Strange properties of water: (a) Cohesiveness enables water to travel upward from roots to 

 leaves, against gravity, (b) High surface tension makes liquid water behave as if coated with an 

 invisible film, which explains why insects can walk on it. (c) Water exists in all three phases — gas, 

 liquid, and solid — at temperatures and pressures that are common on Earth. This familiar property 

 is actually quite unusual, (d) Ice floats on liquid water; unlike most substances, water is most dense 

 in its liquid phase. Lakes and even oceans would otherwise freeze solid in winter, (e) Water's 

 abundance and heat capacity are, in part, responsible for the moderation of global temperature 

 fluctuations and the gradual change of the seasons, (f) Water can dissolve a variety of substances, 

 including acids, bases, and salts, earning it the moniker "universal solvent." 



A 



the melting point. (Ordinary solids remain solid under 

 pressure.) Even though these and other unusual bulk prop- 

 erties 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 understand- 

 ing and control is 

 to study water on 

 a molecular scale: 

 how water mol- 

 ecules arrange 

 themselves, how 

 they interact, and 

 how they dance 

 with other kinds 

 of molecules. We 



Ice (left) consists of a collection of hydrogen-bonded water molecules 

 anu our colleagues arranged in a pattern of hexagonal rings: oxygen atoms make up the vertices 

 in the growing and hydrogen bonds the edges. In liquid water (center), the water molecules 

 held of molecular are linked to their neighbors by three to four hydrogen bonds, which continu- 

 science hope that a "y Drea k and re-form. Water vapor (right), an important constituent of the 

 by understanding atmosphere, consists of weakly interacting water molecules. 



exactly what happens at very small scales (around 10" 10 

 meter, or a billionth of a meter), we can zoom out by a 

 factor of a billion or so to understand and predict phe- 

 nomena 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. 



nyone 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 keep- 

 ing seasonal tem- 

 perature 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 w arm a given mass 



November 2007 natur al HISTORY 



33 



