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Rime of the Ancient Mariner - Part V
And soon I heard a roaring wind :
It did not come anear ;
But with its sound it shook the sails,
That were so thin and sere.
The upper air burst into life !
And a hundred fire-flags sheen,
To and fro they were hurried about !
And to and fro, and in and out,
The wan stars danced between.
_____________________________
AIR
When you stand beneath the sky and on top of the world, you are really standing at the bottom of an
ocean hundreds of kilometres deep made of air. It is a gaseous soup, dominated by nitrogen (4/5th) and oxygen
(1/5th) but containing a rich variety of compounds, some of which are essentially for our daily lives and some
of which are undesirable species.
That air is a gas is something that we can easily recognize but this was not always the case. Indeed,
while the ancients understood air to be an essential element, they often dismissed it from consideration and
certainly were not interested in assaying its chemical composition. As in any gas, the molecules in air are in
continuous chaotic motion. Indeed, the word “gas” comes from the same Greek root as “chaos”.
Air molecules hurtle through space at about the speed of sound - some 1130 km/hr or 340 m/s. It is
actually the speed that air molecules move that dictates the speed of sound. But they don’t all move in the same
direction at once, otherwise we would be flattened! Instead, the gas molecules in the air engage in a mammoth
version of a demolition derby, colliding full force into one another, only to bounce off and go charging off in
another direction a fraction of a second later.
The incessant impact of this storm of molecules on the surface of a container - including your
container, your skin - is experienced as a virtually constant pressure (about 14 pounds per square inch or 1 atm
or 760 mmHg or 101.3 kPa at sea level). On a still, warm summer’s day, or in a quiet room you are continually
in the centre of an unseen, unfelt storm of molecules pressing in and down upon you.
Actually, the pressure of the atmosphere is not totally unfelt and is certainly observable as changes in
the pressure are associated with the “highs” and “lows” that bring about weather. A high pressure ridge results,
literally, from the crest of a wave in the ocean of that is the atmosphere. As the crest passes over, there is more
air above and a greater pressure exerted beneath. Conversely, a low pressure ridge is the trough that invariably
follows the wave.
As further evidence of the activity and perpetual pounding that we take from the air around us, it is
possible to hear the air molecules pounding off your ear drums. It is the low hiss that can be heard - a sort of
static - when there is no other noise about. Within a sound proof room or on a quiet and still day, each little
molecule bouncing upon your drums can be heard.
When the wind blows, the molecules stream predominantly in one direction and strike that side of your
body. It is easy, then, to hear the effect of air but not individual molecules. Still, the collective force of moving
air can be quite powerful - witness the power of a hurricane which is more than sufficient to knock down trees
and buildings. Ditto the tornado. Yet, all they are is moving air.
It is not clear where our atmosphere came from or how it has changed, although one guess about the
latter is shown in the illustration below. There is general agreement that an early atmosphere was formed as
an outgassing of the rocks and planetesimals that aggregated to form our initially molten and primitive planet.
A similar kind of outgassing occurs today at volcanoes and it is surmised that the gases they release - largely
water vapour, hydrogen, hydrogen chloride, carbon monoxide, carbon dioxide, nitrogen, and sulfur containing
molecules - were abundant in the first atmosphere. Of these, only nitrogen is abundant now. Hence, there is
some question about where the rest have gone and where the new gases have come from.
One substance can be dealt with quickly and that is hydrogen. Being very light, hydrogen moves very
fast. Indeed, it is fast enough that it can escape the pull of gravity, leave the planet, and head off for inter-stellar
space. Perhaps, there, it will one day contribute to the formation of a new star, new planets, and new life.
Adapted from “Molecules” by P.W. Atkins, Scientific American Library, 1987.
