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PHASE EQUILIBRIA INVOLVING VAPOUR PRESSURE 229

Why does a cup of hot coffee yield more steam than
above a cup of boiling water at the same temperature?

The effects of poor mixing (immiscibility)

Prepare two cups: put boiling water into one and boiling coffee in the other. The
temperature of each is the same because the water comes from the same kettle, yet
the amount of steam coming from the coffee is seen to be greater. (We obtain a better
view of the steam by placing both cups on a sunny window sill, and looking at the
shadows cast on the opposing wall as the light passes through the vapour as it rises
from the cups.)
When performing this little experiment, we will probably notice
how the steam above the coffee has an extremely strong smell of The rate of steam pro-
coffee, although the smell dissipates rapidly as the rate of steam duction decreases with
production decreases. time as the water cools
This experiment is a simple example of steam distillation. down because energy
Adding steam promotes the volatilization of otherwise non-volatile is lost from the cup as
components, simplifying their extraction. For simplicity, we will water molecules enter
the gas phase.
say that the smell derives from a single sweet-smelling chemical
‘coffee’. Coffee and water are not wholly miscible, with some of
the essential oils from the coffee existing as tiny globules – we call the mixture a
colloid (see Chapter 10). We have generated a two-phase system. Both phases, the
water and the coffee, are saturated with each other. In fact, these globules would
cause strong coffee to appear slightly misty, but for its strong colour blocking all
light. We never see phase separation in the coffee cup, with a layer of oil floating
above a layer of water, because the coffee’s concentration is never high enough.
We say a pure liquid boils when its vapour pressure equals the
external, atmospheric pressure (see p. 188). Similarly, when boiling The boiling of such a
a mixture, boiling occurs when the sum of the partial pressures mixture requires the
O
(p (water) + p (coffee) ) equals p . It is for this reason that the steam sum of the pressures,
above the coffee cup smells strongly of coffee, because the vapour not just the pressure
contains the essential oils (e.g. esters) that impart the smell. But the of one component, to
O
water generates steam at a pressure of p O when the water added equal p .
to the cup is boiling, so the partial pressure of the coffee p (coffee)
is additional. For this reason, we produce more steam than above
the cup containing only water.

How are essential oils for aromatherapy extracted
from plants?

Steam distillation

The ‘essential oils’ of a plant or crop usually comprise a mixture of esters. At its
simplest, the oils are extracted from a plant by distillation, as employed in a standard