Distillation separates liquids by exploiting a simple fact: different compounds boil at different temperatures, so boiling a mixture and recondensing the vapour tends to concentrate the more volatile, lower-boiling component. Run that cycle enough times, in a proper fractionating column, and most mixtures can be pushed to very high purity. Ethanol and water don't cooperate past a certain point. Somewhere around 95 to 96 percent ethanol by weight, ordinary distillation simply stops working — not because the equipment is inadequate, but because the underlying chemistry runs out of anything left to separate.
Why boiling and recondensing usually works, and then stops working
In an ordinary mixture, the vapour produced by boiling is enriched in the more volatile component compared with the remaining liquid — that gap between vapour composition and liquid composition is exactly what a still exploits, condensing the vapour into a stream purer than what it started from, then repeating. As an ethanol-water mixture gets more concentrated in ethanol, though, that gap keeps shrinking, and at a specific composition it closes entirely.
The point where vapour stops being any different from the liquid
At that specific ratio — called an azeotrope — boiling the liquid produces vapour with exactly the same composition as the liquid it came from. Condense that vapour and reboil it, and nothing changes; there's no further enrichment left to extract, because the vapour was never different from the liquid to begin with at that point. This is precisely why standard distilled spirits and lab-grade ethanol from ordinary distillation top out close to the mid-90s percent range rather than reaching 100 percent — the azeotrope is a genuine thermodynamic ceiling, not a limit of technique or patience.
What we're still unsure about
Breaking an azeotrope to reach fully anhydrous ethanol is possible, but it requires abandoning ordinary distillation for a different technique altogether — adding a third substance that changes the mixture's behaviour, exploiting the fact that some azeotropic compositions shift under different pressures, or using molecular sieves that physically trap water molecules. None of these is a universal best choice. Each trades off energy cost, added chemical complexity, achievable purity, and safety differently depending on the specific mixture and the scale of the operation, which is why choosing between them remains a genuine, case-by-case engineering judgement in chemical plant design, rather than a single settled best practice that applies everywhere.
This sits inside Mass Transfer & Separation Processes, one of eight topics in Chemical Engineering, one of four domains in Engineering, one of seventeen subjects the app can quiz you on.