Fractional distillation is the process used to separate the many liquid components of crude oil; and nitrogen, oxygen and argon from air. Simple distillation apparatus in the laboratory can be adapted to separate mixtures of miscible liquids.
miscible describes two liquids that mix such as ethanol and water
immiscible refers to liquids that do not mix such as oil and water
When a mixture of water (bpt 100°C) and ethanol (bpt 78°C) are heated, the mixture will boil at 78 °C as the ethanol reaches its boiling point. If this mixture is distilled using standard apparatus the distillate will be high in ethanol but will also have some water in it too. This is because, although water does not boil at 78 °C there will still be lots of water molecules capable of escaping at that temperature (see Phase Transition).
As with the distillation of crude oil and air, the secret is to include a fractionating column in the apparatus. This is typically a glass tube with a high internal surface area. This provides plenty of below 100 °C places for any water molecules that have escaped to condense and drip back down into the mixture. Below is my attempt to draw this apparatus on the computer but it is possible to buy specially made glassware that snaps together rather more elegantly than my poor drawing!
Over time a range of temperatures occurs within the fractionating column from almost 100 °C at the bottom to 78 °C at the top. This allows all the ethanol vapour to escape but none of the water as it will condense on the high surface area of the fractionating column and drip back down into the flask.
The graph rises until it reaches the boiling point of ethanol. At this point pure ethanol vapour is entering the condenser. Once all the ethanol has been separated from the mixture the temperature rises again until it reaches the boiling point of water. Evaporative cooling (as discussed in the phase transitions post) prevents the temperature rising above 100°C.
If you have any questions about the process above please add them in the comments section below.
I hope you have a graduated cylinder in your drawer. I know I do!
Questions…
- Name something produced when crude oil is distilled.
- What industrial uses might any pure gases from air have?
- How could you demonstrate that sample of water was pure?
- Suggest a feature that a fractionating column has to encourage vapour to condense within it.
- What is the boiling point of ethanol?
- Why do you think I have used an electric heater in the diagram rather than a Bunsen burner?
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