Distillation

Section 2 of 4

Distillation separates liquids by their boiling points: boil the mixture, catch the vapour and cool it back into a liquid.

Wine is about 12% ethanol, and nearly all the rest is water. Ethanol boils at 78 °C and water at 100 °C. As wine heats up, ethanol molecules escape into the vapour more easily than water molecules, so the vapour that rises is richer in ethanol than the wine it came from.

In a still, the vapour flows into a condenser, a tube with cold water flowing round it. There it cools and condenses back into a liquid, the distillate, which drips into a second flask. A thermometer at the top of the first flask reads the temperature of the vapour as it leaves.

Distilling wine: boil, cool, collect A round flask of wine is heated from below. A thermometer in the top of the flask reads the temperature of the vapour as it leaves. The vapour passes into a sloping tube, the condenser, which has a jacket of cold water round it: cold water flows in at the lower end and out at the upper end. The vapour cools, turns back into liquid, and drips from the end of the condenser into a second flask as the distillate. wine cold water in water out condenser thermometer distillate heat
A still for wine. The vapour that boils off is rich in ethanol, and the condenser cools it back into a liquid, the distillate.

Brandy is distilled from wine, and whisky from a beer-like brew of malted barley. Each is watered down to about 40% ethanol for the bottle. Distil sea water and the salt stays in the flask, while pure water drips out of the condenser. Some ships and desert countries make their fresh water this way.

One distillation only partly separates two liquids. A fractionating column, like the tall tower that sorts crude oil, repeats the boiling and condensing many times on the way up, and brings ethanol up to about 96%. There it stops: at 96% ethanol and 4% water, the vapour has the same make-up as the liquid.

Steam distillation: scents from plants

The smells of lavender flowers and orange peel come from oily molecules held in tiny pockets in the plant. Lavender oil is mostly linalool and linalyl acetate, an ester like the ones that give bananas and pears their smell. Orange peel oil is about 90% limonene.

(R)-limonene, C₁₀H₁₆The main molecule in orange peel oil. A six-membered carbon ring with one C=C double bond, a methyl group on one end of the double bond, and on the far side of the ring a three-carbon branch, -C(CH₃)=CH₂.
(R)-⁠limonene, C₁₀H₁₆, the main molecule in orange peel oil. It boils at 176 °C, and it comes over with steam below 100 °C.

These molecules boil at about 175 to 220 °C, and many scent molecules break down when heated that hot. In steam distillation, steam is blown through the flowers or peel. The oil and the water stay apart as two separate liquids, so each one sends its own molecules into the vapour, and together they boil a little below 100 °C. The steam carries the oil into the condenser. In the collecting flask, the oil floats on the water in a thin layer and is drawn off. The water keeps a faint scent too, and it is sold as lavender floral water.

Most orange oil is simply pressed out of the peel, and some is steam distilled from peel left over from making juice.

Quick check

You heat wine in a still. What collects first in the flask at the end of the condenser?