Water and Seawater
Ever wonder why temperature extremes are
found at places far from the ocean, while
those close to the ocean are moderated?
worldbook.com
Winter in South Dakota…and Long Island
http://www.polarbearclub.org/polarbears/gallery_c.htm
Water and Seawater
• The mild climates found in coastal regions are
made possible by water’s unique thermal
properties
• These and other properties of water, which
stem from the arrangement of its atoms and
how its molecules ‘stick’ together, give water
the ability to store vast quantities of heat, to
stick together, and to dissolve almost
anything
Some general chemistry….
• Water is a molecule
– A molecule is a group of 2
or more atoms held
together by mutuallyshared electrons
– Water consists of 1 Oxygen
atom and 2 Hydrogen
atoms (H2O)
Some general chemistry….
• An atom is the smallest unit
of matter; contains protons
(+), electrons (-) and
neutrons (=)
electrons
• The protons and neutrons are
located in the atom’s nucleus
• An element contains a
specific set of protons and
electrons
www.dkimages.com/discover/Home/Science/
Physics-and-Chemistry/Matter/Atoms-and-Molecules/
Atoms-and...-197.html
Nucleus: protons (+), neutrons (=)
General chemistry continued
• Elements in their pure form are quite rare in
nature; instead they frequently combine to
form compounds
• A compound consists of 2 or more elements
• The electron arrangement determines the
chemical properties of an element
– Atoms whose shells are incomplete tend to
interact with other atoms to gain, lose, or share
electrons
My name is bond, ionic bond
• If an atom has 1 electron too many, it will
donate that electron to an atom that is 1
electron short (of a complete outer electron
shell)
• The donation of this electron, and its
acceptance results in 2 charged atoms, or ions
– The donating element has a (+) charge, having 1
more proton than it does electrons
– The accepting element has a (-), having 1 more
electron than it does protons
Formation of an ionic bond
Transfer of
electron
Na
Sodium atom
Cl
Chlorine atom
+
–
Na+
Sodium ion
Cl–
Chloride ion
Sodium chloride (NaCl)
• The electrical attraction between the 2 ions
(Na+ and Cl-) holds them together
My name is bond, covalent bond
• Two atoms may also share electrons
• When atoms share their outermost electrons,
they form covalent bonds
• The resulting compound is called a molecule
• The resulting bond is very strong
• Elements in need of 4 electrons, and those
with only 1 electron to share (=Hydrogen)
frequently form covalent bonds
Water is covalently bonded
• Water (H2O) is a covalently-bonded molecule,
in which Oxygen shares its electrons with 2
Hydrogen atoms (completes the outer
electron shells of both the Hydrogen atoms
and the Oxygen atom)
A water molecule is a polar molecule
(–)
(–)
O
H
(+)
H
(+)
The oxygen atom in water is highly electronegative; it attracts the shared
electrons much moreso than does the Hydrogen atoms, so the shared
electrons spend more time near the Oxygen than the Hydrogen atoms
Water is a polar molecule
• A polar molecule results from the unequal
distribution of electrical charges
• Atoms in a covalently-bonded molecule are in
a constant ‘tug-o-war’ for their shared
electrons
• An atom with a high electronegativity (its
attraction for shared electrons) can pull
electrons towards its nucleus and away from
the other, sharing atom
• Recall that in a water molecule, two hydrogen
atoms share electrons with an oxygen atom,
completing the outer shell of all 3 atoms
O
H
H
• But the electrons do not spend an equal
amount of time with hydrogen as they do
oxygen
++
+
+
O
++ +
+
+
H
+
H
• This is because oxygen has 8 protons in its
nucleus, while hydrogen only has 1
• Remember that opposites attract. The
difference in positive
charges pulls the
shared electrons
++
+
+
O
toward oxygen, and
++ +
+
H
+
away from the two
hydrogen atoms
+
H
• The abundance of electrons near oxygen
makes the oxygen atom in a water molecule
slightly negative
(-)
• Likewise, the 2
hydrogen atoms
become slightly
positive since their
shared electrons
spend so little
time near them
++
+
+
++ +
+
+
(+)
+
(+)
• A water molecule is said to be polar in that it
has two ends of opposing charges: a slightly
positive charge near the hydrogen atoms, and
a slightly negative charge near the oxygen
atom
(-
)
O
H
(+)
H
(+)
Water is a polar molecule
• The bonds that exist between 2 polar
