Freezing of Water
From the time we are in diapers until we are ferried across the River Styx, water is so much a part of life that we mostly take it for granted.
More than half of each living thing on Earth is water. In addition to being the all-pervading fluid of life, water is the custodian of most of the Earth's available energy. More so than most substances, water is capable of storing much heat energy. As water moves within the global circulation of the atmosphere and the oceans it helps transport solar energy received in the tropics to the polar regions and thereby moderates the environment of the whole planet.
Water is said to be the universal solvent, able to dissolve most other substances. Also, it is the best wetting agent by virtue of its ability to bond onto and coat the surfaces of many materials, even hard, slick substances such as glass.
Water owes its array of strange properties to the unusual way oxygen and hydrogen atoms cling together to form water molecules. The atoms in most molecules are bonded together by only one of two kinds of forces, but a water molecule is held together by both kinds. This causes a tricky balancing act because the two forces each try to establish different spacings between the hydrogen and oxygen atoms. Furthermore, at least one of the forces is temperature-dependent and extends outside the molecule to affect nearby molecules.
This complex interplay of forces causes water to behave in a strange way when it is frozen into ice. Most substances become more dense as they are cooled, and are more dense in the frozen state than when liquid. But when water is cooled below +4°C it begins to expand. When the water freezes, the crucial bonding force that sets up the ice crystal array actually pushes the randomly milling crowds of liquid molecules apart into less-dense, uniform ranks. This is why ice floats on water and why frozen plumbing breaks.
In school we learned that a stirred beaker of ice and water remains exactly at 0°C (32°F), and therefore that this temperature is the freezing point. (The water tries to thaw the ice, and the ice tries to freeze the water, so the mixture must go to an equilibrium temperature which is the freezing point.) But water found in Nature can undergo the establishment of fixed ordering of the molecules--the process we call freezing--over a wide range in temperature. In some circumstances, water will remain in liquid form down to -40°C (also -40°F, the two scales being equal at this particular temperature).
Thin layers of water molecules around protein molecules or hydrocarbon molecules will freeze at temperatures as high as +4°C (+39°F). Thus plants, being partly made up of protein, can receive frost damage at temperatures above "freezing"--something the northern gardener should keep in mind on cool spring or fall nights.
