The ancient Egyptians attempted to purify and sweeten muddy river water by adding crushed almonds. Sanskrit writing on a 3,900-year-old tablet from India informs that water "may be given a wondrous and different new taste by dipping seven times into it a piece of hot copper." Today many cities treat water with chlorine to kill bacteria, and some add fluoride to reduce tooth decay. Because water is probably the most important single chemical in the world, people have always sought new ways to use it and improve it—and to understand just what it is.
One of the earliest attempts to explain the nature of water was made by the Greek philosopher Aristotle in 335 B.C. when he issued his Theory of Matter. According to Aristotle's theory, every substance was a combination of four basic elements—fire, earth, air and water. This idea was to keep alchemists busy for centuries trying to change common metals into gold by rearranging their "elements." They were not very successful, however, since no one was ever able to break down a substance and find out just how much fire, earth, air and water was in it. Not until the eighteenth century did scientists decide that air, earth and fire were not really elements at all, and in 1781 a British chemist proved that water wasn't an element either.
That year Joseph Priestley exploded a mixture of air and hydrogen in a bottle as "a mere random experiment to entertain a few philosophical friends," and noted with interest that the explosion caused moisture to condense inside the bottle. He repeated the experiment until he was certain that the moisture was produced from the reaction of air and hydrogen, thereby proving that water, rather than being a basic element, was itself composed of other chemicals.
In 1783 the French chemist Antoine Lavoisier elaborated on Priestley's experiment and discovered that the hydrogen was not combining with air to form water, but only with the oxygen in air. Further experiments determined that a molecule of water contained two atoms of hydrogen and one atom of oxygen, a chemical combination which has been given the symbol H2O.
The discovery that water was H2O or "hydrogen oxide" raised more questions than it answered, for chemists learned that despite its apparent simplicity water is actually a most puzzling and paradoxical substance which seems to defy the rules of chemistry.
Compared to other liquids, water requires a great deal of heat to raise its temperature even a few degrees, and once it is warm it loses its heat slowly. This ability to "store" heat enables water to modify the world's climate. Oceans don't warm up as fast as the coastlines they touch, nor do the)' cool off as fast, with the result that coastal areas usually have milder winters and cooler summers than areas inland. For example, Bismarck, North Dakota has average temperatures of 8 degrees in January and 70 degrees in July, while Seattle, at the same latitude, has averages of 41 degrees and 66 degrees for the same two months.
Water is the only substance found abundantly in all three natural forms—liquid, solid and gas. But in changing from one form to another some more peculiarities become evident. One of water's most paradoxical qualities is that when it changes into a solid it expands (and so becomes relatively lighter for its volume). Other substances, with the exception of bismuth, contract or condense and so become relatively heavier. Of course, if water condensed when it froze the world would have some serious problems. Ice would form on the bottom of a lake because it would be denser and therefore heavier than the liquid water. And there it would stay, growing upwards, killing all marine life. Underneath the surface, shielded from much of the sun's heat, very little of it would melt in the spring and eventually the rivers, lakes and seas in cooler parts of the world would become solid ice.
The world would have different but even greater temperature problems if there were no water vapor in the air to absorb the sun's heat during the day and retain it during the night. Daylight hours would be miserably hot, and nightfall would bring freezing temperatures. In southern states where winter crops are grown, farmers dread a cold, dry, clear night for this same reason. Without clouds and moist air to act as insulation, the earth's heat quickly escapes, permitting the temperature to drop unusually low and sometimes ruin citrus fruit and tender plants with frost.
Water's peculiar way of expanding when it freezes gives it another important but little-known role in agriculture. Because it collects in the minute cracks of rocks, it gradually breaks them up in the process of alternately freezing and thawing, and thereby assists in the lengthy weathering process that reduces rocks to soil.