molecules are called hydrogen bonds - very
weak (compared to ionic and covalent) but
collectively are very strong
Hydrogen bonds
Water: Cohesion
(-)
O
H
(+)
H
(+)
(-)
(-)
O
O
H
H
(+)
(+)
H (+)
H (+)
I’m getting thirsty…
• Hydrogen bonds make water cohesive; its
molecules tend to ‘stick’ together
• This cohesion allows plants to transport water
and nutrients from their roots to their leaves
(think of a giant redwood)
• Water has an extremely high surface tension
(the measure of the difficulty to stretch or
break the surface of a liquid)
A water strider walks on water
…and let’s not forget Jesus Christ Superstar, er, Lizard
http://www.youtube.com/watch?v=45yabrnryXk&feature=related
Water, water everywhere
• Water’s hydrogen bonds moderate
temperature
– Water is able to absorb a great deal of heat energy
without an increase in temperature because of its
hydrogen bonds
– Heat must be absorbed to break hydrogen bonds
and heat is released when hydrogen bonds form
– To increase temperature of water, heat energy
must first disrupt the hydrogen bonds; when
water is cooled, more hydrogen bonds form
Water: Heat Capacity
HEAT
Water: Heat Capacity
HEAT
Water: Heat Capacity
• We experience the benefits of water’s high
heat capacity every time we sweat.
• Our body provides the heat to evaporate
water from a liquid to a gas (water vapor),
keeping us cool
• This is the same reason why
dogs pant and pigs wallow in
the mud!
H2O
How much heat required to
induce evaporation
Water is a moderator of climate
Hurricane path: Camille 1969
Day minus night temperature difference
Water’s amazing properties
• Water exists in all phases (gas, liquid, solid) on
Earth
• Jeopardy question: What is the molecular
formula of ice???
• Because of hydrogen bonds, ice is less dense
than water
– This prevents lakes, ponds, and even the ocean
from freezing solid; fish, dolphins, walruses very
grateful
Thank you Hydrogen Bonds!!!
Water is the solvent of life
• Because of water’s polar properties (its
hydrogen bonds), water is an incredible
solvent
• A solvent is the dissolving agent of a solution,
a liquid consisting of uniform mixture of 2 or
more substances
• The substance that is being dissolved is called
a solute
Ion in
solution
Salt
crystal
Seawater
• Just how salty is seawater?
• The oceans contain enough salt to cover the
entire planet with a layer more than 500 feet
thick (the height of a 50-story skyscraper)!
(but we have a lot of ocean….)
• The total amount of solid material dissolved in
water is a measure of its salinity
Salinity of seawater
• The salinity of seawater is typically 3.5%
• A salinity of 3.5% indicates that seawater
contains 96.5% pure water and 3.5% solutes
• The major solutes of seawater are chlorine,
sodium, sulfur, magnesium, calcium, and
potassium (=99% of the dissolved solutes),
but >80 other elements have also been
identified in seawater
Major dissolved components in
seawater
Salinity of seawater
• Sodium and Chlorine make up 85% of the
solutes found in seawater!
• Salinity is typically expressed in parts per
thousand
• A salinity of 3.5% is equivalent to 35‰
• 35‰ seawater has 35 grams of salt in every
1000 grams of seawater
• But where does all this salt come from???
Could you please pass the salt?
• Rivers supply most of the ‘salt’ to the oceans
• Volcanic activity on land and underwater
contributes to the amount of ions in seawater
• The weathering of rocks by rainwater
contribute sodium, magnesium and calcium
(remember water is an incredible solvent!)
• Out-gassing through volcanoes and rift vents
provide carbon dioxide, chlorine, sulfur,
hydrogen, fluorine, nitrogen and water vapor
The ocean as an Earth tea
• Some of the ocean’s solutes
are hybrids of the
weathering and outgassing
processes; table salt (NaCl)
• Recent evidence suggests
that mid-oceanic rifts increase the content of
calcium and potassium of seawater, but may
decrease the content of magnesium and sulfur
– Water circulating through new ocean floor at these
sites is apparently stripped of these elements
The Principle of Constant Proportions
• When samples from the Challenger expedition
were analyzed, the salinity of seawater
changed slightly from location to location, but
the relative amounts of ions did not vary
significantly
– Regardless of the salinity of seawater, the ratio of
solutes remain constant
– How could this be so?
