Geology Reference
In-Depth Information
ally high surface tension—a fascinating property that allows small insects literally to walk
on water. Surface tension also leads to capillary action, which causes water to rise through
thestemsofvascularplantsandallowstreestosoarhundredsoffeetabovetheland.Roun-
ded raindrops, pulled together by the strong mutual attraction of water molecules, are yet
another manifestation of surface tension and a vital link in maintaining Earth's unusually
rapidwatercycle.Nonpolarvolatilemoleculeslikemethaneandcarbondioxidecan'tform
such droplets. They would just float in the atmosphere as an ultrafine, pervasive mist, so
“rain” would be unknown on a planet dominated by those atmospheric gases.
Strongbondsbetweenmolecules resultinanotherofwater'smostcuriousandimportant
properties: liquid water is about 10 percent denser than solid ice. In almost every known
chemical compound, the solid sinks in its liquid—a situation that is intuitively logical be-
cause the regular, repeated packing of molecules in solids contrasts with their random dis-
tribution in liquids. Think about storing shoe boxes in the back room of a shoe store. Neat
stacks and rows of boxes (like perfectly aligned molecules in a solid crystal structure) take
up much less volume than a random pile (like chaotically tumbling molecules in a liquid).
But in water, the molecules actually pack more efficiently in their random liquid state than
they do in orderly ice crystals.
The important consequence is that ice, whether it is a cube in your drink, a layer on a
frozen river or stream, or a giant iceberg, floats. Were it not for this unusual characteristic,
many bodies of water would freeze solid, bottom to top, rather than forming a thick, pro-
tective surface layer of ice every winter. In such a solidly frozen world, aquatic life would
be severely challenged, while the vital water cycle would all but come to a halt. Curiously,
the same phenomenon is one of several factors that facilitate ice-skating and skiing. The
high pressure exerted by the blade of your skate pressing down on solid ice helps to pro-
duce a thin layer of denser liquid water over which you can glide. If the temperature be-
comes too cold, typically below about -100 degrees Fahrenheit, the lubricating water layer
doesn't form, and ice-skating and skiing become much more difficult.
Yet another distinctive characteristic of “pure” water is its lack of purity. No matter how
carefully filtered or distilled, water is
never
made entirely of H
2
O molecules. A small frac-
tion of those three-atom units inevitably splits apart into positively charged hydrogen ions
(hydrons, or H
+
ions, which are actually just individual positively charged protons without
any electrons attached), plus negatively charged hydroxyl groups (OH
−
ions). Hydrons
quickly latch on to water molecules to produce H
3
O
+
hydronium ions. What we call pure
water at room temperature contains equal numbers of positive hydronium and negative hy-
droxyl groups, at a concentration that translates to a pH of 7 (a “power of hydrogen” of
10
−7
moles of hydronium groups per liter, in chemistry terms).
A prime focus of speculation on Earth's earliest oceans is their pH and salt content.
Water easily dissolves lots of impurities, some of which are positively charged like sodium
