Steam Distillation Definition and Principle in Chemistry

What Is Steam Distillation?

Steam distillation is used to separate two liquids that have different boiling points.
Steam distillation is used to separate two liquids that have different boiling points. Joanna KoĊ›mider, public domain

Steam distillation is a separation process used to purify or isolate temperature sensitive materials, like natural aromatic compounds. Steam or water is added to the distillation apparatus, lowering the boiling points of the compounds. The goal is to heat and separate the components at temperatures below their decomposition point.

The advantage of steam distillation over simple distillation is that the lower boiling point reduces decomposition of temperature-sensitive compounds.

Steam distillation is useful for the purification of organic compounds, although vacuum distillation is more common. When organics are distilled, the vapor is condensed. Because water and organics tend to be immiscible, the resulting liquid generally consists of two phases: water and the organic distillate. Decantation or partitioning may be used to separate the two layers to obtain the purified organic material.

Principle Behind Steam Distillation

When a mixture of two immiscible liquids (e.g., water and organics) is heated and agitated, the surface of each liquid exerts its own vapor pressure as though the other component of the mixture was absent. Thus, the vapor pressure of the system increases as function of temperature beyond what it would be if only one of the components was present. When the sum of the vapor pressures exceeds atmospheric pressure, boiling begins. Because the temperature of boiling is reduced, damage to heat-sensitive components is minimized.

Uses of Steam Distillation

Steam distillation is the preferred method used to isolate essential oils. It is also used for "steam stripping" in petroleum refineries and to separate commercially important organic compounds, such as fatty acids.