The Principle of Constant Proportions
• The addition of solutes (“sources”) is balanced
by the removal of solutes (“sinks”)
• The sources = the sinks
• The rate at which an element is added to the
ocean equals the rate at which it is removed
(steady state condition)
• Materials added to the ocean are
counteracted by processes that cycle dissolved
substances out of seawater
How to measure salinity
• Because the ratio of solutes in seawater is
constant, salinity can be measured by
chlorinity; the measure of chlorine, bromine
and iodine ions in seawater
– Easy to measure
– Salinity = 1.80655 x chlorinity
How to measure salinity
• Because increases in the amount of dissolved
solutes increases the conductivity of water,
the salinity of seawater is easily obtained by
measuring the electrical conductivity (the
ability of a substance to transmit electric
current) of
seawater
– Very accurate
Dissolved gases in seawater
• Gases in the air easily dissolve in seawater at
the ocean’s surface
• Organisms living in the ocean require these
dissolved gases to survive
• The major gases in seawater are:
– Nitrogen
– Oxygen
– Carbon dioxide
Dissolved gases in seawater:
Nitrogen
• Nitrogen constitutes ~48% of the dissolved
gases in seawater; the upper surface waters of
the ocean are saturated (no additional
Nitrogen can dissolve)
• Nitrogen is essential for organisms and can be
a limiting factor for phytoplankton
• Most dissolved Nitrogen is unusable since it
exists as N2 gas, a form that cannot be
assimilated by most organisms
Dissolved gases in seawater:
Oxygen
• Oxygen constitutes ~36% of the gases
dissolved in seawater
• Oxygen is essential for all aerobic life forms
• Dissolved oxygen comes from photosynthesis
and diffusion from the atmosphere
Dissolved gases in seawater
• Carbon dioxide is very soluble in water and
comprises ~15% of the dissolved gases in
seawater
• Because CO2 combines chemically with water
to form H2CO3 (carbonic acid), water can hold
a tremendous amount of CO2
• In fact, much more CO2 moves from the
atmosphere into the ocean, than from the
ocean to the atmosphere
• CO2 is used
near the
surface and
produced
at depth
• O2 is
produced
near the
surface and
used at
depth
Dissolved gases in seawater
• Concentrations of oxygen and carbon dioxide
vary with depth
• Moreover, gases dissolve more readily in cold
water than warm
• Cold, polar water contains a greater volume of
dissolved gases than warm, tropical water
• As a result, when deep, cold water is upwelled
to the surface, it provides the nutrients many
organisms need to survive (we return to this)
The Acid-Base Balance
• Water (H2O) can separate to form hydrogen
ions (H+) and hydroxide ions (OH-), which will
be present in equal concentrations in pure
water
• An imbalance of these ions, however,
produces an acidic or basic solution
• An acid is a substance that releases H+ ions in
solution; a base is a substance that combines
with a H+ ion in solution
The Acid-Base Balance
• The acidity or alkalinity (basic) of a solution is
measured on a pH scale
• pH measures the concentration of H+ ions in
solution; an excess of H+ ions makes the
solution acidic; an excess of OH- ions makes
the solution basic (alkaline)
• Seawater is slightly alkaline with an average
pH of 7.8
The Carbonate Buffering System
• If CO2 readily dissolves into seawater and
forms carbonic acid, why is seawater slightly
alkaline, and not slightly acidic???
