TABLE OF CONTENTS The use of steam to remove lower-boiling or lighter components from a liquid is one of the oldest methods of distillation. Sometimes called steam distillation, this technique relies on a combination of two simple effects.
The first effect is illustrated when we blow across a bowl of hot soup to cool the soup. Our breath displaces the steam vapors that are on top of the soup. This encourages more molecules of steam vapors to escape from the soup; that is, the vapor pressure of the steam above the liquid soup is diminished, because steam is pushed out of the soup bowl with air. The correct technical way to express this idea is to say, The partial pressure of the steam, in equilibrium with the soup, is diminished.
But our breath itself does not remove heat from the soup. The evaporation of steam from the soup, promoted by our breath, takes heat. Converting one pound of soup to one pound of steam requires 1000 Btu. This heat of evaporation comes not from our breath, but from the soup itself. The correct technical way to express this second effect is, The sensible-heat content of the soup is converted to latent heat of evaporation.
For example, if we have 101 lb of soup in a rather large bowl, and cause one pound to evaporate by blowing across the bowl, the soup will lose 1000 Btu. This heat of evaporation will come at the expense of...
