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What is water? Like any of the best recipes, it has simple ingredients: two hydrogen atoms and one oxygen atom. “When oxygen is placed next to hydrogen, the force of attraction strengthens,” says Dr. Teruo Yoshino, professor in the Department of Chemistry, Division of Natural Sciences at International Christian University in Tokyo. “That’s what is known as the hydrogen bond. It’s a type of intermolecular force or molecular attraction.”
This bond makes water unique in the natural world. For example, most substances are denser — heavier — in their solid state. Not so with water. “Liquid water has a higher density than ice,” Dr. Yoshino points out. “Its structure is not as dense as water’s.” An easy experiment to illustrate this: Put an ice cube into a glass of water.
Ice floats because it is less dense than liquid water. (In fact, a liter of ice weighs less than a liter of water!) This is why lakes and rivers freeze at the top only, allowing fish and aquatic plant life to survive in winter. Temperatures of the water in a frozen lake, by the way, range between 0ºC (32ºF) at the surface and 4ºC (39ºF) at the bottom. So the highest temperature is at the lake bed, where the water density is at its maximum. “If water behaved like other substances,” Dr. Yoshino explains, “the ice would sink and the temperature of the entire lake would reach 0ºC (32ºF). The lake would be completely frozen from top to bottom and no organic life could survive. The same is true of the oceans.”
Structures of water and ice
Because ice’s structure has larger spaces (left), it is less dense than water. However, as ice melts, H2O molecules start flowing out, filling in the spaces (right), which increases density. This is why ice cubes float on the water’s surface.
Think about the glass of water in our ice cube experiment. Where did that water come from? How old is it? It might have fallen from the sky as rain two weeks ago, but the water itself has been around for billions of years. We have the water cycle to thank for this.
The Earth has a limited amount of water that is continuously processed through the water cycle, which has four distinct phases. In the first step, evaporation, the sun heats the water in oceans, rivers and lakes, turning it into steam or vapor that rises into the atmosphere. Next is condensation, where the vapor cools and changes back into liquid, or clouds. When so much water has condensed that the air cannot hold it any longer, precipitation, the third stage, begins: The water falls back to earth as rain, snow, hail or sleet. During collection, the water returns to rivers, lakes and land, and finally to oceans. If it falls on land, it will either soak into the earth and become ground water or run off to replenish the rivers, lakes and oceans... where the cycle begins again.
Unit: 1 km³ = 1,308 cu. yd.
Grand Water Cycle
(Courtesy of Professor Teruo Yoshino, International Christian University)
Dr. Yoshino says the water cycle “has repeated since the birth of the Earth.” Moreover, it gave rise to the first life forms that developed in the Earth’s primeval oceans. “Over time, such lightweight molecules as hydrogen, ammonia and methane were vaporized with water in the atmosphere,” he explains. “Mixtures of these substances were destroyed by sunlight and restructured into amino acids and glucose which dissolved in the sea and developed into rudimentary cells. This is a widely supported hypothesis regarding the origins of life. Without water, this miracle could not have happened.”
The oceans were the original melting pot in which all these ingredients coalesced and mixed into a primordial soup. It took several billion years for the first single-celled organisms to evolve into multicellular creatures and, eventually, the ultimate life form, human beings. “No one can explain why this occurred,” Dr. Yoshino remarks. “Life is a result of coincidence upon coincidence.”
