WATER’S UNIQUE bonds help make it the stuff of life.
As of Saturday, March 1, 2014
Our recent winter weather has kept me thinking about ice, snow and water lately. We spend every day surrounded by water in many different forms, rarely considering its critical role in our lives both past and present.
Water is the most common substance on Earth, covering over two-thirds of the planet’s surface, and life here almost certainly first began in the ancient oceans.
Detecting water on a distant exoplanet would be considered a key marker of possible life there as well. Although other liquids could theoretically support life different from Earth’s, water’s chemistry and resulting special properties make it an ideal choice.
A single molecule of water, the smallest trace of water that still exists as water, consists of two hydrogen atoms and one oxygen atom, or H2O. These are held together by covalent bonds, a type of strong chemical bond that involves sharing pairs of negatively charged electrons between atoms.
Because of some specific qualities of oxygen and hydrogen, these shared electrons end up spending a little more time on the oxygen side of a water molecule. The oxygen becomes slightly negatively charged, leaving the hydrogens slightly positively charged, and the molecule has what’s known as polarity.
Polarity results in water molecules attracting each other like tiny magnets, naturally positioning themselves with their positive and negative poles in alignment. This attraction creates hydrogen bonds, and explains much of what makes water so special.
Water has a high specific heat capacity, meaning it takes a lot of heat to warm water up and longer for it to cool down. When liquids heat up, their molecules move around faster as they gain energy. But because of hydrogen bonding, it takes more energy to get water molecules to change speed. This means lakes and oceans don’t warm or cool as quickly, making them a more stable environment. For animals, it’s easier to maintain a steady internal temperature.
Hydrogen bonds also explain surface tension, when water bulges above the top of a glass or a bug walks on water. They make water molecules “sticky” enough to hang together when many liquids won’t. This also allows water to defy gravity and rise up narrow tubes. Called capillary action, it’s a critical property for plants moving water from roots to leaves.
Water is often called the universal solvent, because it dissolves so many substances. Polarity again plays a key role, surrounding and helping separate (dissolve) other charged molecules like salt.
Our blood plasma, mainly water, can transport sugars, proteins and many other substances thanks in part to polarity.
The last example is probably the most familiar. As liquids cool, the molecules slow down and move closer together as they lose energy, becoming more dense. Water does this only until it reaches about 39 degrees, when the molecules get so close together polarity begins to repel instead of attract. This keeps water molecules farther apart as they crystallize and freeze, with the result that ice is less dense than water. It floats.
Thanks to floating ice, lakes and rivers freeze from the top down, rather than from the bottom up. Surface ice acts like an insulating blanket, helping keep water beneath from freezing and allowing aquatic life to survive. Sea ice also serves as a platform for bears, seals, birds and other creatures to feed and travel.
That’s just the tip of the iceberg when talking about water. We live around water in snow and streams, use it daily in our homes, breathe in water vapor, depend on water for our body functions, and probably originated in a primordial soup whose main ingredient was water. Not bad for a clear, colorless and odorless fluid.
Lifelong Oregonian Fred Schubert, a The Dalles biologist, has a lifelong interest in general science and science writing. Feel free to submit any comments on this article or suggestions for new topics to email@example.com.