The vapor pressure of a pure substance is the pressure exerted by the gaseous phase on the liquid or solid phase in a closed container.
A liquid boils when the vapor pressure of the liquid is equal to the atmospheric pressure.
A miscible solution contains substances that can form a homogeneous solution, whereas an immiscible mixture is composed of substances that cannot be mixed, and therefore form a heterogeneous mixture.
In an immiscible mixture, the sum of the individual pure component vapor pressures equals the total vapor pressure of the solution.
The steam distillation setup has a water reservoir, which lowers the boiling point of the mixture by forming an immiscible mixture with the organic compounds.
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