Distilling Isopropyl Alcohol

As a follow-up to last time, when we separated a mixture of sand, salt, iron filings, and rice, today we separated a mixture of two liquids: water and isopropyl alcohol. We took 10 mL from a bottle of rubbing alcohol, which is 70% isopropyl alcohol in water, and ran a distillation in order to separate it into its pure components. Distillation is a separation technique which takes advantage of liquids having different boiling points. Because IPA boils at a lower temperature than water, we were able to boil the mixture and collect the vapor using a condenser.

From 10 mL of the mixture, we were able to recover about 3 mL of isopropyl alcohol. Because no separation technique ever recovers 100% of the components, the students were not surprised to learn that we did not get back all 7 mL of alcohol. What was surprising was that the recovered alcohol was only about 91% instead of 100% pure. This is because some of the water evaporated with the alcohol, so the vapor we condensed was actually a mixture of the two. In addition, it turns out that isopropyl alcohol and water form an “azeotrope,” which is a mixture that, when it boils, produces a vapor with the same composition as the mixture. Thus, it is impossible to completely separate isopropyl alcohol from water using simple distillation; the best we can ever get is a more concentrated alcohol at 91% instead of 70%.