theory it could be distilled at any temperature by collecting and condensing its vapors. However, ordinary distillation below the boiling point is not practical because a layer of vapor-rich air would form over the liquid, and evaporation would stop as soon as the partial pressure of the vapor in that layer reached the vapor pressure. The vapor would then flow to the condenser only by diffusion, which is an extremely slow process. Simple distillation is generally done by boiling the starting material, because, once its vapor pressure exceeds atmospheric pressure, that still vapor-rich layer of air will be disrupted, and there will be a significant and steady flow of vapor from the boiling flask to the condenser. In steam distillation, that positive flow is provided by steam from boiling water, rather than by the boiling of the substances of interest. The steam carries with it the vapors of the latter. The substance of interest does not need to be miscible water or soluble in it. It suffices that it has significant vapor pressure at the steam's temperature. If the water forms an azeotrope with the substances of interest, the boiling point of the mixture may be lower than the boiling point of water. For example, bromobenzene boils at 156 °C (at normal atmospheric pressure), but a mixture with water boils at 95 °C. However, the formation of an azeotrope is not necessary for steam distillation to work. Applications Steam distillation is often employed in the isolation of essential oils, for use in perfumes, for example. In this method, steam is passed through the plant material containing the desired oils. Eucalyptus oil, camphor oil and orange oil are obtained by this method on an industrial scale. Steam distillation is a means of purifying fatty acids, e.g. from tall oils. Steam distillation is sometimes used in the chemical laboratory. Illustrative is a classic preparation of bromobiphenyl where steam distillation is used to first remove the excess benzene and subsequently to purifiy the bominated product. In one preparation of benzophenone, steam is employed to first recover unreacted carbon tetrachloride and subsequently to hydrolyze the intermediate benzophenone dichloride into benzophenone, which is in fact not steam distilled. It one preparation of a purine, steam distillation is used to remove volatile benzaldehyde from nonvolatile product. Equipment On a lab scale, steam distillations are carried out using steam generated outside the system and piped through the