© J. Dirk Nies, Ph.D.
Water is the most common chemical compound on earth. It is essential for life and has long been perceived as foundational to the formation, composition and function of the world. The creation story in Genesis begins with the Spirit of God hovering over the face of the waters. The ancient Greeks held that water along with earth, air and fire were the four elements of which all things were made. Traditional Chinese philosophy assigns water as one of five interacting essences of the world, the others being earth, fire, wood, and metal.
Many ancient civilizations had their origins near water. Along the coastlines of oceans, bays, and lakes, and especially along the banks of great rivers—the Nile of Egypt, the Tigris-Euphrates of Mesopotamia, the Indus of India, and the Yangtze and Huang He of China—human cultures and empires were founded. To a great extent, history is the narrative of agriculture, trade and conquests made possible by water’s life-giving properties and by our travels upon its surface. Many chapters of our human story have been shaped by the prolonged deficit or the sudden surplus of water, by punishing drought or devastating flood.
In this series of articles on water, I will highlight its continuing importance in our lives, our environment and our economy, illuminating how precious, useful and essential water is. In doing so, we shall come to appreciate that water is, in many ways, the most extraordinary and unusual compound we know.
The planet’s inventory of water resides overwhelmingly in saline oceans and salty seas. Only three percent of the earth’s water is fresh; and this stock of fresh water is locked up primarily as ice, with the remainder stored away as groundwater. Of all the fresh water on earth, just 0.3 percent is found in lakes and rivers. The United Nations Environment Program (UNEP) estimates that the total usable freshwater supply for humans and ecosystems is less than one percent of all freshwater resources.
Unique among natural substances, water is the only one that plays essential roles in the environment as a solid, as a liquid and as a gas. If the earth were to orbit a little closer to or farther away from the sun, environmental conditions would no longer permit the three forms of water to be as widespread as they presently are.
As we begin this story of water, let’s review its chemistry. Water is a molecule comprising three atoms; two of hydrogen and one of oxygen (H2O). Most molecular compounds containing only a few atoms, such as hydrogen (H2), oxygen (O2), nitrogen (N2), carbon dioxide (CO2) and methane (CH4), are gases at room temperature. Many of these gases must be cooled down to hundreds of degrees below 0 to become liquids, and even colder to become solids. This is fortuitous too, because for example, if atmospheric oxygen were to become liquid or solid when it got very cold outside, we would all suffocate. So water’s most amazing property relative to other compounds of similar size and weight is that it is a liquid at room temperature.
Why is water a liquid at temperatures high enough to cause other similar small molecules to be in the gaseous state? The answer lies in its atomic makeup and structure. The two hydrogen atoms of water are chemically bonded to the single oxygen atom in such a manner that the three atoms make the shape of a wide open “V,” where the oxygen is located at the tip formed by the two lines (the internal bond angle is 104.45 degrees). Because oxygen strongly attracts electrons, more so than does hydrogen, electrons swirling around these atoms tend to reside more on the oxygen than on the hydrogens. This leads to a non-uniform distribution of negative and positive charges within the water molecule. These electrical charges promote linkages, called hydrogen bonds, between the positively charged hydrogens of each water molecule with the negatively charged oxygens of adjacent water molecules. These intermolecular hydrogen bonds (which do not exist in the other gaseous compounds mentioned earlier) hold water molecules together so tightly that they do not easily fly apart to become a gas. This hydrogen bonding phenomenon is strong enough for water to exist as a liquid even on the hottest summer days.
Why is this so important? Being a liquid allows water to be contained and yet flow. Being a liquid permits water to be a solvent capable of dissolving and transporting minerals, nutrients and other materials vital for life. Being a liquid at biologically benign temperatures makes water a suitable reaction medium for all the reactions that occur within the cells of living organisms.
Another special feature of water is that it is one of only a handful of substances that expands upon freezing, occupying 9 percent more volume as ice than does ice-cold liquid water. This extraordinary property has shaped the landscape of earth, breaking up larger rocks into smaller stones as water freezes and thaws, and in a more fundamental way, made life on earth much more prevalent than it otherwise would be. Imagine what the world would be like if ice sank to the bottom of the ocean, a lake or a river when it froze. Their surface waters would be continually exposed to the air, un-insulated by a layer of ice from the freezing cold. Consequently, as more and more ice formed on the surface and descended into the depths, all bodies of water located in the higher latitudes would become solid from the bottom up in winter. Over time, they would become thick blocks of ice, with a thin veneer of water appearing on top only during warmer months as the ice at the bottom remains frozen, shielded from the warm air above.
Water’s ability to hold heat, that is, its heat capacity, also is remarkably large. For example, the amount of energy needed to raise the temperature of a pound of water by one degree is more than 30 times greater than the energy required to achieve a one-degree rise in a pound of gold. Stated another way, the energy necessary to raise the temperature of gold by a blisteringly hot 360 degrees would raise the temperature of an equal weight of water by only a paltry 12 degrees! This amazing energy-holding property of water gives rise to the aphorism ‘a watched pot never boils.’
Via persistent ocean currents and water’s enormous capacity to store heat, vast quantities of tropical warmth are transported to colder climes. For example, the Gulf Stream, by giving up its water-borne heat to the atmosphere as it flows off the coast of Great Britain, keeps London’s average monthly lows above freezing year round even though London is hundreds of miles north of Duluth, Minnesota.
Despite water’s great capacity to absorb energy, to transform boiling water from liquid to vapor is even more energy intensive. In fact, to change 212 degree Fahrenheit liquid water into 212 degree steam, that is, to change its phase without increasing its temperature, requires five times as much energy as is needed to heat ice-cold water by 180 degrees to its boiling point!
This high heat of vaporization has a practical importance greater than one might appreciate at first glance. Water expands more than a thousand fold when it changes from a liquid to a gas. If only a little more heat were required to make this phase change from water to steam, the ‘watched pot’ would explode like a bomb just after the boiling point of water was reached.
In the next article, I will explore the hydrological cycle of water: its continuous movement on, above, and below the surface of the earth. Until then, while savoring your favorite cup of tea, delight as well in water’s wondrous properties that make its brewing possible. And just for fun, let your imagination wander to places on earth the water in your teapot has been.