Page 342 - Physical chemistry understanding our chemical world

ACTIVITY 309

approximate and say it contains just one component in a water-based solution. Any
incursions into reality, achieved by extending our thoughts to encompass a multi-
component system, will not change the nature of these arguments at all.
Adding table salt to gravy causes its lovely smell to become less intense. This is a
general result in cooking: adding a solute (particularly if the solute is ionic) decreases
the smell, in just the same way as adding table salt decreased the smell of brandy in
the example directly above.
The ability to smell a solute relies on it having a vapour pres-
sure above the solution. Analysing the vapour above a gravy dish The pressure above
shows that it contains molecules of both solvent (water) and solute a solution relates to
(gravy), hence its damp aroma. The vapour pressures above the the composition of
gravy dish do not alter, provided that we keep the temperature con- solution, according
stant and maintain the equilibrium between solution and vapour. to Henry’s law; see
The proportion of the solute in the vapour is always small because Section 5.6.
most of it remains in solution, within the heavier liquid phase.
As a good approximation, the vapour pressure of each solute in the vapour above
the dish is dictated by the respective mole fractions in the gravy beneath. As an
example, adding water to the gravy solution dilutes it and, there-
fore, decreases the gravy smell, because the mole fraction of the
An electrochemist asses-
gravy has decreased.
ses the number of ions
Putting ionic NaCl in the gravy increases the number of ions in
and their relative influ-
solution,eachofwhichcantheninteractwiththewaterandthesolute,
ence by means of the
which decreases the ‘perceived concentration’ of solute. In fact, we ‘ionic strength’ I (as
can now go further and say the thermodynamic activity a represents defined below).
the concentration of a solute in the presence of interactions.

Why add alcohol to eau de Cologne?

Changing the perceived concentration

Fragrant eau de Cologne is a dilute perfume introduced in Col-
The word ‘perfume’
ogne (Germany) in 1709 by Jean Marie Farina. It was probably
comes from the Latin
a modification of a popular formula made before 1700 by Paul per fumem, meaning
Feminis, an Italian in Cologne, and was based on bergamot and
‘through smoke’.
other citrus oils. The water of Cologne was believed to have the
power to ward off bubonic plague.
Eau de Cologne perfume is made from about 80–85 per cent These esters are sta-
ble in the dark, but
water and 12–15 per cent ethanol. Volatile esters make up the
degrade in strong sun-
remainder, and provide both the smell and colour.
light, which explains
The vapour pressure of alcohol is higher than that of water, so
whyso manyperfumes
adding alcohol to an aqueous perfume increases the pressure of the
are sold in bottles of
gases above the liquid. In this way, the activity a of the organic darkened or frosted
components imparting the smell will increase and thereby increase glass.
the perceived concentration of the esters. And increasing a (ester)