#2388. Fractional Distillation

Science, level: Middle
Posted Sun Sep 23 18:15:28 PDT 2001 by Dave Lane (dave.lane@uq.net.au).
Wavell State High School, Brisbane, Australia
Materials Required: alcohol, water, glycerol, good quality (pyrex) test-tube, Bunsen burner, test-tube clamp.
Activity Time: 10 minutes
Concepts Taught: Separation of substances using differences in boiling point

Because of the difficulties and risks associated with the fractional distillation of mineral oil mixtures, I concocted the following procedure as a simple introductory demonstration before proceeding to fractional distillation of the same mixture using standard quick-fit appratus.

Preliminary discussion/demonstration

Explain (or demonstrate) that alcohol and glycerol will burn - eg small amount on glass wool held over the Bunsen burner.
Note (or demonstrate) that the three liquids have different boiling points.

The Demonstration

Mix small (>10 mL) equal quantities of the three liquids (water, alcohol, glycerol) in a test-tube.

Heat over the Bunsen burner. Ignite the alcohol as the mixture begins to boil. (Take care!)

Continue heating. The alcohol flame extinguishes as the mixture continues to boil and steam is produced.

Continue heating. The glycerol now ignites. Allow to burn for a few seconds and then extinguish the flame by covering the mouth of the test-tube with card. Set aside to cool. Turn off the Bunsen.

Hints:

The flame produced is affected by the rate of heating. Adjust accordingly. This is a demonstration that must be rehearsed. Workplace Health & Safety/ Risk Analysis should be completed beforehand.