Friday Jan. 17, 2014
A little Cajun music to check out experiment materials to from
the Lost Bayou Ramblers: "Blue Moon
Special" (recorded during an appearance in Sweden) and
"Moi
J'Connais Pas" (filmed at an appearance in France).
About 45 sets of
Experiment #1 materials were checked out before class today.
Those of you that have materials will eventually find your name on
the Expt. #1 signup list. Even
though your report isn't due until Feb. 10, the experiment can
take several days, maybe even a week, to run to completion so
don't wait too long to get it started; this weekend would be a
perfect time. You may need to check the experiment fairly
frequently at the beginning (every hour or two). It slows
down somewhat as it progresses and eventually you will only need
to look at it once or twice a day. You'll find more
information about the experiment here.
Once you have collected your data, return your
materials and pick up the supplementary information handout.
Try to do this before the experiment report is due because the
handout will help with the analysis portion of your report.
It will also make materials available for someone else that wants
to do the experiment. Your name should turn this rust color
when you have returned your materials.
Signup sheets for the remaining
experiments were circulated in class. So some of you may not
be signed up so I'll try to bring the lists again next week.
You can check on the appropriate
list to see if your name is there (it will take a day or two
for that to happen)
We spent a few minutes reviewing a couple of items that weren't
covered in class on Wednesday.
The air in the classroom contained some water vapor (the air was
pretty dry and the dew point temperature was pretty low).
Water vapor (a gas) is also invisible. However when it moves
and comes into contact with the cold dry ice and liquid nitrogen
the air is cooled. When you cool air to or below its dew
point temperature, the water vapor will condense and form small
drops of liquid water or solid ice crystals (a gas to solid phase
change is called deposition). We are able to see the water
droplets and ice crystals. That is what forms the whitish
cloud that you see surrounding the dry ice and hovering over the
liquid nitrogen.
Here's the list again of the 5 main gases in air and what
we learned about them in class on Wednesday.
This present day atmosphere is very different from the earth's
original atmosphere which was mostly hydrogen and helium with
lesser amounts of ammonia and methane.
This original atmosphere probably both escaped (the earth was
hot and the gases were moving around with enough speed that they
could overcome the pull of the earth's gravity) and was swept
into space by the solar wind
(click on the link if you are interested in learning more about
the solar wind, otherwise don't worry about it).
With the important exception of oxygen (and
argon), most of our present atmosphere is though to have come
from volcanic eruptions. In addition to ash, volcanic
eruptions send a lot of water vapor, carbon dioxide, and sulfur
dioxide into the atmosphere. Carbon dioxide
and water vapor are two of the 5 main gases in our present
atmosphere.
Volcanoes also emit lots of other gases, many of them are
poisonous. Some of them are shown on the right side of the
figure (I found the gases in the "also" list mentioned in a lot of
online sources, the gases in the "perhaps" list were mentioned
less frequently).
As the earth began to cool the water vapor condensed and began to
create and fill oceans. Carbon dioxide dissolved in the
oceans and was slowly turned into rock. Smaller amounts of
nitrogen (N2) are also emitted by volcanoes (the N is highlighted
in the lists above). Because nitrogen is relatively
nonreactive it remained in the air and its concentration was able
to built up over time.
Now before we move on you should really have
a look at some amazing
pictures of the Eyjafjallajökull volcano
in Iceland which erupted in Spring
2010. It caused severe disruption of airline travel
between the US and Europe. Here's another
set of photos also from the Boston Globe.
Volcanoes didn't add any of the oxygen that is in the
atmosphere. Where did that come from?
1st source of atmospheric oxygen
The oxygen is thought to have first come from
photo-dissociation of water vapor and carbon dioxide by
ultraviolet (UV) light (the high energy UV light is able to split
the H20 and CO2 molecules into pieces). The O and OH then
react to form O2 and H.
By the way I don't expect you to remember the chemical formulas
in the example above. It's often easier and clearer to show
what is happening in a chemical formula than to write it out in
words. If I were to write the equations down, however, you
should be able to interpret them. Ultraviolet is a
dangerous, high energy, potentially deadly form of light and it's
probably also good to remember that ultraviolet light is capable
of breaking molecules apart.
