consequences for foods will be discussed in Section 16.3. Another
consequence of the density difference is that dispersed ice crystals tend to
cream.
The expansion tends to be smaller if solutes are present. The main
reasons are that generally (a) not all of the water will freeze and (b) that
(most of the) freezing occurs at lower temperatures, whereDrwill be smaller
(see Figure 15.13). For highly concentrated solutions, e.g., of sucrose, the
expansion can even be negligible.
The high values of the thermal conductivity and the thermal diffusivity
of ice (see Appendix I) imply that ice can conduct heat better than
(immobilized) water.
Impurities. Water is never absolutely pure. For one thing, it
contains Hþ(or H 3 Oþ, rather) and OH. Upon crystallization, most of
these ions form H 2 O, but some ions remain; the concentration of each in ice
is about 10^10 molar. This gives ice a finite electrical conductivity. Air is
another impurity that is nearly always present. Upon freezing, the remaining
water becomes gradually more supersaturated, often resulting in the
formation of air bubbles in the ice.
Particulate impurities are needed for heterogeneous nucleation, and
these are always present (natural waters tend to start freezing at, say, 18 C).
The formation of air bubbles is also dependent on impurities, some of which
may contain a tiny air pocket (see Section 14.4); some natural ice is opaque,
due to the numerous small bubbles it contains. Other impurities, be they
dissolved substances or particulate matter, can greatly affect the size and
morphology of the ice crystals.
Growth Rate. The growth of ice from ‘‘pure’’ water is not diffusion
limited: the crystal is bathed in water molecules. The growth rate then is
expected to be proportional toDm, hence toðTeqTÞ. Moreover, there is
no competition, and the fitting difficulty of water molecules is only slight.
Growth will thus be very fast, e.g., some cm?s^1 at 10 K undercooling.
In the presence of solutes, growth rate can be much reduced. Most
substances that are well soluble in water do this, and at high concentration
(as will occur when most of the water has become frozen) the growth rate
may be decreased by, say, two orders of magnitude. Causes can be
The solute causes a freezing point depression, and at the same
temperature the undercooling will be smaller for a higher molar
solute concentration.
The absolute value ofDmwill be smaller, since now thewater activityis
decreased, the more so for a higher molar solute concentration.