PBS/Nova on water
So why is liquid water the indispensible ingredient for life as we know it? Liquid water may sound
redundant, but planetary scientists insist on using the qualifier, for solid or vaporous water won't do. The
biochemical reactions that sustain life need a fluid in order to operate. In a liquid, molecules can dissolve
and chemical reactions occur. And because a liquid is always in flux, it effectively conveys vital
substances like metabolites and nutrients from one place to another, whether it's around a cell, an
organism, an ecosystem, or a planet. Getting molecules where they need to go is difficult within a solid
and all too easy within a gas—vapor-based life would go all to pieces.
And why is water the best liquid to do the job? For one thing, it dissolves just about anything. "Water is
probably the best solvent in the universe," says Jeffrey Bada, a planetary scientist at the Scripps
Institution of Oceanography in La Jolla, Calif. "Everything is soluble in water to some degree." Even gold
is somewhat soluble in seawater. (Before you get any ideas about extracting gold from the oceans, I
should add that, according to Bada, the value of dissolved gold in a metric ton of seawater comes to
about $0.0000004).
Water plays another key role in the biochemistry of life: bending enzymes. Enzymes are proteins that
catalyze chemical reactions, making them occur much faster than they otherwise would. To do their
handiwork, enzymes must take on a specific three-dimensional shape. Never mind how, but it is water
molecules that facilitate this.
Water's ability to so successfully further the processes of life has a lot to do with just how unusual a fluid it
is. Not long ago, if I had to guess, I would have said that water is about as normal a liquid as they come.
In fact, despite its ubiquity and molecular simplicity, H2O is abnormal in the extreme.
For starters, while other substances form liquids, precious few do so under the conditions of temperature
and pressure that prevail on our planet's surface. In fact, next to mercury and liquid ammonia, water is our
only naturally occurring inorganic liquid, the only one not arising from organic growth. It is also the only
chemical compound that occurs naturally on Earth's surface in all three physical states: solid, liquid, and
gas. Good thing, otherwise the hydrological cycle that most living things rely on to ferry water from the
oceans to the land and back again would not exist. As science journalist Philip Ball writes in his
informative book Life's Matrix: A Biography of Water, "This cycle of evaporation and condensation has
come to seem so perfectly natural that we never think to remark on why no other substances display such
transformations."
Compared to most other liquids, water also has an extremely large liquid range. Pure water freezes at
0°C (32°F) and boils at 100°C (212°F). Add salt and you can lower the freezing temperature; natural
brines are known with freezing points below -50°F. Add pressure and you can raise the boiling
temperature; deep-sea vent waters can reach over 650°F. Water also has one of the highest specific
1
heats of any substance known, meaning it takes a lot of energy to raise the temperature of water even a
few degrees.
Water's broad liquid range and high heat capacity are good things, too. They mean that temperatures on
the Earth's surface, which is more than two-thirds water, can undergo extreme variations—between night
and day, say, or between seasons—without its water freezing or boiling away, events that would throw a
big wrench into life as we know it. As it is, the oceans serve as a powerful moderating influence on the
world's climate.
th
[Physical Chemistry by Daniels and Alberty, 4 edition, p. 86 confirms the relatively large liquid range of
water compared to most other liquids. It has the largest liquid range of all liquids that could be plentiful. As
an example, methane and ammonia, that are in liquid form on Saturn’s moon Titan, have a liquid range of
22 C and 44 C compared to 100 C for water.]
Liquid water has yet another unusual property that means the difference between life and essentially no
life in cold regions of the planet (or on most of the planet during ice ages). Unlike most other liquids when
they freeze, water expands and becomes less dense. Most other frozen liquids are denser than their
melted selves and thus sink. If it sank, ice, being unable to melt because of the insulating layer of water
above it, would slowly fill up lakes and oceans in cold climates, making sea life in those parts of the world
a challenging prospect.
[Since it floats, the surface freezes and it remains liquid and insulated underneath, where life can
continue.]
2
The expansion upon freezing comes from the fact that water crystallizes into an open hexagonal
form. This hexagonal lattice contains more space than the liquid state. (last two sentences and the
drawings are from Georgia State Univ physics website). (Liquid is random motion, solid falls
into the standard lattice framework like above.)
