Stories About Chemistry
16. “Ice, Not Yet Firm, on the Cold Little River…”
In 1913 news of a grave tragedy spread all over the world. The huge ocean liner “Titanic” ran into an iceberg and sank. Experts gave various reasons for the catastrophe. It was said that owing to a fog the captain could not see the immense floating mountain of ice in time, and the ship collided with it and so perished.
If we consider this tragic event from the standpoint of a chemist, we come to a quite unexpected conclusion: the Titanic fell victim to another anomaly of water. Frightful mountains of ice - icebergs - float like cork on the surface of water, though these mountains weigh tens of thousands of tons. This is possible only because ice is lighter than water.
Try melting any metal and throwing a piece of the solid metal into the liquid: it will sink immediately. The density of any substance in the solid state is higher than in the liquid.
Ice and water are an astonishing exception to this rule. But were it not for this exception, all the bodies of water in the middle latitudes would soon freeze in winter right down to the bottom and all living things in them would perish.
Remember Nekrasov’s poem (translated by Juliet M. Soskice):
“Ice, not yet firm, on the cold little river
When the frosts come the ice hardens. A winter road can be laid on the river. But under the thick layer of ice water continues to flow as before. The river never freezes to the bottom. Ice, the solid state of water, is a very peculiar substance.
There are several kinds of ice. The one found in nature is that which melts at zero (on the Celsius scale). By using high pressures scientists have obtained six more varieties of ice in the laboratory. The most fantastic of them (ice VII), occurring at pressures over 21,700 atmospheres, might be called red-hot ice. It melts at 192°C above zero when the pressure is 32,000 atmospheres. It would seem that there could hardly be anything more familiar than the picture of ice melting. But what surprising things it involves!
After melting any solid begins to expand. But the water that forms when ice melts behaves quite differently: it contracts and only afterwards, if the temperature continues to rise, does it begin to expand. This is again due to the ability of water molecules to attract one another. At four degrees above zero this ability becomes especially pronounced, and therefore at this temperature the density of water is at its highest; that is why our rivers, ponds and lakes do not freeze to the bottom even in the coldest weather. The coming of spring makes everyone glad; we have all taken pleasure in the golden days of autumn. The joyous spring thaw and the crimson attire of the woods….
Again the result of an anomalous property of water!
A great deal of heat is required to melt ice, much more than to melt the same quantity of any other substance. When water freezes this heat is evolved, and in returning it, ice and snow warm the Earth and the air. They soften the sharp transition to severe winter and enable autumn to reign for several weeks. In the spring, on the contrary, the melting of the ice holds back the sultry weather for a time.