Question Video: Separating and Collecting Four Liquids That Are Mixed Together Depending on Their Boiling Point Using the Fractional Distillation Technique Chemistry

The given table shows the boiling points of four liquids that are mixed together. a) In what order would each liquid be collected if fractional distillation was used to separate them? b) Ethanol has a boiling point of 78°C. Why would fractional distillation Not separate ethanol from the mixture?

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Video Transcript

The given table shows the boiling points of four liquids that are mixed together. In what order would each liquid be collected if fractional distillation was used to separate them?

Distillation is a technique that can be used to separate liquids by their boiling point. Fractional distillation, in particular, is great at separating liquids whose boiling points are somewhat close together, like we see in this problem. The boiling point of a substance is a reflection of how easy it is for molecules of that substance to leave the liquid and become a gas. That means that the liquids in our mixture will be collected in order of increasing boiling point since the liquids that have a lower boiling point are more likely to become a gas first.

So that means to answer this part of the problem, we simply need to put our liquids in order of increasing boiling point. The lowest boiling point of our list is 65 degrees Celsius, which corresponds to methanol. The next lowest is 77 degrees Celsius, which corresponds to ethyl ethanoate. Next is 98 degrees Celsius, which is the boiling point of 2-butanol. And then our highest boiling point is 118 degrees Celsius, the boiling point of ethanoic acid. So if we distill this mixture of liquids, the order that we collect them in would be methanol, ethyl ethanoate, 2-butanol, and finally ethanoic acid.

Ethanol has a boiling point of 78 degrees Celsius. Why would fractional distillation not separate ethanol from the mixture?

Though fractional distillation is quite good at separating a mixture of liquids whose boiling points are close together, it’s not perfect. The boiling point of ethanol, 78 degrees Celsius, is quite close to the boiling point of another liquid in our mixture, ethyl ethanoate, which has a boiling point of 77 degrees Celsius. Since these two liquids have a boiling point that’s so close together, they would boil at about the same time, which means that when we perform distillation with a mixture of these two liquids, we would be collecting them at about the same time as well.

Fractional distillation is simply not capable of separating liquids whose boiling points are only one degree Celsius apart. So the reason that fractional distillation could not separate ethanol from the mixture is because ethanol has a very similar boiling point to ethyl ethanoate.

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