Could life as we don't know it have gotten a start without water? Some planetary scientists have
suggested that on certain very cold planetary bodies liquid ammonia might serve in place of water to
incubate life. But even though it's the most common non-aqueous solvent, liquid ammonia would seem to
have several other things going against it as a medium for life. Its liquid range is small, only about 30
degrees. Also, when it freezes, it sinks, and we know what that would do.
Some have suggested that oceans of methane or other hydrocarbons on places like Saturn's moon Titan
could also serve the purpose. But, again, we're talking temperatures so low that chemical reactions as we
know them could only proceed at a glacial pace. "At minus 150 degrees," says Bada, "most of the
reactions that we think about in terms of being important in the origin of life probably wouldn't take place
over the entire age of the solar system." Moreover, compounds like amino acids and DNA would not be
soluble (able to be dissolved; that is, becomes part of the solution) in these other liquids. "They would just
be globs of gunk," Bada says.
For these and other reasons, liquid water is still the Holy Grail for planetary scientists, who, based on
what they know today, consider it likely that liquid water is essential to all life, terrestrial and extra-. Says
Neil de Grasse Tyson, an astrophysicist and director of the Hayden Planetarium at the American Museum
of Natural History, "Given that life on Earth is so dependent on water, and given that water is so prevalent
in the universe, we don't feel that we're going out on a limb to say that life would require liquid water."
3
Sources: http://www.chem1.com/acad/sci/aboutwater.html and wikipedia.
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons
between atoms.
Oxygen has 8 protons and 8 electrons. Hydrogen has 1 and 1. Water has 10 protons and 10 electrons.
Oxygen has two electrons in the 1s orbital (spin up and spin down), two in the 2s orbital, and then there
are three 2p orbitals. Note that these orbitals come naturally from the quantum mechanics study of the
various energies and angular momentums that the electrons can have. The electrons can only exist in
states that satisfy the quantum formulas, integral wavelengths or they decay. Oxygen has two electrons
in one orbital and one in each of the other two. So it has room for two more electrons to complete its 2p
orbitals. These two electrons come from the hydrogen atoms and result in a lower energy state if they
are bonded.
Electron orbitals (UC Davis). Up and down spin electrons are allowed in each.
4
5
The molecule of water
A molecule is an aggregation of atomic nuclei and
electrons that is sufficiently stable to possess
observable properties— and there are few molecules
that are more stable and difficult to decompose than
H2O. In water, each hydrogen nucleus is bound to the
central oxygen atom by a pair of electrons that are
shared between them; chemists call this shared electron pair a covalent chemical bond. In H2O,
only two of the six outer-shell electrons of oxygen are used for this purpose, leaving four
electrons which are organized into two non-bonding pairs. The four electron pairs surrounding
the oxygen tend to arrange themselves as far from each other as possible in order to minimize
repulsions between these clouds of negative charge. This would ordinarly result in a tetrahedral
geometry in which the angle between electron pairs (and therefore the H-O-H bond angle) is
109.5°. However, because the two non-bonding pairs remain closer to the oxygen atom, these
exert a stronger repulsion against the two covalent bonding pairs, effectively pushing the two
hydrogen atoms closer together. The result is a distorted tetrahedral arrangement in which the
H—O—H angle is 104.5°.
Oxygen attracts electrons much more strongly than hydrogen, resulting in a net positive charge on the
hydrogen atoms, and a net negative charge on the oxygen atom. The presence of a charge on each of
these atoms gives each water molecule a net dipole moment. Electrical attraction between water
molecules due to this dipole pulls individual molecules closer together, making it more difficult to
separate the molecules and therefore raising the boiling point.
Water has a very high specific heat capacity – the second highest among all the heteroatomic species
(after ammonia), as well as a high heat of vaporization (40.65 kJ/mol or 2257 kJ/kg at the normal boiling
point), both of which are a result of the extensive hydrogen bonding (sharing the electrons between
hydrogen and oxygen) between its molecules. These two unusual properties allow water to moderate
Earth's climate by buffering large fluctuations in temperature. According to Josh Willis, of NASA's Jet
Propulsion Laboratory, the oceans absorb one thousand times more heat than the atmosphere (air) and
are holding 80 to 90% of global warming heat.
