It’s the second most abundant substance in the universe. It dissolves more materials than any other solvent. It stores incredible amounts of energy. Life as we know it would not be possible without it. And although it covers more than 70 percent of the Earth’s surface, many parts of the world are in dire straits for lack of it. What is it?
The answer, of course, is water. In some ways, water is one of the substances we know best, in part because it makes up about 75 percent of our bodies. Every day we drink it, bathe in it, clean with it and use it to dispose of our wastes. Yet scientists are still striving to understand many of water’s remarkable properties, and the 21st century will force us to think about water like we never have before.
What Makes Water So Remarkable?
For most of human history, water was considered to be one of the four elements, along with air, earth, and fire. It was only in the 18th century that chemist Antoine Lavoisier passed an electrical current through water and realized that it gives off two gases: hydrogen (literally, “water maker”) and oxygen.
The formula of water is H2O—two atoms of hydrogen and one of oxygen. One of water’s most remarkable properties is traceable to the hydrogen bonds that continually form and reform between its slightly negatively charged oxygen and slightly positively charged hydrogen components. Thanks to these bonds, water molecules attract one another far more strongly than those of almost any other substance.
These hydrogen bonds give water a very high specific heat, meaning that it takes a great amount of energy to warm it. It also has a remarkably high boiling point compared to other chemically similar molecules, such as hydrogen sulfide. These properties enable human beings to dissipate large amounts of heat during exercise by perspiring.
Another consequence of hydrogen bonding is capillary action (the topic of Einstein’s first paper), which occurs, for example, when a liquid is drawn up between the hairs of a paintbrush. The water molecules attract one another so strongly that they defy the force of gravity. When water evaporates from the highest leaves of a tree, it draws up other water molecules from the roots far below.
Still another consequence of hydrogen bonding is water’s high surface tension. This accounts for its tendency to form droplets and enables some insects literally to walk on water. This force can be so strong that premature infants, who lack surfactant, a substance that lessens it, can become exhausted just trying to inflate their lungs. Fortunately, surfactant is now available as a medication.
The fact that water has slightly positively and negatively charged poles also makes it the “universal solvent,” perfect for dissolving salts, sugars, acids, alkalis, and even gases such as carbon dioxide, accounting for the fizz in sodas. Such substances are known as hydrophilic (water-loving), precisely because they dissolve so easily in water.
By contrast, fats and oils are classified as hydrophobic, because they do not have electrical charges at their ends. As a result, they are attracted more strongly to one another than to water. To wash such substances from our hands or clothes, we need soaps, which have both hydrophobic and hydrophilic ends that help break them up into tiny droplets that can be carried away by water.
From One State to Another
Even more remarkably, water is practically the only substance known to man that, as it cools from its liquid to solid state, actually expands. Virtually every other substance becomes denser as it “freezes,” but thanks to this remarkable property, ice cubes float in our drinks. More importantly for living organisms, lakes and other bodies of water freeze from the top down.
