A Lightweight among Solids:
Water, which often appears to follow a set of natural laws all its own, behaves most outlandishly when it forms ice. For one thing, unlike most other compounds,it is lighter in this solid form than it is as a liquid. As a result,it floats when it freezes. If this did not happen in nature and ice were heavier than water,it would continuoulsy sink to the bottom, where the sun’s rays could not melt it. slowly an ice pack would build upward until the world’s oceans,rivers and lakes became frozen solid.
Even as it changes from a liquid to a solid, water acts contrary to expectations. At first it follows the universal pattern of cooling:It contracts,and grows heavier and more dense. But when cooled below 39deegree F.(3.9deegree c.), it begins to expand and grow lighter and less dense. The reason for this turnabout lies,again,in the hydrogen bonds that exist between water molecules. As they cool,the molecules slow up and begin crowding together. At 32 deegree F.(0 deegree c.),the bonds bring them to a halt and fix them at arm’s length from one another in lightweigh crystals of ice.
A Compound Slow to Boil:
Homeowners whose water pipes have burst when the temperature suddenly fell need no other proof that water,unlike most liquids,releases tremendous energy when it freezes. Conversely,it must absorb a great deal of energy-in the form of heat-before its temperature is raised even slightly. An iron kettle used to boil water will be blistering hot long before the water in it is lukewarm. This property accounts for water’suse as a cooling agent in automobile engines:It soaks up an enormous amount of haeat without boiling.
In this manner,large bodies of water and the moisture in the atmosphere can regulate extremes of temperature, absorbing heat on hot days,and giving off heat on cold days. Where there is little natural water,as on the desert, temperatures can range from a searing 140 deegree F.(60deegree C.)to well below freezing at night.
Before the temperature of a substance can be raised,its molecules must be prodded into vigorous motion. But in water molecules,the firm grip of hydrogen bonds must first be loosened-a task which requires considerable amounts of heat. If the hydrogen bonds did not put up such firm resistance,water would boil at temperatures lower than those at the North Pole,and all of the world’s water would immediately evaporate.