A special brew: investigators still can't completely explain the strange molecular workings of water

Natural History, Nov, 2007 by Christopher J. Mundy, Shawn M. Kathmann, Gregory K. Schenter

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As children, we have all lain in the grass and looked up at the clouds. Sometimes they seemed to take on the shape of an animal, a favorite plaything, a familiar face. For many of us, such daydreaming segued into a deeper curiosity. What are the clouds? we wondered. What are they made of?

From an adult perspective, the answer seems obvious: water. Stand among the clouds on a mountaintop, and you can feel their moisture. Watch the plump white clouds of a sunny day transform into dark, daunting behemoths, and before long, sheets of water come pouring down. The common wisdom that clouds presage the weather is grounded in a less well known fact: the unique properties of water--in particular, its capacity to transport enormous quantities of energy--are what give the weather its variability, its energy, and its occasional violence.

Of course, our relationship with water goes far beyond the weather. We have fun with it whenever we go skiing or skating, boating, fishing, or swimming. The pleasure of a cold glass of thirst-quenching water on a hot summer day has a more serious basis, though. Without water, a human being can live only a few days. Every organism is made up mostly of water, and the substance covers nearly three-quarters of the Earth's surface.

Yet this commonplace, familiar, and essential stuff of life is also quite peculiar, as substances go. For example, if the water molecule ([H.sub.2]O) acted in bulk like other small molecules--oxygen ([O.sub.2]), carbon monoxide (CO), nitrogen ([N.sub.2])--it would be a gas under the conditions prevailing on Earth. Instead, water occurs in all three states of matter: solid, liquid, and gas. Furthermore, water reaches its maximum density in its liquid form, at 39.2 degrees Fahrenheit (four degrees Celsius), just a few degrees above the freezing point. Thus water stays at the surface as it starts to freeze, and ice floats--a rare property shared by very few other substances. If its nature were otherwise, all temperate-zone lakes, ponds, rivers, and even oceans would eventually freeze solid from the bottom up, and life as we know it could not exist. Instead, a floating skin of ice cocoons the life in the liquid water beneath a layer of insulation, enabling it to persist under the frozen surface.

Another unusual and related property of ice is that, for a given temperature, increasing the pressure decreases the melting point. (Ordinary solids remain solid under pressure.) Even though these and other unusual bulk properties of water have been described in detail, a complete picture of how and why water acts the way it does is still lacking. It is not possible, for instance, to completely predict the properties of materials that incorporate water in their structure, either physically or chemically, or to design and tune their responses to various conditions. Perhaps the key to achieving that level of understanding and control is to study water on a molecular scale: how water molecules arrange themselves, how they interact, and how they dance with other kinds of molecules. We and our colleagues in the growing field of molecular science hope that by understanding exactly what happens at very small scales (around [10.sup.-10] meter, or a billionth of a meter), we can zoom out by a factor of a billion or so to understand and predict phenomena on a human scale.

But we don't stop there. Because water is fundamental to all life on Earth, we also want to zoom out by another factor of 10 million to study its properties on a global scale.

Anyone who has visited the San Francisco Bay area has experienced local climate moderation. The city of San Francisco maintains a mild climate year-round, but just a few miles inland, where hills guard the bay, temperatures can soar to 90 degrees F (32.2 degrees C) in the summer and plummet to near freezing in the winter. The reason for this contrast, as most residents are well aware, is that the ocean moderates large temperature fluctuations. The same effect, on a global scale, is a factor in keeping seasonal temperature changes gradual rather than abrupt.

What governs the ocean's moderating effect is the large quantity and heat capacity of water. Heat capacity is the amount of heat energy that must be absorbed or released to raise or lower the temperature by a given amount. For example, it takes four times as much energy to warm a given mass of water by one degree Celsius as it does to warm the same mass of dry air by that amount. The heat capacity of water acts as a buffer, or perhaps a heavy flywheel, on climate, smoothing out what would otherwise be sharp changes in temperature.

Heat capacity is a good example of a macroscopic property of water that can be explained by what takes place at a molecular level. The chemical formula [H.sub.2]O, instantly recognized round the world, indicates that the water molecule is a bound system of three atoms, two of hydrogen and one of oxygen. When you add heat (a form of energy) to a macroscopic sample of water molecules, the molecules increase their average speed and collide more often. The temperature of the sample is simply a measure of their average speed. Any energy added to or subtracted from the energy stored as such "translational" motion--movement from one place to another--changes the temperature.