AIR
Air is, for the most part, a mixture of four simple gases. It’s nominal composition is:
78.084 ± 0.004% Nitrogen N2
Molecular Weight:
28.0134 g/mole
Melting Point: -210.01/C (63.14 K)
Boiling Point:
-195.79/C (77.36 K)
Normal physical state is a gas at room temperature
20.946 ± 0.002% Oxygen O2
Molecular Weight:
31.9988 g/mole
Melting Point: -218.40/C (54.75 K)
Boiling Point:
-182.96/C (90.19 K)
Normal physical state is a gas at room temperature
0.033 ± 0.001% Carbon dioxide
CO2
Molecular Weight:
44.01 g/mole
Sublimation Temp.: -78.48/C (194.67 K)
Normal physical state is a gas at room temperature
0.934 ± 0.001% Argon
Ar
Molecular Weight:
39.948 g/mole
Boiling Point:
-185.86/C (87.29 K)
Normal physical state is a gas at room temperature
Within the troposphere (the first 10 kilometres or so above the surface), air is composed
predominantly of these four gases but it also naturally contains a smattering of other gases (Helium
[He], Neon [Ne], Krypton [Kr], Xenon [Xe], Hydrogen [H2], Methane [CH4], and Nitrous Oxide or
‘Laughing Gas’ [N2O]) and a fair amount of water vapour (depending upon weather conditions,
temperature, and local climate). At higher altitudes, other gases are present - most notably ozone
which is located in a band of significant concentration about 20 km thick and centred about 30 km
above ground.
Argon
Although argon is not the most abundant gas in the atmosphere, it is one of the simplest of
all compounds consisting of just a single atom. Argon means “the unreactive one” and it is the third
most abundant compound in every breath. It is because argon is so unreactive that it exists in the air
as atoms and forms no stable compounds.
It is also the reason that all of the argon on earth is re-cycled from person to person. As a
calculation, the average person breathes about 14 times per minute, pumping in and out about 1 litre
of gas or 10 mL of Argon (1 litre = 1000 mL). Over the course of a lifetime, say Leonardo da Vinci’s,
that represents about 67 years of breathing, a total of 493 million breaths or about 4.93 million litres
of Argon. This might seem like a lot but it is very little compared with the total volume of air
surrounding earth, which is some 8.12 x 1020 litres. What this means, though, is that in every litre of
air that you breath, there is 6.07 x 10-15 litres of Argon that was also breathed by Leonardo da Vinci.
How much is that? Well, about 155 million Argon atoms - which, if you think about it, is quite a lot.
Why do this calculation? Not to show off my calculator but to make the point that the
atmosphere is one of the ways that we are all connected. We share the atoms of this planet with all
things, living and dead, that have existed on this planet, be they a grub in the earth, a chunk of
limestone, or Leornardo da Vinci. Oh, yeah, and it really only works with people who have been dead
for a while. Local mixing - or the lack thereof - means that in a classroom, for example, we are all
sharing a lot more than just 155 million Argon atoms each time we breathe!
Argon was introduced into the air from below the surface of the Earth. An Argon atom is
formed when the nucleus of a potassium atom in a mineral such as potash captures an electron
released during the radioactive decay from another atom in the surrounding material. The electron
“transmutes” the potassium nucleus into an Argon nucleus, which subsequently loses an electron
through chemical means (read: non-radioactively). Since the Argon can not bind to any of the
surrounding elements, it leaks out of the earth and into the atmosphere. This process is ongoing and
as the earth ages, the concentration of argon in the atmosphere is slowly increasing. (Very slowly!)
Argon is harvested from the atmosphere, which is our sole and barely sufficient source. It is
either obtained directly from the distillation of air or as a by-product from the manufacture of
ammonia. Large amounts are used in the steel industry to dilute the more reactive oxygen that is
introduced into the molten metal to burn away impurities. It is used as the gas in incandescent light
bulbs since it is inert and does not react with the white hot tungsten filament. Argon is mixed with
mercury vapour to fill fluorescent lights as well. In this case, the Argon is actually part of the light
making process as its high energy fluorescence helps to excite the phosphor coating the inside of the
glass tube causing it to glow with a characteristic white light.
Argon can be made to glow blue. The colour originates from the electric current which passes
down the tube as a storm of electrons. These electrons smash into atoms of the gas and excite them
to states of increased energy in which their own electrons are rearranged slightly. An excited atom
shrugs off this extra energy almost immediately as its electrons collapse back into a lower energy
arrangement. This discarded energy is radiated away as light. The greater the amount of energy to
be lost, the shorter the wavelength or the bluer the emitted light. Light is only lost at one wavelength
because the energy is specific for the “distance” that the electron has to fall.
