I understand that boiling water with salt in it, you end up with just salt. But, How does it work with two liquids? Wouldn’t they just mix and end up with the same boiling point?
In: Chemistry
No.
When you mix alcohol and water, as you warm the solution the alcohol evaporates first. If you carefully control the temperature and cool the gas released into a different container, you can separate the alcohol in a new flask while leaving the water in the initial container – just like the salt.
If you do this while raising the temperature, you boil the alcohol and the water, leaving the initial flash empty and the second flask with the same mixture. That’s just not doing distillation by fraction correctly.
That would be how distillation works, for example alcohol and water. The liquid that boils at a lower point will tend to boil off first, so for example distilling alcohol and water you may double the concentration of alcohol because alcohol boils first. You can then do it again and again each time getting a higher concentration of alcohol. However you are correct in the end they will boil at the same rate and you can’t get a higher concentration, this would be called an azeotrope.
Both liquids will evaporate at the same time, one will just evaporate faster. If you want to separate the two liquids you need to find out what the dew point of each liquid is and cool down the vapor to whatever has the higher dew point first.
It *can* get more complicated, but not always. However, in general, it’s pretty simple. One of the components boils and the other does not.
In reality both components produce vapor, one just produces *more* vapor. Your distillate will favor the one with a higher vapor pressure, but you get both.
By doing multiple distillations you can improve your results, to a point. Some substances do actually ‘seek out’ a particular mixture when you distill them. Water and alcohol, for instance, and this makes alcohol distillation above 96% impossible.
Even when you “mix” two liquids, their individual particles (molecules) don’t change. Picture a ball pit with red and yellow balls. No matter how much you mix them up, you’re never going to get orange balls, you just get a jumble of red balls and yellow balls. If the red ones boil at 40 degrees and the yellow ones at 75 degrees, then you can raise the temp to 50 and the red ones will (mostly) evaporate and the yellow ones (mostly) won’t.
Usually liquids are chemically bonded together, it’s just a mixture of the two liquids, each retaining their own boiling point. If one of the liquids has a sufficiently lower boiling point than the other, you can heat the solution to be hotter than one liquid’s boiling point, but not the other, resulting in one liquid boiling off. This process isn’t perfect and some of the other liquid does escape, which is why often you distill something multiple times until you reach the desired purity.
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