Fractional distillation

I teach about the fractional distillation of crude oil. Crude oil is a mixture of liquids with different boiling points. The bigger the hydrocarbon chain the more energy that has to be supplied to overcome the forces holding the particles in the liquid. You heat them to vapourise all fractions except one and then send them up a column away from the heat source so that the temperature drops. One by one, each fraction falls below its boiling point and condenses back to a liquid. I demonstrate with a water and ethanol mixture. Both are boiled at the bottom and both vapours head up the column. It is fixed so that the temperature at the top is 78 degrees Celsius so that the ethanol condenses there and liquefies. The ethanol flows out down the side pipe. Much of the water vapour has already condensed as it fell below 100 degrees Celsius. It dribbles back down the inside of the fractionating column. There will still be some water vapour left at 78 degrees C but the ethanol solution is now strong enough to catch fire.