The solid form of most substances is denser than the liquid phase; thus, a block of most solids will sink in
the liquid. However, a block of ice floats in liquid water because ice is less dense. Upon freezing, the
density of water decreases by about 9%.[19] This is due to the 'cooling' of intermolecular vibrations
allowing the molecules to form steady hydrogen bonds with their neighbors and thereby gradually
locking into positions reminiscent of the hexagonal packing achieved upon freezing to ice Ih.
Water is one of the few known substances whose solid form is less dense than the liquid. The
plot at the right shows how the volume of water varies with the temperature; the large increase
6
(about 9%) on freezing shows why ice floats on water and why pipes burst when they freeze. The
expansion between –4° and 0° is due to the formation of larger hydrogen-bonded aggregates.
Above 4°, thermal expansion sets in as vibrations of the O—H bonds becomes more vigorous,
tending to shove the molecules
farther apart.
These properties of water have
important consequences in its role in
Earth's ecosystem. Water at a
temperature of 4°C will always
accumulate at the bottom of
freshwater lakes, irrespective of the
temperature in the atmosphere. Since
water and ice are poor conductors of
heat[22] (good insulators) it is unlikely
that sufficiently deep lakes will
freeze completely, unless stirred by
strong currents that mix cooler and
warmer water and accelerate the
cooling. In warming weather, chunks of ice float, rather than sink to the bottom where they
might melt extremely slowly. These properties therefore allow aquatic life in the lake to survive
during the winter.
[Note that all these properties are extremely important for complex life, but result from the basic
physical laws, working themselves out in the Universe.]
Liquid water:
H2O molecules attract each other through the special type of dipole-dipole interaction
known as hydrogen bonding
A hydrogen-bonded cluster in which four H2Os are located at the corners of an imaginary
tetrahedron is an especially favorable (low-potential energy) configuration, but...
The molecules undergo rapid thermal motions on a time scale of picoseconds (10–12
second), so the lifetime of any specific clustered
configuration will be fleetingly brief.
Ice, like all solids, has a well-defined structure; each water
molecule is surrounded by four neighboring H2Os. Two of
these are hydrogen-bonded to the oxygen atom on the
central H2O molecule, and each of the two hydrogen atoms
is similarly bonded to another neighboring H2O.
The hydrogen bonds are represented by the dashed lines in
this 2-dimensional schematic diagram. In reality, the four
bonds from each O atom point toward the four corners of a tetrahedron centered on the O
atom. This basic assembly repeats itself in three dimensions to build the ice crystal.
7
When ice melts to form liquid water, the uniform
three-dimensional tetrahedral organization of the
solid breaks down as thermal motions disrupt,
distort, and occasionally break hydrogen bonds.
The methods used to determine the positions of
molecules in a solid do not work with liquids, so
there is no unambiguous way of determining the
detailed structure of water. The illustration here is
probably typical of the arrangement of neighbors
around any particular H2O molecule, but very little is known about the extent to which an
arrangement like this gets propagated to more distant molecules.
About two-thirds of the weight of an adult human consists of water. About two-thirds of
this water is located within cells, while the remaining third consists of extracellular water,
mostly in the blood plasma and in the interstitial fluid that bathes the cells. This water,
amounting to about five percent of body weight (about 5 L in the adult), serves as a
supporting fluid for the blood cells and acts as a means of transporting chemicals between
cells and the external environment. It is basically salt (NaCl) water containing smaller
amounts of other electrolytes, the most important of which are bicarbonate (HCO3–) and
protein anions.
The water content of our bodies is tightly controlled in respect to both total volume and
its content of dissolved substances, particularly ions. Drinking constitutes only one
source of our water; many foods, especially those containing cells (fruits, vegetables,
meats) are an important secondary source. In addition, a considerable amount of water
(350-400 mL/day) is produced metabolically — that is, from the oxidation of glucose
derived from foods. (see respiration equation in the carbon discussion earlier)
Water expands as it freezes. It is because of this effect that we can have complex life on Earth.
Because the oceans stayed liquid under the layer of ice, life could flourish, while the surfaces
were frozen. Otherwise the entire oceans would have frozen.
8