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.