Oxygen
Oxygen accounts for about 20 percent of the volume of the atmosphere. It is also the most
abundant element in the earth’s crust, accounting for almost half its total mass. However, within the
crust, it does not occur in molecular form but in combination with other elements forming water,
silicates, and metal oxides. It is also found in abundance in moon rocks, occurring as silicates, metal
oxides, and according to recent results, water. It does not occur in a free state due to the low gravity
found of the moon which allows gas molecules to easily achieve escape velocity.1
Although oxygen is abundant in the atmosphere and is obtained industrially from this rich
source by distillation of liquid air, it is a relative newcomer on a global scale. The atmosphere of the
newly formed earth did not contain oxygen. Some of the molecular oxygen was formed when water
molecules that had outgassed from the rocks were broken apart by the intense radiation from the sun
(without the atmosphere, there was nothing blocking the intense UV light - a situation that we may
yet again face). In their mono-atomic forms, some of the oxygen and hydrogen atoms combined to
provide O2 and H2 gas.
But the bulk of our oxygen arrived when the first photosynthezing cells evolved. These were
the prokaryotes2 that we call “blue-green algae” or “cyanobacteria”. These single-celled life forms
acquired hydrogen from water (discarding the oxygen) and carbon and oxygen from carbon dioxide
to build their own carbohydrates. Thus, the oxygen that we now prize so highly, that is essential to
most animal life, and that must be carried whenever we explore alien environments, was originally
a pollutant in an atmosphere that favoured a different form of life. That great pollution left its imprint
on the earth, for the surge of oxygen that accompanied the emergence of photosynthesis oxidized the
iron dissolved in the seas. The earth rusted, and the great deposits of red iron ore that can be
observed around Cache Creek are a mute testimony to that epoch.
Oxygen itself is an odourless, colourless, tasteless gas that condenses to a pale blue liquid. The
colour change comes about when pairs of molecules cooperate in the absorption of light - a
phenomenon that is possible only when the molecules are close together as in a liquid or solid state.
Oxygen also has the unusual property of being magnetic. This is most clearly shown by the ability of
a magnet to pick up liquid oxygen, but the gas is also magnetic. One application of this property is
to the measurement of oxygen concentrations in artificial atmospheres, such as in incubators for
premature babies. The magnetism of the atmosphere is monitored and the reading is converted to the
concentration of oxygen molecules.
Oxygen is highly reactive. When its molecules are torn apart (by heat or light), the liberated
atoms can form strong bonds with atoms of other elements. The strength of these bonds is due to the
smallness of the oxygen atom. The central nucleus can exert a strong force on neighbouring electrons,
including those of other atoms, because it can approach them quite closely. Because it is highly
reactive, oxygen can only exist in the atmosphere in a molecular form - either the “oxygen” molecule
or as ozone.
1
Earth’s much higher gravity (about six times that of the moon) allows it to retain an
atmosphere (which is a good thing) but some gases still escape. Both hydrogen and helium travel
faster than the escape velocity, allowing them to leave the atmosphere so the world’s supply is
slowly bleeding off into space.
2
A cell without a membrane-bound nucleus as opposed to the eukaryotes which have a
membrane-bound nucleus and compose most higher functioning organisms such as you and me.
The prokaryotes evolved a long time before the eukaryotes.
Ozone
Ozone is present in the upper atmosphere in the ozone layer which is a region in the
atmosphere about 20 km thick centred between 25 and 35 km above the surface of the earth. If all
of it were collected and compressed to the atmospheric pressure characteristic of the earth’s surface,
it would form a layer about 3 millimetres thick - about half of the width of a pencil.
Ozone is formed when the sun’s UV radiation is absorbed by molecules containing oxygen most commonly, this is molecular or diatomic oxygen. In absorbing the radiation, the molecule is
blasted apart generating atomic oxygen which subsequently bond to any O2 molecules that they strike
(or anything else, for that matter, except nitrogen). Once formed, the ozone molecule absorbs more
ultraviolet radiation at a different wavelength and is itself blasted apart. Both processes, ozone
formation and ozone depletion, absorb UV radiation and help to protect the living organisms on the
surface below. The absorption is so efficient that at wavelengths near 250 nm (harsh ultraviolet), only
1 part in 1030 of the incident solar radiation penetrates the ozone layer. A being with eyes able to see
only 250 nm light would see the sky as pitch black at noon.
