Heteroazeotrope

When we mix different liquids together, sometimes they act funny and don't separate like they're supposed to. This can happen when we mix two liquids that have different boiling points.

If we heat up this mix of liquids, the liquid with the lower boiling point will turn into a gas first, and the liquid with the higher boiling point will stay behind. But sometimes, the gas that forms will have a different composition than the original mix of liquids, which means it won't behave like a normal mixture when it cools down again. This is what we call a "heteroazeotrope."

For example, if we mix water and ethanol, they normally separate if we heat them up. The water turns into steam, and the ethanol stays behind as a liquid. But if our mix of water and ethanol has a certain proportion of 95.6% ethanol and 4.4% water, something strange happens. When we heat up this mixture, the steam that forms will also have 95.6% ethanol and 4.4% water, instead of just being pure water vapor. This means it will behave differently than a normal water-ethanol mix when it cools down again, in ways that scientists can study and use for things like making certain chemicals or cleaning products.