Question

What is the function of water in a steam distillation of the spice oil from the ground spices? What determines the distillation temperature of the distillate in a steam distillation and why does it stay constant?

Answer 1

Steam distillation is a method used for distilling immiscible liquids for which steam enables one of the immiscible phases. The two substances mix in the gaseous phase and co-distill however on cooling, the desired component separates from water since it is immiscible.

When a mixture of two practically immiscible liquids is heated, each constituent independently exerts its own vapor pressure as a function of temperature as if the other constituent were not present. Consequently, the vapor pressure of the whole system increases. Boiling begins when the sum of the vapour pressures of the two immiscible liquids just exceeds the atmospheric pressure.

For example, the boiling point of bromobenzene is 156 °C and the boiling point of water is 100 °C, but a mixture of the two boils at 95 °C. Thus, bromobenzene can be easily distilled at a temperature 61 °C below its normal boiling point.

It stays constant because water which has a boiling point of 100 oC sets the temeperature up and the mixture always boils out below this temperature as the vapour pressure of the whole systems is kept higher by the vapour pressure of water.