Ozone is a blue, pungent gas (ozein is Greek for “to smell”) that condenses to an inky blueblack explosive liquid. Its smell can be detected near electrical equipment and after lightning, since
it is also formed by an electric discharge through the atmosphere or, more accurately, through
oxygen. It is not only unpleasant to smell but can also lead to respiratory distress, headaches, and
nausea. For this reason, the cabin air supply in commercial airliners is passed through filters that
catalytically decompose the ozone to ordinary oxygen.
Nitrogen
Nitrogen is the most abundant gas in the atmosphere and probably resulted from the
outgassing of rocks and minerals, just as did the other gases of the early atmosphere. However,
nitrogen molecules are too heavy and too slow to have escaped the earth, and they are too unreactive
to have combined with other substances to any great extent, so what was abundant has remained
abundant.
Like oxygen, nitrogen naturally forms diatomic molecules. However, the atoms in N2 are
bound together by a triple bond. This results in one of the most strongly bonded molecules known,
one that can survive collisions with other molecules which, for double-bonded oxygen, would have
led to a reaction. Nitrogen’s relative lack of reactivity allows it to act as a dilutant for the dangerous
oxygen in the air. Without atmospheric nitrogen, one spark would long ago have ignited all the
vegetation of the earth in a massive and annihilating fire storm.
Nitrogen’s inertness, however, must not be confused with Argon’s, which can be attributed
to its individual atoms having no tendency to react with any other. Nitrogen’s inertness is a property
of the molecule, not the atoms, and is due to its atoms having formed three very strong and difficult
to break bonds. However, once those bonds are broken, nitrogen is highly reactive and forms
numerous compounds.
Nitrogen is essential to the growth of plants as many of the molecules in living cells,
particularly the proteins, contain nitrogen atoms. Its incorporation into living organisms, first into
plants and then into animals, began with its conversion into nitrogen oxides by lightning or solar
radiation in the upper atmosphere. These more reactive compounds were then washed out of the
atmosphere and into the soil.
However, the major highway for the movement of nitrogen from the air to living organisms
is nitrogen fixation. This is achieved biologically by certain prokaryotes, including bacteria
(particularly cyanobacteria) and actinomycetes (branching, multicellular mould-like organisms). Some
of these bacteria (particularly Azotobacter and Clostridium) can exist and operate individually but the
most important (Rhizobium) form symbiotic associations with higher plants, particularly the legumes
(clover, pulses such as peas and beans, alfalfa, acacia, etc.), whose roots they colonize. In all cases,
the agent responsible for fixing nitrogen is the enzyme system nitrogenase. This enzyme consists of
two protein molecules which are basically an organic scaffolding that supports an active site
consisting of molybdenum and iron atoms linked by sulphur. This enzyme converts water and nitrogen
to a combination of ammonia, oxygen, and hydrogen. It is a sobering thought to realize that this
enzyme, crucial to the food chain, is present as only a few atoms in a million in soil.
Ammonia is also generated industrially through the Haber process. Nitrogen and hydrogen
are reacted together at high temperatures and pressures to give ammonia. Ironically, nature does this
at atmospheric pressures and temperatures with nitrogenase and mimicking this process industrially
is a major goal of industrial chemists. The Haber process must also use a lot of energy to overcome
the nitrogen triple bond which makes it an “expensive” process. However, it is absolutely necessary
to support the burgeoning production of crops in our farmlands. Ammonia or one of its salts is
sprayed into the soil as fertilizer and is necessary for the level of agricultural production that we
presently require.
Nitrogen lost from the atmosphere, naturally or otherwise, is replenished by the
decomposition of vegetation and flesh, for when protein molecules decompose, the nitrogen atoms
are released as ammonia molecules, which in due course degrade back to gaseous, diatomic nitrogen.
Carbon Dioxide
Carbon dioxide is a gas we exhale, for it is one end product of the consumption of the organic
compounds that we ingest as food. When an organic compound burns - which, in a very slow form,
is metabolism - each carbon atom is excised from its molecule by two oxygen atoms and carried away
as CO2. (If insufficient oxygen is provided, the carbon is carried away as carbon monoxide, CO.) In
a flame, the disruption of the molecule and the formation of strong carbon-oxygen bonds is
accompanied by the release of energy as heat. Carbon dioxide is the end of the road for the
combination of carbon with oxygen and its formation corresponds to the maximum release of energy.
