Chapter 3
Chemical and Physical Features
of Seawater and the World Ocean
Brownian motion
•
General Brownian motion
•
Brownian motion of water molecules
•
Brownian motion of fat globules in milk
– http://www.youtube.com/watch?v=6VdMp46ZIL8
– http://www.youtube.com/watch?v=w1aVqKDAx6o
Movement of solute through water
•
http://www.youtube.com/watch?v=B944o4m3Rp4
•
The movie captures 5 picoseconds of a molecular dynamics simulation of a
metastable "hydrophobic" hydrogen atom dissolved in liquid water at 300 K and
ambient pressure conditions. Shown is the fluctuating hydrogen bond network of
water embedding the H-particle. The oxygen atoms of the water molecules represent
the vertices of the network. Note that the small H particle is quickly exploring the
accessible free volume left over by the water molecules. The "open" water network
structure permits the H-particle to adopt a diffusion coefficient being about three to
four times larger than the diffusion coefficient of water itself.
Unique Nature of Water
• Water molecules are unique in many
ways.
• First of all, each water molecule has slight
positive and negative electrical charges.
• The positive charges are near the
hydrogen atoms and the negative charges
exist around the oxygen atom.
Unique Nature of Water
• Due to these slight electrical charges,
water molecules are attracted to one
another.
• The negative charge of one molecule is
attracted to the positive charge of other
molecules (remember: opposites attract!)
• This attraction of one water molecule to
another is known as hydrogen bonding.
Surface tension
Surface tension
• Water molecules are attracted to each
other
• Inner molecules are pulled in all directions
• Surface molecules are pulled towards the
other molecules and away from air
Unique Nature of Water
• Hydrogen bonding between water molecules
does not impede movement of molecules and
diffusion through water is rapid
• The lifetime of the hydrogen bond between
water molecules is a picosecond and weak
bonds between water and solutes have a
lifetime of nanoseconds
– The blink of an eye is 300300-400 msecs
• Diffusion is the primary means by which
molecules move through water and organisms.
Unique Nature of Water
• Water and its solutes are highly mobile
• Its viscosity allows flow through both large
and small vessels.
Unique Nature of Water
• Hydrogen bonds help keep water molecules as
a cohesive group at most temperatures found on
earth.
• This is the reason we have liquid water.
• The liquid form is just one of the three states
matter can have.
• The other two are gas and solid.
• Water is the only substance on earth that exists
in all three states naturally – solid (ice), liquid
and gas (water vapor).
Figure 03_03
Unique Nature of Water
• Cold water sinks underneath the warmer
water.
– This is important for ocean circulation
• But ice is less dense than seawater
– This is fortunate
Unique Nature of Water
• If ice did not float, a body of water would freeze
from the bottom up and eventually the whole
body of water would freeze.
• This would not be great for all the organisms in
that water.
• Since ice floats, the floating ice creates a barrier
between the air temperature and the water
below the ice, keeping it from freezing.
Unique Nature of Water
• Can function as a “universal solvent”
solvent”
• The main solutes dissolved in ocean water
are sodium and chloride (which gives it a
salty taste).
• Salinity is measured in parts per thousand
(ppt). This means that for every 1000
grams of water, there are 35 grams of salt.
Unique Nature of Water
• Salinity is normally around 35 ppt,
however, this can vary by location.
• Ocean water near a location where a river
meets the sea can be 1515-25 ppt.
• Other areas can be higher if evaporation is
high and no rivers are bringing freshwater
to that area.
The Composition of Seawater at
35 ppt Salinity
Unique Nature of Water
• Water is also relatively transparent (you can see
through it).
• This means that sunlight shining on the surface
can penetrate the surface (great for those
photosynthetic organisms living under the sea)
• This level of penetration varies greatly
depending on the amount of solutes in the
water.
Figure 03_11
Figure 03_12
Adding Water and Solutes to the
Oceans
• Water comes from rivers and from
precipitation such as rain and snow.
• And, to a lesser extent, from melting of
polar ice.
• Solutes come from the weathering of
rocks, hydrothermal vents and solutes that
were delivered in rivers from land runrun-off.
Figure 03_06
Gases in Seawater
• Many gases are also dissolved in seawater
including:
– Oxygen
– Carbon dioxide
– Nitrogen
• Gases dissolve at the sea surface from the
atmosphere. Occasionally, the reverse
happens.
• Like other aerobic organisms, many organisms
in the ocean utilize oxygen and expel carbon
dioxide.
Carbon in the ocean (gigatons)
organisms
dissolved organic
surface ocean
deep ocean
sediment
3
700
1020
38100
150
39973
Carbon in outside the ocean (Gt)
soil
1580
vegetation
610
atmosphere
750
2940
Conditions are not the Same at All
Depths
• Conditions at the ocean floor, in the water
column and at the surface can vary greatly.
• Oxygen content, temperature, salinity and other
factors are often very different from one depth to
another.
• Pressure is also greater the deeper the depth.
Water at the bottom has the weight of the water
above it pushing down on it (which means that
organisms living there also experience this
pressure).
Pressure
Increases with
Increasing Depth
Figure 03_15