Forest Preserve District of Cook County, Illinois
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Nature Bulletin No. 661-A   january 7, 1978
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

There was a time when ice, cut on frozen ponds and lakes, was transported by fast clipper ships from New England to New Orleans where it was worth its weight in gold. Nowadays this cold brittle colorless substance is commonplace everywhere. Few people, however, know that ice is one of the strangest of all solids; and that, because of its unique properties, life on earth is what it is.

Those properties are due to the distinctive structure of a molecule of water, formed of three elemental particles or atoms -- two of hydrogen and one of oxygen -- expressed by the familiar symbol, H2O. The three atoms are held together by two chemical bonds expressed by another symbol, H-O-H. Briefly, the unique properties of water, water vapor, and ice arise from that bonding and the arrangement of electron pairs around the oxygen atom.

The strangest and perhaps the most important property is that water expands as it freezes and a cubic foot of water increases almost 10 percent in volume. Consequently, whereas a cubic foot of water weighs about 62.4 lbs. at ordinary temperatures, a cubic foot of ice weighs only 57.2 and it floats. The blanket of ice that forms and floats on a pond in winter makes it possible for aquatic plants and animals (fish, etc.) to remain alive in the water underneath.

If, like all other substances except bismuth, water contracted and became denser as it solidified, ice would be heavier than water and sink to the bottom. More ice would form on the surface until the pond was frozen solid. Only the top would be melted into shallow slush by the heat in spring and summer; the ice below would never thaw. In the cooler parts of the world the rivers, ponds, lakes, and even the oceans would all be permanently frozen.

Water has a far greater capacity for absorbing and storing heat than other substances. It gives up that heat as it cools -- in autumn, for instance -- and continues to do so as it freezes. Conversely, as ice melts, the ice water absorbs heat from the air or objects around it, and that is the basic principal of refrigeration. Also due to that capacity, a large body of water such as Lake Michigan tends to moderate the climate in its vicinity.

The temperature at which water freezes varies with the amount of pressure upon it and whether or not it contains anything in solution. Chemically pure water under atmospheric pressure at sea level, freezes at 32 Fahrenheit, Sea water does not freeze until its temperature drops to between 29 and 28 F; and the freezing point of brine -- water saturated with salt -- is 7 degrees below zero.

When the pressure upon water is increased, its freezing point is lowered. If a heavy weight is suspended from a loop of wire passing around a block of ice, the wire slowly cuts all the way through it, leaving the block perfectly solid. The pressure of the wire melts a pathway which freezes again as soon as the pressure is removed. Likewise, in skating, pressure of the skate blade melts a thin slippery film of water. By subjecting ordinary ice to enormous pressures, other kinds of ice can be produced and the freezing point of one of them is 40 degrees below zero ! If it were not for ice we would all be Eskimos.

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