http://www.ck12.org Chapter 15. Water
Ice - H
Water in its solid form is generally referred to as ice. Other common terms for solid water include snow, sleet, hail,
or frost. The use of a different term generally depends on how and where the water was frozen. Ice plays a major
role in activities as diverse as skiing and skating, rain making, polar marine transportation, and offshore drilling. It
also plays an important role in the degradation of concrete and other porous materials. Liquid water gets into the
cracks of such materials, which can then fracture if the water freezes and expands. Icing on airplanes and electrical
transmission lines can also interfere with their functioning.
Water can freeze into both crystalline and non-crystalline (amorphous) forms.Crystalline solidsrefer to solids in
which the atoms or molecules are packed in a precisely arranged order. The environmental conditions under which
the transition occurs dictate whether this phase change produces regular crystals or not. In its crystalline form, water
molecules are generally arranged in a hexagonal array, as shown inFigure15.2.
FIGURE 15.2
Ice on Earth generally forms when liquid water freezes. However, it can also be produced by the deposition of water
vapor (i.e., H 2 O(g)→H 2 O(s)). When this is the case, the ice is often referred to asfrost(Figure15.3).
When water freezes in a non-crystalline, amorphous form as frost, the water molecules are fixed in place, but
not in a regular, repeating pattern. Frost and otheramorphous solidsare characterized by their unorganized and
unpredictable structure. Because ice comes in both amorphous and crystalline forms, there are differences in the
physical properties of ice such as its hardness and density.
Example 15.2
Can you think of another substance that has an amorphous geometry? Can you think of another substance that has a
crystalline geometry?
Answer: