.

This sequence happens because as each fraction condenses, the composition of the remaining mixture changes, altering the vapor pressure and therefore the temperature at which distillation occurs. Additionally, inserting a cold thermometer into the mixture can cause a temporary drop in temperature as the alcohol in the thermometer initially contracts before expanding due to the heat of the liquid.