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In steam distillation, water and the organic compound are immiscible, so each contributes its full vapor pressure independently. The total vapor pressure reaches atmospheric pressure at a much lower temperature than either component's individual boiling point. This allows separation of temperature-sensitive compounds that would decompose under the high heat required for simple distillation.

Miscible liquids form homogeneous solutions where each component's vapor pressure depends on its mole fraction, following Raoult's law. Immiscible liquids are insoluble in each other and contribute their full vapor pressures independently. This key difference means immiscible mixtures boil at lower temperatures, making steam distillation possible for heat-sensitive organic compounds.

A solution boils when its total vapor pressure equals atmospheric pressure. For miscible mixtures, total pressure is the sum of partial pressures calculated from each component's pure vapor pressure multiplied by its mole fraction. For immiscible mixtures, total pressure is simply the sum of each pure component's vapor pressure, allowing boiling at lower temperatures.

As steam distillation proceeds, both water and the volatile organic compound vaporize and are collected as distillate. The non-volatile materials remain in the flask. The immiscible organic component is slowly distilled along with water, while contaminants stay behind. After distillation, liquid-liquid extraction separates the organic compound from the water.

If the organic compound were soluble in water, it would form a miscible mixture where its vapor pressure depends on mole fraction, not its pure vapor pressure. This would require higher temperatures to achieve boiling. Immiscibility ensures the compound contributes its full vapor pressure independently, enabling distillation at lower temperatures suitable for temperature-sensitive molecules.

The water reservoir replenishes water throughout the steam distillation process. As water vaporizes along with the organic compound, the reservoir maintains a continuous supply so the mixture remains heated and boiling. This allows sustained distillation of the volatile organic compound until separation from non-volatile substances is complete.

The boiling point of an immiscible mixture occurs when the sum of the pure vapor pressures of each component equals atmospheric pressure. For example, benzene and water boil together at 69.3 °C, where water's vapor pressure is 227 mm Hg and benzene's is 533 mm Hg, totaling 760 mm Hg. This is far below either component's individual boiling point.