Once molecular oxygen (O2) begins to accumulate in
the air UV light can split it apart to make atomic oxygen
(O). The atoms of oxygen can react with molecular oxygen
to form ozone (O3).
Ozone in the atmosphere began to absorb the dangerous and
deadly forms ultraviolet light and life forms could then begin
to safely move from the oceans onto land (prior to the buildup
of ozone, the ocean water offered protection from UV
light. A molecule of O3 absorbs some UV
preventing it from reaching the ground.
O3 + UV light ---> O2 + O
You might think the O2
and O would recombine, but they usually go flying off in
different directions.
2nd source of atmospheric oxygen.
Photosynthesis is now the main source of atmospheric
oxygen.
Photosynthesis in its most basic form is shown in the chemical
equation above. Combustion is really just the opposite of
photosynthesis and is shown below.
We burn fossil fuels (dead but undecayed plant material) to
generate energy. Water vapor and carbon dioxide are by
products. Combustion is a source of CO2. We'll
see these two equations again when we study the greenhouse effect
(CO2 is
a greenhouse gas ) and global warming.
And a detail that I didn't mentioned
in class (and something you probably don't need to
remember). The argon we have in the atmosphere apparently
comes from the radioactive decay of potassium in the ground.
Three isotopes of potassium occur naturally: potassium-39 and
potassium-41 are stable, potassium-40 is radioactive and the
source of the argon in the atmosphere.
OK back to the top part of the list of the 5 main
gases in air.
We've added some new information to the oxygen
column.
The
following figure is the first page in the packet of
photocopied ClassNotes.
This somewhat confusing figure shows some of the important
events in the history of the earth and evolution of the
atmosphere. There were 5 main points I wanted you to take
from this figure, and really 1-3 are the most important.
First, Point 1: the
earth is thought to be between 4.5 and 4.6 billion years
old. If you want to remember the earth is a few billion
years old that is probably close enough. Something I
didn't mention in class, it's in small type above.
The formation of a molten iron core was important because it gave
the earth a magnetic field. The magnetic field deflects the
solar wind and keeps the solar wind from blowing away our present
day atmosphere.
Stromatolites (Point
2) are column-shaped
structures made up of layers of sedimentary rock,
that are created by microorganisms living at the top of the
stromatolite (I've never actually seen a stromatolite, so this
is all based on photographs and written descriptions).
Fossils of the very small microbes (cyanobacteria = blue green
algae) have been found in stromatolites as old as 2.7 B years
and are some of the earliest records of life on earth.
Much older (3.5 to 3.8 B years old) stromatolites presumably
also produced by microbes, but without microbe fossils, have
also been found.
Blue green algae grows at the top of the column, under water
but near the ocean surface where it can absorb sunlight. As
sediments begin to settle and accumulate on top of the algae they
start to block the sunlight. The cyanobacteria would then
move to the top of this sediment layer and the process would
repeat itself. In this way the stromatolite column would
grow layer by layer over. Now, this isn't a geology class,
we're learning about stromatolites because the cyanobacteria on
them were a very early form of life on the earth and were able to
produce oxygen using photosynthesis.
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Living stromatolites are found in a few
locations today. The two pictures above are from Coral Bay
(left) and Shark's Bay (right) in Western Australia. The
picture was probably taken at low tide, the stromatolites would
normally be covered with ocean water. It doesn't look like a
good place to go swimming, I would expect the top surfaces of
these stromatolites to be slimy.
Point 3 refers to
the banded iron formation, a type of rock formation. These
rocks are 3 billion years old (maybe older) and are evidence of
oxygen being produced in the earth's oceans. Here are a
couple of pictures of samples of banded iron formation rock that I
will pass around in class at some point (thanks for being careful
with them and not stealing them).
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The main thing to notice are the
alternating bands of red and black. The next paragraph
and figure explain how these formed.
