Why does it work? For two immiscible liquids, each exerts its own vapor pressure independently (Dalton's law for immiscible systems — NOT Raoult's law, which applies to miscible/solution).

Total vapor pressure = P(water) + P(compound) at any temperature.

Since total P exceeds atmospheric pressure at a temperature BELOW the boiling point of either pure component, the mixture boils at a lower temperature.

Calculation: At the co-distillation temperature T: P(water at T) + P(compound at T) = 1 atm Mole ratio in distillate = P$\frac{water}{P}$(compound) = n$\frac{water}{n}$(compound) Mass ratio = [P(water) x MW(water)] / [P(compound) x MW(compound)]

Requirements for the technique to be useful:

1. Compound must be immiscible with water (otherwise Raoult's law applies)
2. Compound must have sufficient vapor pressure at ~100 degC (must be "steam-volatile")
3. Compound must not react with water or decompose under steam conditions

Solids like naphthalene can also be steam-distilled if they have sufficient vapor pressure.