Physical Chemistry of Foods

any mechanical external energy applied to them, say due to

streaming of a liquid. This is illustrated when we calculate the

average velocity of particles from (4.8) and their mass; some results

at 300 K are:

Hydrogen atom 2700 m?s
^1

Protein molecule, e.g., 15

Emulsion droplet, e.g., 0.001

Billiard ball, about 10
^10

An emulsion droplet can readily attain a velocity relative to the

liquid of greater than 1 mm per second due to stirring and a very

slight touch may move a billiard ball at a speed of 10
^1 m?s
^1.

Although the velocities are high for small species, they do not

say anything about the distances that the molecules travel. For

water molecules at 0C the average velocity is 614 m?s
^1 , and this is

equally true in ice, liquid water, and water vapor. But in ice the

molecules vibrate over a very small distance only (order of 10
^10 m);

in water they also vibrate, but they move with respect to each other

FIGURE4.3 The Maxwell–Boltzmann distribution of the translational kinetic
energy U of molecules or other particles. Nis number of molecules, kB is
Boltzmann’s constant, andTis temperature (K). (a) Normalized distribution. (b)
Distribution for two temperatures.