Why don't water and alcohol separate?

Why don’t water and alcohol separate? What keeps them so tightly bound together in drinks and in water/isopropyl 30/70 blends used in rubbing alcohol.

Alcohols by definition have a -OH group at the end of the rest of the molecule. The -OH group is polar and makes the alcohol molecule soluble in H[sub]2[/sub]O.

Because both are polar molecules. And in chemistry, “like dissolves like”. The OH group on alcohol (the formula for ethanol is H[sub]3[/sub]C-CH[sub]2[/sub]-OH) has a slightly negative charge, and water (H-O-H) has slightly positive charges on the hydrogens and a slightly negative charge on the oxygen. These slight electric charges keep everything attracted to each other.

Hoever, if you do distillation, you can separate the two. But only to a certain percentage–I beleive its 195 proof. That’s because water and ethanol form an aezeotrope. I could go into it more, but it’s been 2 years since I took that chem. engr. course, and I don’t have the text handy. I’m sure someone else will come along to explain.

I think the azeotrope is closer to 95% (190 proof), which is why grain alcohol is 95% (or used to be).

You can distill to higher concentrations by several means, none of which are cheap. You can “get around” the azeotrope by distilling twice at different pressures, you can use molecular sives that “Absorb” the water out of the alcohol.

My question is why don’t the oxygen and nitrogen separate in the atmosphere?

I remember 95% being the limit with ordinary distillation. I also recall that alcohol and water mix so well that the volume of the mixture is slightly less than the sum of the volumes of each separately–something like 50 ml of alcohol mixed with 50 ml of water yields 98(?) ml of mixture.

Water forms strong hydrogen bonds. Alcohol forms strong hydrogen bonds. The two liquids form strong hydrogen bonds with each other. To separate the molecules you have to disrupt this network of bonds. That takes more energy than is normally available in solution, so the two compounds remain as a mixture.

Because they’re both symmetrical, diatomic molecules, and so neither one has any polarity. Just like polar compounds mix together, nonpolar ones do too.

But I think the more important reason is that, for the most part, gases all just mix together. I’m guessing that in a gas, the molecules are moving faster and not held together very tightly, so the mixing effect of entropy is stronger than any polarity difference. Polar water vapor mixes in with the rest of the primarily nonpolar atmosphere just fine.

Exactly correct. Gases are going so fast relative to the strength of the forces that might serve to keep them together or apart. In fact, the only force that has much of an effect on gases is gravity (good thing, as that keeps the atmosphere on Earth). I have to say all of this with fingers half-crossed, though, because sometimes gases can be affected by electromagnetism, but not normal (dare I say ‘Ideal’ :)) gases under STP (Standard Temperature and Pressure, or zero degrees Celsius at one atmosphere).

At other (lower, or very much higher for plasmas) temperatures or other (much, much higher) pressures, the rules of thumb (and the Ideal Gas Law) begin to break down.

that they act a lot like pinballs at the molecular level, no mutual forces of any consequence between them.

I can write coherent, complete sentences sometimes.


actually the more interesting question from a chemical point of view is why do two liquids or solids separate in the first place, but thats another thread