Distilling Well: One Plate, Ten Plates, and the Wall You Cannot Pass

**Never heat a closed system.** There must be an opening to atmosphere somewhere. A sealed apparatus that is heated becomes a bomb, and this is the single most dangerous mistake in the whole of practical chemistry. **Never distil to dryness.** Peroxides, and in some cases residues that decompose violently, concentrate in the pot. Stop with liquid still in it, always. **Boiling chips or a stirrer bar, added COLD.** A smooth-walled flask of a superheated liquid bumps — it erupts as a slug of boiling liquid — and adding a chip to an already-hot liquid causes exactly that eruption. **Water into the condenser at the BOTTOM, out at the top.** Fed the other way it never fills and barely cools. **Secure every joint, and clamp the flask, not the condenser.** Glassware that shifts pulls joints apart. **No naked flame anywhere near a flammable solvent.** A heating mantle or a hot water bath, never a bunsen under ether. **Thermometer bulb level with the side arm**, so it reads the vapour that is leaving rather than the vapour above the liquid. Ventilation, eye protection and a clear path away from the bench.

The temperature at the still head is the whole record of what is happening, and it reads like a graph. **A slow rise, then a plateau.** The plateau is a fraction coming over, and it sits at that component's boiling point. Collect while it holds steady. **The plateau ends and the temperature climbs.** That fraction is finishing. Change the receiver. **No plateau at all, just a steady climb.** The components are too close to separate with this apparatus. Either accept a mixture or fit a column. **The temperature falls.** The heating is too low, or the vapour is no longer reaching the head. On a column this usually means it has flooded. **Collect in three parts, always.** The **fore-run** — the first few per cent, containing the lowest boiling impurities and any residual solvent. The **main fraction**, over the plateau. The **tail**, as the temperature climbs again. Combining them is how a careful distillation is thrown away. **Go slowly.** A fast distillation gives the column no time to equilibrate and loses most of its plates. One to two drops a second at the receiver is the usual guidance.

**Vacuum distillation.** Lowering the pressure lowers the boiling point, which is how a substance that would decompose at its atmospheric boiling point is distilled at all. A rough rule: halving the pressure drops the boiling point by roughly 15 C, and the first 10 mmHg matters more than everything above it. Use thick-walled glass rated for vacuum, never a flat-bottomed flask, and let it down slowly at the end. **Steam distillation.** Passing steam through carries over compounds that are immiscible with water at a temperature BELOW 100 C, because the two partial pressures add. This is how essential oils are obtained without cooking them, and the lavender blueprint is exactly this. **Short-path and Kugelrohr.** For small quantities, where the surface area of ordinary apparatus would hold most of the product. **Simple distillation is enough** when the boiling points are far apart and the impurity is non-volatile — removing a solvent, for instance, which is the commonest distillation there is and needs no column at all. Choose by the gap and by the thermal stability, and reach for a column only when the plate count says you must.