Water not only refuses to obey the usual rules of chemical behavior in freezing and melting—it sometimes defies even its own rules. For example, water can be cooled below its freezing point of 32 degrees F. without freezing. If kept absolutely still it will remain a liquid to within a few degrees of zero, but once disturbed freezes instantly. Similarly, water can be heated above its boiling point of 212 degrees F. without turning to steam. Under laboratory conditions it has been heated as high as 380 degrees F. When it finally does boil, however, it makes up for lost time by boiling with explosive violence.
Water's solvency gives it another unusual property. Most people consider water a good conductor of electricity, but in fact it's a terrible conductor and a good insulator. When someone standing in water gets a shock, it's not the water that conducted the electricity but the impurities dissolved in it. Pure water doesn't carry current, but it is such a good solvent for other substances that any moist surface is usually a good conductor.
Water's ability to dissolve most substances makes it a highly corrosive liquid, yet it is not only harmless to plants and animals but highly essential to their lives. It assists in all body processes, lubricates the joints, and is the key to the cooling system that keeps the human machine at just the right working temperature. In fact, the human body is nearly 60 per cent water, and a person drinks about 20,000 gallons of the liquid in an average lifetime.
Water's component parts, hydrogen and oxygen, are themselves interesting. Oxygen is a colorless, odorless, tasteless gas and by far the most abundant element. It can combine with practically every other element, and composes 21 per cent of the volume of the atmosphere, about 90 per cent of the weight of water, and about half the weight of the rocks in the earth's outer crust. Because it supports both respiration and combustion, it is the world's chief source of energy.
Hydrogen, also a gas, is the lightest and simplest of all elements. The ordinary hydrogen atom contains a single proton and electron, which puts it in first place on the atomic chart of elements. Because the gas is so light it was once widely used in balloons, but because it is so inflammable and has resulted in some disastrous explosions balloons today are filled with inert helium.
Hydrogen, however, is not quite the simple element it was once thought to be, and because of this scientists have made another surprising discovery about water—that its chemical formula is not always H2O. In 1934 the American chemist Harold Urey discovered a second kind of water that contained deuterium, a special type of hydrogen. Instead of containing the usual one proton and one neutron, a deuterium atom also contains a neutron which nearly doubles its atomic weight. Thus water made of deuterium and oxygen is called "heavy water" and is given the symbol D2O. Heavy water exists in ordinary water at a ratio of about 1 to 7,000, or about one ounce in fifty gallons, and it has slightly higher freezing and boiling points. It has proved extremely valuable in atomic research as a moderator to slow down nuclear chain reactions by absorbing the free neutrons that keep a chain reaction going.
Surprisingly, seeds won't sprout in heavy water and rats will die of thirst rather than drink it. According to a report in the Soviet newspaper Isvestia, the Russian biophysicist Boris Rodymov has made "pigs grow fatter, cows give more milk and chickens lay more and bigger eggs" by watering them only with melted snow, which contains less D2O than ordinary water.
After scientists had adjusted to the idea of there being two kinds of water, English and U.S. researchers discovered there was a third. This is a "super-heavy" water containing tritium, which is a hydrogen atom with two additional neutrons in its nucleus. Tritium oxide is extremely rare, only one part occurring in one million million million parts of ordinary water. Tritium is radioactive and is important in the making of hydrogen weapons and atomic research.
The newest discoveries about water have been as curious as ever. Botanists have found, for example, that corn may show frost effects when the temperature is as high as 40 degrees, which is eight degrees above water's usual freezing point. Another discovery has been that natural gas pipelines can become clogged with a slushy "snow" containing water, which is puzzling since natural gas is practically insoluble in water. Stranger yet was the discovery that such inert gases as argon and krypton, which do not react chemically, can sometimes combine with water to form what resembles a chemical compound.
From past experience, water seems to have an unlimited amount of surprises in store for the chemists and other scientists who will be working with it in the future. Water was a common and simple enough substance when Joseph Priestley amused his friends by exploding some hydrogen in a bottle a century ago, but it hasn't been that way since.