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Name: Robert
Status: other
Grade: other
Country: USA
Date: Spring 2014


Question:
Please explain how some mixtures such as isopropyl alcohol may separated by using the different freezing point of those substances water and alcohol. While salt water and also a mixture seems to have one freezing point? or does it??



Replies:
Mixtures have a freezing range, not a sharp point. There is a temperature at which solid will first form and a lower temperature at which the whole thing is solid. These temperatures depend on the composition of the mixture. This is true for salt water as well as for alcohol and water mixtures.

Richard E. Barrans Jr., Ph.D., M.Ed. Department of Physics and Astronomy


Robert,

Actually, both alcohol/water and salt/water solutions can be made concentrated (or dilute) by a process called "fractional freezing". The basic idea here is that if a solution is very slowly made to freeze, at the point that material start to solidify, there will be a mixture of liquid and solid. The solid will be richer in one of the substances (and consequently poorer in the liquid state). For example, if the temperature of a water/alcohol mixture were to be slowly lowered, at a certain point, solids will start to form. This solid will be rich in water and poor in alcohol (consequently the liquid which has yet to freeze is rich in alcohol and poor in water). If the solid is then separated, we can then do more fractional freezing of the now alcohol-rich liquid, and further solidify out more water than alcohol. After several stages, we will have obtained an alcohol rich solution (still containing water) at the expense of collecting solids that are still alcohol/water mixtures but rich in water.

This is possible because different concentrations of water/alcohol have different freezing points. This is true for all solutions however so a salt/water solution can also be fractionally frozen and made richer in salt. For some solutions this concentration technique is not practical because the difference in freezing points of the different concentrations is not sufficiently broad that a fine control of temperature is needed.

Greg (Roberto Gregorius) Canisius College


Hi Robert,

Thanks for the question. If you very slowly cool a mixture of water an alcohol, you may start to see chunks of ice floating on top. You can then pick out these ice chunks. This method is a very crude way to separate water and alcohol. If you take a closer look at the chunks of ice you recovered above, you will find that they are mostly water, but there will be some alcohol in them. For two miscible liquids, the freezing point is not a single temperature, but a temperature range which can be rather broad, say 10 degrees C.

Let us contrast the above situation with that of salt water. Salt water has one freezing point. If you cool salt water slowly, it will form ice cubes with salt on the outside. Try this in your freezer. When you lick an ice cube formed from salt water, it will taste a bit salty on the outside since the salt is excluded from the crystal. The main difference of this situation as compared to the water/alcohol situation above is that salt and water are not miscible liquids. Salt is a solid while both alcohol and water are miscible.

I hope this helps. Thanks Jeff Grell


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