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If a mixture of water with acetone, methanol, ethanol, or other water miscible solvent, is cooled below 0 degrees, would the water freeze and aggregate out as ice, separating out of the mixture. Or would the frozen 'water particles' be dissolved in the solvent, similar to how solid organics dissolve in solvents?

I suppose it really depends on the concept of solubility vs miscibility. I've been researching this for a while now and haven't been able to find a clear definite answer.

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The short answer is


This is exactly how acetic acid is purified: Acetic acid freezing distilation

In this case, acetic acid (freezing point $16\ ^\circ \text{C}$) freezes first, leaving a water-enriched fluid behind. Conceivably you could remove water from methanol or acetone in the same way (except that water would freeze first). In practice, fractional distillation can be done more economically on a large scale. Fractional distillation requires 1 step (with lots of theoretical plates), while you need to repeat fractional freezing multiple times to get to high purity. Acetic acid is purified by fractional freezing perhaps because it may azeotrope with water.

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Can you explain why you would need to repeat the freezing multiple times to get a high purity? Assume we want to purify the solvent and not the water. Wouldn't the freezing only need to be repeated to purify the frozen (water), not the solvent? Some solvent would be trapped in the ice, sure, which would be scooped out, but pretty much all of the water would freeze into ice, and float to the top (if solvent is dense), allowing it to be removed. If all ice is removed this way, how does water remain in the solvent? If the liquid isn't dense, the mass could be frozen and solvent decanted off. – Tyliero Aug 24 '14 at 21:43
It isn't as simple as the water simply freezing out. A mixture of miscible liquids typically doesn't behave as the separate components. At the point at which freezing begins (which may be higher or lower than that of either component), both the frozen material and the liquid still contain both components. The frozen is enriched in the higher melting point component and the liquid is depleted. Barring a eutectic point, one can remove the solid and freeze the liquid again to enrich it further, repeating until the desired purity is attained. – Michael D. M. Dryden Aug 24 '14 at 23:39

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