Why solutions boil at different points, but freeze at one point?

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Offline nuclear

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For example, we have liquid solution of water and alcohol. When we warm it, first the alcohol boils at 78 degrees, and the remaining water boils at 100 degrees. The same is, if we liquefy a gas mixtures - for example the air. Each of it's parts has a different boiling point. So each fraction will liquefy at different temperature. So they can be separated.
But when we cool the solution, it freezes (both liquids freeze together) at a temperature between the freezing point of both pure liquids, depending on their concentration?
In other words, why they freeze together (example: The antifreeze in the cars - it's freezing point depends of the concentration of water in the antifreeze), but boil one by one (for example, water and alcohol)?


Offline Soul Surfer

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Your assumption is untrue there is no difference between boiling or freezing solutions.  For example when salt solutions and water alcohol/antifreeze mixtures freeze the first things to crystallize out are pure water and pure water can be created or the concentration of salt/alcohol/antifreeze in the solutions can be increased in this way.

Freezing has been used as an illicit method of concentrating alcohol in fermented liquor beyond that possible by fermentation but it has the big disadvantage in that it does not remove any methyl alcohol in the liquor.  Methyl alcohol is a very dangerous poison and causes death or blindness in people who consume strong drinks from which it has not been removed.
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