Rain would first of all wash iron ions from the earth's land
surface into the ocean (at a time before there was any oxygen
in the atmosphere). Oxygen from the cyanobacteria living
in the ocean water reacted with the dissolved iron (the iron
ions) to form hematite or magnetite. These two minerals
precipitated out of the water to form a layer on the sea
bed. This is what produced the black layers.
Periodically the oxygen production would
decrease or stop (rising oxygen levels might have killed the
cyanobacteria or seasonal changes in incoming sunlight might
have slowed the photosynthesis). During these times of low
oxygen concentration, red layers of jasper would form on the
ocean bottom. Eventually the cyanobacteria would recover,
begin producing oxygen again, and a new layer of hematite or
magnetite would form. The rocks that resulted, containing
alternating layers of black hematite or magnetite and red layers
of jasper are known as the banded iron formation. In addition to the red and black layers,
you see yellow layers made of fibers of quartz in the samples
passed around class. The rocks are fairly heavy
because they contain a lot of iron, but the most impressive
thing about them in my opinion is their age - they are a few
billion years old!
Eventually the oxygen in the ocean reacted with all of the iron
ions and was free to move
from the ocean into the atmosphere. Once in the air, the
oxygen could react with iron in sediments on the earth's
surface. This produced red colored (rust colored)
sedimentary rock. These are called "Red Beds" (Point 4). None of these
so-called red beds are older than about 2 B years old. Thus
it appears that a real buildup up of oxygen in the atmosphere
began around 2 B years ago.
Red State Park near Sedona Arizona. An example of "red beds"
that formed during the Permian period 250-300
million years ago.
Oxygen concentrations reached levels that are about the same as
today around 500 to 600 million years ago (Point 5 in the figure).
I didn't have time to write down the list below.
It adds several important trace gases to the list of
the 5 main constituents of air. Trace gases are gases found
in low concentrations (and often the concentrations vary with time
and location). Low concentrations doesn't mean they aren't
important, however.
Carbon monoxide, nitric oxide, nitrogen dioxide, ozone,
and sulfur dioxide are some of the major air pollutants.
We'll cover 3 of these in more detail next week.
Water vapor, carbon dioxide, methane, nitrous oxide (N2O = laughing gas),
chlorofluorocarbons, and ozone are all greenhouse gases.
Increasing atmospheric concentrations of these gases are
responsible for the current concern over climate change and
global warming. We'll discuss this topic and learn more
about how the greenhouse effect actually works later in the
course.
Ozone has sort of a Dr. Jeckyl and Mr. Hyde personality
(i)
Ozone in the stratosphere (a layer of the atmosphere between
about 10 and 50 km altitude) is beneficial because it absorbs
dangerous high energy ultraviolet (UV) light coming from the
sun. Without the protection of the ozone layer, life as we
know it would not exist on the surface of the earth. It
was only after ozone started to buildup in the atmosphere that
life could move from the oceans onto land.
Chlorofluorocarbons are of concern in the atmosphere because
they destroy stratospheric ozone.
(ii) In
the troposphere (the bottom 10 kilometers or so of the
atmosphere and where we live) ozone is a pollutant and is one of
the main ingredients in photochemical smog.
(iii) Ozone is also a
greenhouse gas.
Finally, I wasn't being
entirely honest when I said that gases are
invisible.
Some gases can be seen, here are some examples. I
would like to bring some actual samples to class, but some
are very toxic and require careful handling.
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Bromine in
both liquid and gaseous phases. Bromine and mercury
are the only two elements that exist as liquids at
room temperature. The bromine is in a sealed
glass ampoule inside an acrylic cube. Bromine
could be safely brought to class in a container like
this.
This photo was taken by Alchemist-hp and was
Picture of the Day on the English Wikipedia on Oct. 29,
2010.
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Chlorine (Cl2)
I found this image here
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Iodine
Also an element that is normally found in solid
form. The solid sublimates, i.e. it changes
directly from solid to gas (you would probably need to
heat the solid iodine to produce gas as dense as seen
in the picture above). source
of this image
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Nitrogen dioxide (NO2)
An important pollutant.
source
of this image
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