Steam distillation

If, as is usually the case, the volatiles are not miscible with water, they will spontaneously form a distinct phase after condensation, allowing them to be separated by decantation or with a separatory funnel.

[1] Steam distillation can be used when the boiling point of the substance to be extracted is higher than that of water, and the starting material cannot be heated to that temperature because of decomposition or other unwanted reactions.

In direct steam distillation, the starting material is suspended above the water in the boiling flask, supported by a metal mesh or perforated screen.

[4] Steam distillation is used in many of the recipes given in the Kitāb al-Taraffuq fī al-ʿiṭr ('Book of Gentleness on Perfume'), also known as the Kitāb Kīmiyāʾ al-ʿiṭr wa-l-taṣʿīdāt ('Book of the Chemistry of Perfume and Distillations'), attributed to the early Arabic philosopher al-Kindi (c. 801–873).

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.

Illustrative is a classic preparation of bromobiphenyl where steam distillation is used to first remove the excess benzene and subsequently to purifiy the brominated product.

[11] It one preparation of a purine, steam distillation is used to remove volatile benzaldehyde from nonvolatile product.