Carbon dioxide is a “dead” form of carbon.
But it is not inert, for green vegetation uses the energy of sunlight to pluck carbon dioxide
from the skies, combine it with hydrogen obtained from water, and build carbohydrates in a process
called “photosynthesis”. This process is the single most important process for the maintenance of life
as we know it on this planet. And, in combination with the dissolution of carbon dioxide into sea
water, it helps to maintain the balance of carbon dioxide in the atmosphere.
When carbon dioxide is formed, whether in a muscle or a brain, the energy released may be
used to raise a weight or produce an idea (respectively!). Carbon dioxide is also an end product of
the partial consumption of carbohydrates during fermentation, an incomplete form of respiration that
forms alcohol as another principal product. Hence, carbon dioxide is the gas in the head of beer and
the bubbles in champagne. It is also the fizz in a bottle of pop and can be released through nucleation.
In water, carbon dioxide forms the very weak acid carbonic acid, which tingles the tongue,
is a taste enhancer, and acts as a mild bactericide. In the ocean, carbonic acid is neutralized to various
mineral carbonates. Carbonic acid is also said to encourage flow from the stomach to the intestine
and from the intestine to the bloodstream, which perhaps accounts for the rapid inebriating effect of
champagne. (Or maybe champagne has this reputation because it is the drink of choice when we
celebrate.)
Carbon dioxide is the fourth most abundant component of the dry atmosphere - under certain
conditions, the concentration of water vapour can exceed that of CO2. However, it is the most
abundant gas in the atmospheres of Mars and Venus. A great deal of carbon dioxide was removed
from earth’s early atmosphere as the oceans fell from the skies, for the gas is readily soluble in water.
Now, most of the carbon dioxide of the early planet lies beneath our feet in the form of carbonate
rocks - chalk and limestone. But a great deal of carbon dioxide is still suspended in the oceans, as
carbonate ions.
No similar precipitation of water occurred on the hot surface of Venus or the cold surface of
Mars, so on those planets the carbon dioxide remains in the atmosphere. It has been calculated that
the mass of carbonate rock on earth, plus the amount of carbon dioxide in the atmosphere and
dissolved in the oceans, is approximately the same as the mass of carbon dioxide that now hangs in
the skies of Venus and make that planet a “hothouse hell”. Had the earth been only 10 million
kilometres closer to the sun than its present 140 million kilometre location, the temperature of its
surface would have been too high for the oceans to have formed and the earth would have evolved
into a planet like Venus.
Carbon dioxide in the atmosphere acts partly to trap the infrared radiation emitted from the
warm surface fo the earth. Because carbon dioxide is transparent to visible light it penetrates to the
surface. As the surface warms, it emits infrared radiation that cannot escape back into space as it is
absorbed by the carbon dioxide molecules. Hence, carbon dioxide in our atmosphere acts like a one
way mirror - letting in light but trapping the returned heat. The trapped energy warms the atmosphere
in a process called the greenhouse effect.3
Carbon dioxide is used as a leavening agent in baking for such foods as pancakes(!) Typical
baking powders consist of sodium bicarbonate (NaHCO3), an acid (or, typically, two acids such as
tartaric acid and sodium aluminum sulfate which is an acidic salt), and starch. The latter acts as a
filler, keeping the acid(s) and bicarbonate separate until it is time to react. But even so mundane a
3
In a real greenhouse, the heat buildup is due more to the glass preventing a convective
mixing of the warm air inside with cold outside air than to the absorption of infrared radiation but
the term is still used by climatologists and astronomers.
product as baking powder has an engaging science, because it has to provide separate bursts of action
or carbon dioxide release. The first production of carbon dioxide occurs at room temperature as a
result of the action of the moistened tartaric acid, and it produces many tiny cavities in the batter. The
second burst of activity is due to the action of the aluminum salt, and it occurs at high temperature.
This second flux of carbon dioxide swells the cavities to give the desirable final light texture.
The carbon dioxide used in bread making is usually formed by the action of yeast on sugar
or other small carbohydrate molecules. Such yeast particles are present in the air, but to achieve more
uniform characteristics in baking, a particular strain, Saccharmyces cerevisiae, is normally cultured
in dilute molasses and then used to do the job.
Adapted from “Molecules” by P.W. Atkins, Scientific American Library, 1987.