Steam distillation

Steam distillation separates heat-sensitive, water-insoluble compounds by vaporizing them alongside water vapor, often below their normal boiling points.

Dalton's law of partial pressures: The total pressure of a gas mixture equals the sum of the partial pressures of its components. Water and the organic vapor jointly reach atmospheric pressure before either pure liquid reaches its boiling point.
Essential oil: A concentrated mixture of volatile compounds produced by plants, often responsible for their characteristic aromas. Steam distillation is a standard way to extract many plant essential oils.
Immiscibility: The inability of two liquids to mix into a single homogeneous phase. The target compound must form a separate liquid phase from water for the classic method.
Simple distillation: A distillation method that separates a volatile liquid from less volatile components using one vaporization and condensation step. It is unsuitable when a high boiling target would decompose before vaporizing.
Vapor pressure: The pressure exerted by a vapor in equilibrium with its liquid or solid at a given temperature. The combined vapor pressures determine the temperature at which the mixture boils.
Eugenol: A fragrant phenolic compound found in clove oil and other plants. Clove oil can be isolated from plant material by steam distillation.
Volatility: A substance's tendency to vaporize, commonly reflected by its vapor pressure. The target must contribute enough vapor to be carried over with steam.
Vacuum distillation: Distillation performed below atmospheric pressure to lower boiling temperatures. It lowers boiling temperatures by reducing pressure, unlike steam distillation's co-vaporization mechanism.
Heterogeneous azeotrope: A boiling mixture whose vapor condenses into two immiscible liquid phases. Steam distillation vapor often condenses into separate water-rich and organic-rich layers.
Limonene: A volatile cyclic terpene found in citrus peels and other plants. Steam distillation can recover limonene from citrus material.

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