A SPECIAL BREW 



Investigators still can't completely explain 

 the strange molecular workings of water. 



By Christopher J. Mundy, Shawn M. Kathmann, and Gregory K. Schenter 



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 pour- 

 ing 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 oc- 

 casional 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 



32 NATURAL HISTORY November 2007 



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 (H2O) acted in bulk like other 

 small molecules — oxygen (Ot), carbon monoxide (CO), 

 nitrogen (N2) — 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 



