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

****:ICE

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.