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By the end of this video, you should be able to describe how
fractional distillation can be used to separate a mixture of liquids.
It is a bit of a tricky topic so you may need to watch this video a couple of times.
In the last video, we saw that simple distillation
can be used to separate a liquid from a solid if we want to keep the liquid.
First we heat our solution to evaporate the liquid, turning it into a vapour.
Then we cool the vapour to condense it back to the liquid.
We use the apparatus I am showing you here.
In fractional distillation, we separate a mixture of different liquids.
However, the key point is that these liquids must have different boiling points.
So let us see how this is done.
Here is the apparatus that we use.
Now you will notice that this is very similar to the apparatus for simple
distillation but there is one key difference.
We have here in our flask a mixture of two different liquids but this technique
would work just as well with a mixture containing lots of different liquids.
As long as these liquids have different boiling points then this technique will work.
The flask containing our mixture is attached to a long column containing hundreds of glass beads.
This is called the fractionating column.
As you will notice, the fractionating column was not used for simple distillation.
We are going to see what it does in a minute.
At the top of the fractionating column, we have a thermometer
and then we have got a condenser just like we saw with simple distillation.
Remember that the condenser contains circulating cold water.
So let us see how fractional distillation works.
Imagine that we are trying to separate two liquids:
one has a boiling point of 80oC and one has a boiling point of 100oC.
We start by gently heating our mixture just as we did with simple distillation.
Now both of our liquids will start to evaporate
but the one with the lower boiling point will evaporate more easily.
So we have a mixture of two different vapours making their way into the fractionating column.
When the vapours hit the fractionating column,
they condense and drip back into the flask where the liquids evaporate again.
This repeated evaporation and condensation increases
the amount of the lower boiling point chemical in the fractionating column.
So here, the red arrows show the lower boiling point chemical making its way up the fractionating
column ahead of the higher boiling point chemical which I am showing in orange.
As the warm vapours pass up the column, they reach
the thermometer and the temperature on the thermometer begins to rise.
This means that a mixture of two different vapours are passing over the thermometer
although the mixture will contain more of the chemical with the lower boiling point.
These vapours now pass into the condenser and they turn back to
a liquid but again this is still a mixture of the two chemicals.
There comes a point when the temperature on the
thermometer reaches the boiling point of the first chemical and it stops rising.
In this case around 80oC.
At this point, we now have mainly one chemical passing into the condenser.
So this condenses and we can collect it in a fresh beaker.
This is our first proper fraction, in other words, the chemical that boils at 80oC.
After a while the temperature on the thermometer begins to rise again and
that means that once again, a mixture of vapours is passing into the condenser.
But this time, it mainly contains the chemical with the higher boiling point.
Once the temperature becomes constant, in this case around 100oC, we know
that we are collecting a relatively pure sample of the second chemical.
So as you can see, we have now separated our two chemicals based on their boiling points.
There are a couple of facts that you need to consider.
Firstly, if the two liquids have very similar boiling points,
then it is much harder to separate them.
We might need to carry out several rounds of fractional distillation.
Secondly, the equipment I am showing you here is not really useful for separating
a very large volume of liquid for example when we refine crude oil.
In this case, we carry out fractional distillation in a slightly different way,
and you are going to see that in a later video.
The principle is the same but the apparatus is different.
OK so hopefully now, you should be able to describe how fractional distillation
can be used to separate a mixture of liquids.