Modernist-Cuisine-Vol.-1-Small

VAPOR IZATION
A ND CONDENSATION
6
For more on vapor pressure, see Canning,
page 2·75.
For more on the physics of temperature, see
The Nature of Heat and Temperature, page 264.
Water vapor condenses into droplets of
liquid water. Small droplets are roughly
hemispherical, but as they grow by
merging with other droplets, they become
less regular in shape.
In between its freezing point and its boiling point,
water exists as a liquid. But while you’d be surprised
to see a few tiny icebergs floating in water at
room temperature, you should not be surprised to
know that some of the liquid is turning into vapor
right before your eyes. Your eyes can’t see the
vapor, but it’s always there. That goes for all
liquids: some of their molecules have an irrepressible
urge to fly off into the air as a gas or a vapor in
a process called evaporation or vaporization.
As explained in chapter 5 on Heat and Energy,
temperature is a measure of the molecules’ (or
other particles’) velocities within a substance.
But a temperature indicates only an average
velocity. Some of the fastest particles that happen
to find themselves at the surface of the liquid will
simply fly off and evaporate. Even cooking oil
evaporates at room temperature but slowly
enough that it is not noticeable.
How many of a liquid’s molecules are evaporating
at any given temperature? The degree of
evaporation is expressed in terms of the vapor
pressure of the liquid because the flying molecules
that leave it bounce off the walls of its
container, thereby exerting outward force on it.
Inside any given container at any given temperature,
the pressure of a gas is directly proportional
to the number of its molecules.
The more strongly the molecules are bound to
one another in a liquid, the less they tend to
evaporate and the lower the vapor pressure of the
liquid. Because of its hydrogen bonds, water
requires a lot more energy (a higher temperature)
to evaporate than other liquids do. That is, it has
a relatively low vapor pressure. Alcohol and
gasoline have higher vapor pressures than water
and thus evaporate faster.
As we heat a liquid, more and more molecules
acquire enough energy to escape from the surface.
The evaporation rate and vapor pressure therefore
increase. When the vapor pressure reaches the
pressure exerted on the liquid’s surface by its
surroundings (most often the atmosphere),
evaporation becomes vaporization, and the liquid
boils. The boiling point is the temperature at
which the vapor pressure equals ambient pressure.
We can’t see the vapor pressures, of course, but at
this stage many molecules well below the surface
have enough energy to escape. Trapped as they are
in the depths, however, all they can do is form
bubbles, and that’s our visual clue as to what’s
going on.
Boiling
In stove-top cooking, where the heat source is
typically beneath the pot, vaporization happens
first at the bottom, then at the sides of the vessel.
Inside the pot, slight temperature differences arise
between water at the bottom and at the top. The
water at the bottom is hottest and rises to the
surface to be replaced by falling pockets of cooler
water. These movements are called convection
currents. They are slow at first and become more
vigorous as the water gets hotter. Stir a few grains
of ground black pepper into a pot of cold water,
wait until the stirring motions stop, then turn on
the burner. As the water grows hotter, you will see
the grains rising and falling with the currents.
If you look closely, you can see other changes
occurring as the water gets hotter. The first thing
you may notice is that large numbers of tiny
bubbles form on the bottom and sides of the pot.
But wait! They have nothing to do with the boiling
process; they’re simply dissolved air being forced
out of the water because gases are less soluble in
hot water than in cold.
Then an odd thing happens: the pot begins to
make sizzling and rumbling noises. The very first
vapor bubbles form in microscopic cracks or
protuberances on the inner surface of the pot
no matter how smooth it may appearthat act as
Green beans show the two kinds of
bubbles common to the boiling process.
The large, free-floating ones are pockets
of steam released from the bottom of the
pot. The smaller bubbles clinging to the
sides of the beans are air being forced by
the rising temperature from the spaces
between the cells of the beans.
314 VOLUME 1 · HISTORY AND FUNDAMENTALS
THE PHYSICS OF FOOD AND WATER 315