Q.What is the difference between distillation, distillation under reduced pressure and steam distillation?

Concept: Separation Techniques — The choice of distillation method depends on the thermal stability and volatility of the components.

Reasoning:

1. Simple distillation separates liquids with boiling points differing by at least 25–30°C at normal atmospheric pressure. It works by heating the mixture until the more volatile component vaporises, then condensing it.
2. Distillation under reduced pressure lowers the boiling point of a liquid by decreasing the external pressure. This is used for heat-sensitive compounds (e.g., glycerol, organic peroxides) that would decompose at their normal boiling point. …

The Core Idea: Separation by Vaporisation

All three techniques rely on the same physical principle: when you heat a liquid mixture, the more volatile component (lower boiling point) vaporises first. The vapour is then condensed back to liquid and collected separately. The key difference lies in how you make that vaporisation happen — and for what kind of mixture.

Think of it like this: you want to separate two friends who walk at different speeds. Normal distillation is like asking them to walk on a flat road — the faster one pulls ahead naturally. Reduced-pressure distillation is like putting them on a treadmill with less gravity — both walk slower, but the difference remains. Steam distillation is like having a third friend carry one of them — you're not relying on their own speed at all.

1. Simple Distillation

What it does: Separates two miscible liquids whose boiling points differ by at least 25–30°C.

How it works: You heat the mixture in a flask. The liquid with the lower boiling point vaporises first, travels through a condenser (cooled by water), and drips into a collection flask. The higher-boiling liquid stays behind.

Real example: Separating acetone (b.p. 56°C) from water (b.p. 100°C). Heat gently — acetone boils off, water remains.

2. Distillation Under Reduced Pressure

What it does: Separates heat-sensitive compounds that would decompose at their normal boiling point.

The physics: Boiling point depends on pressure. Lower the pressure, lower the boiling point. By attaching a vacuum pump, you reduce the pressure inside the apparatus, so the liquid boils at a much lower temperature.

How it works: The setup is identical to simple distillation, but with a vacuum pump connected to the condenser outlet. A pressure gauge lets you control the vacuum. The liquid boils at, say, 60°C instead of 200°C — safe for the compound.

Real example: Glycerol normally boils at 290°C (where it decomposes). Under reduced pressure (say 10 mmHg), it boils at about 180°C — still hot, but stable.

3. Steam Distillation

What it does: Separates volatile, water-immiscible compounds from non-volatile impurities — without ever reaching 100°C.

The clever trick: When you mix two immiscible liquids (like oil and water), they boil at a temperature lower than either pure liquid's boiling point. This happens because the total vapour pressure is the sum of the individual vapour pressures. When that sum equals atmospheric pressure, the mixture boils.

For water (b.p. 100°C) and an organic oil (b.p. say 180°C), the mixture might boil at 95–98°C. The steam carries the oil vapour with it. On condensation, you get two layers — water and oil — which you separate using a separating funnel.

How it works: Steam is generated (either externally or by boiling water in the flask) and passed through the plant material containing the volatile oil. The steam-oil vapour mixture passes into a condenser. The condensed liquid separates into two layers.

Real example: Extracting eucalyptus oil from eucalyptus leaves. The oil is volatile but would decompose if you tried to distil it directly at 170°C. Steam distillation pulls it out at ~98°C, intact. …