Question

What is `"steam distillation"`?

Answer 1

Steam distillation is a technique that allows the distillation of a volatile liquid at a reduced temperature.

Explanation:

Should water be heated with an immiscible organic liquid, each constituent, the water and the liquid independently exerts its own vapour pressure as a function of temperature. The vapour pressure of the component may be low, however, boiling of the mixture will occur when the sum of the vapour pressures exceeds 1 atmosphere. Of course water comes over with the distillate, but along with it comes the volatile oils, which tend to aggregate on standing in the distillation flask.

When the distillation is over, the receiving flask separates into 2 phases: water, which may be discarded; and the organic distillate, which may be a high value essential oil. Of course you get more water than oil over, but this is immaterial because water is cheap.

On the other hand, if the attempt is made to distill the organic component directly, the high temperature required may cause decomposition. Steam distillation allows temperatures around `100` `""^@C`. Many beautifully smelling perfumes are collected by this technique.

So to finally answer your question, if the stuff is steam volatile, it has a sufficient vapour pressure at about `100` `""^@C` to co-distil with steam.