Geology Reference
In-Depth Information
(Na
+
)orcalcium(Ca
2+
)ions,andothersnegativelychargedlikechlorine(Cl
−
)orcarbonate
(CO
3
2−
) ions. A rule of thumb is that the net electrical charge of any bulk water solution
must be zero: the total number of positive charges must be balanced by an equal number
of negative charges. In pure water at room conditions, 10
−7
moles of H
3
O
+
is perfectly bal-
anced by 10
−7
moles of OH
−
. In acids, however, excess H
3
O
+
has to balance negative ions
(such as chlorine in hydrochloric acid, HCl). In bases, excess OH
−
has to balance positive
ions (such as sodium in the base sodium hydroxide, NaOH).
Thestrengthofacidsandbasesisquantified bythepHscale.Smaller valuesofpHpoint
to acidic solutions with more H
3
O
+
than OH
−
ions. A slightly acidic solution with pH 6
(typical of untreated tap water in many localities) has ten times more hydronium ions than
a neutral solution at pH 7. More strongly acidic liquids include coffee (pH 5, with a hun-
dred times more H
3
O
+
), vinegar (pH 3, with ten thousand times more H
3
O
+
), and lemon
juice (pH 2, with one hundred thousand times more H
3
O
+
). By contrast, bases are liquids
with more OH
−
than H
3
O
+
,andthuswith pHvalues greater than 7.Common bases include
baking soda (pH 8.5), milk of magnesia (a common antacid with pH 10), and household
ammonia cleaners (pH 12). As we shall see, the pH and salinity of Earth's earliest oceans
remains a hotly debated matter.
Water, Water Everywhere
Waterisoneofthemostabundantchemicalsinthecosmos.Themorewelook,themorewe
find, and its presence on other planets and moons and comets provides clues to its abund-
ance on Earth, as well as the possible distribution of water-dependent life in the universe.
Telescopic observations can be tricky, since our water-rich atmosphere tends to obscure all
but the most concentrated deposits of H
2
O on more distant sources. Nevertheless, some
objects in deep space reveal an icy surface via their distinctive absorption of infrared radi-
ation.
This spectroscopic fingerprint reveals that some comets and asteroids incorporate signi-
ficantamountsoffrozenwater.Astronomershavedocumented numerousicyworldsinour
Solar System, from Pluto and its companion moon Charon, to Saturn's luminous ice rings.
All of the gas giant planets, though primarily made of hydrogen and helium, hold signi-
ficant stores of water vapor in their dense atmospheres. And Jupiter's large moons Europa
and Callisto are now thought to feature an ice veneer a few miles thick over much deeper
encircling oceans of water.
Closer to home, the other terrestrial planets at first glance appear to be rather dry. Mer-
cury is so close to the Sun that any near-surface water has likely been baked off, making