• When dissolved in seawater, CO2 is actually
present in different forms; carbonic acid is
only 1 of these
• Some carbonic acid, H2CO3 breaks down to
produce H+ and the carbonate ion, HCO3-
The Carbonate Buffering System
• In this way, carbonic acid and bicarbonate are
buffers; if seawater becomes ‘too’ basic,
reactions will proceed to produce more H+,
increasing the acidity of the solution, and vice
versa
Bicarbonate
• H2O + CO2  H2CO3  HCO3- + H+  H+ + CO3-2
Carbonic acid
• More complex than other gases
Carbonate
The Ocean is density-stratisfied
• The density of water depends mainly on its
temperature and salinity
• Freshwater is less dense than salt water
• Warm water is less dense than cold water
• Much of the ocean is divided into 3 density
zones:
– Surface zone
– Pycnocline
– Deep zone
Temperature & salinity are ~constant
Density
increases with
increasing
depth
Lies below the pycnocline; density
changes little with increasing depth
Temperature and Salinity Affect Density
• The pycnocline’s rapid density increase with
depth is due mainly to decreasing water
temperature
• Between the surface zone and the deep zone
also lies the thermocline; a zone in which
temperature decreases rapidly with depth
Temperature and Salinity Affect Density
• Low salinity can also
contribute to the
pycnocline, especially
in regions where
freshwater runoff
mixes with the surface
water
• The halocline is a
zone of rapid salinity
increase with depth
The Deep Zone
• The deep zone constitutes 80% of the ocean
volume
• Water in the deep layer originates at the
surface where it cools, becomes more dense,
and sinks
• Very cold water;
1-3°C
Temperature and Salinity Affect Density
• Very cold and/or very salty water sinks to the
seabed until it reaches a layer of water that is
equally dense
– Results from the formation of sea ice, or from a
high degree of evaporation
• The pycnocline traps dense water masses at
depth and isolates 80% of the ocean water
from surface circulation
– Density differences power deep ocean currents
(we’ll come back to this….)
Hear no evil, see no evil…
• Light does not penetrate far through the
ocean
• Light is a form of electromagnetic radiation
that travels as waves through space, air and
water (includes radio waves, infrared,
ultraviolet and X-rays
• Water rapidly absorbs the visible wavelengths
of light
Let there be light!
• When light is absorbed by water molecules
(e.g., the ocean), the molecules vibrate and
light energy is converted to heat
• Where light penetrates, photosynthesis
proceeds; very important!
• The thin film of sunlit water at the top of the
surface zone is the photic zone
– <100 meters in open ocean
– <40 meters in coastal regions
– <600 meters in clear tropical regions
Hey, who turned off the lights???
• Ocean below the photic zone lies in blackness
• Except for light generated by living organisms
(who ‘dat?!!?), the region is in perpetual
darkness
• This dark water beneath the photic zone is the
aphotic zone
Water transmits blue light more
efficiently than red
• When light reaches surface waters, some of
the energy is converted to heat
• The top meter of the ocean absorbs nearly all
of the infrared radiation that reaches the
ocean surface, warming it significantly
• This top meter also absorbs 71% of red light;
the dimming light becomes bluer with depth
as the red, orange, and yellow wavelengths
are absorbed
Increasing energy
10–5 nm 10–3 nm
Gamma
rays
X-rays
103 nm
1 nm
UV
106 nm
Infrared
1m
Microwaves
103 m
Radio
waves
Visible light
380 400
500
600
Wavelength (nm)
700
650
nm
750
Light waves with shorter
wavelengths contain more
energy and penetrate deeper
than those with longer
wavelengths. However, by 300
meters, even blue light has been
converted into heat
Deep-sea shrimp
Sound travels much farther than light
in the ocean
• Sound is a form of energy transmitted by rapid
pressure changes
• Sound intensity decreases as it travels through
seawater because of spreading, scattering,
and absorption
– Scattering occurs as sound bounces off bubbles,
suspended particles, organisms, the surface, the
bottom, or other objects
– Sound is also absorbed by molecules into very
small amounts of heat (sound is energy!)
Did you hear that?
• Sound travels nearly 5 times faster
underwater than on land (in the air)
• In water, sound is transmitted by water
molecules; since water molecules are densely
packed (more so than molecules in air), they
transmit sound more quickly
– Sound causes a chain reaction of molecules
bumping into one another in a wave pattern,;
closely packed molecules transmit sound more
efficiently and as a result, faster
Do you hear what I hear?
• Because of sound’s ability to travel so
effectively in water, we can use it to map
ocean contours!!! (sound familiar???)
• And because sound waves travel more
efficiently underwater than light waves,
aquatic organisms frequently rely on sound
(hearing), much more so
than light (vision)
– Other senses, too
A whale of a sound
• Whales regularly communicate with one
another through sound
– Breaching
– Fin- and Tail- slapping
– Bubbles
– Vocalizations
• The larger the whale, the deeper the sound
they can produce
Talk about singin’ the blues…
• Blue whales produce deep rumbling sounds;
deep, low-pitched sounds that are the loudest
noise produced by any animal
• You will feel a blue whale vocalization before
you ever hear it
• Whale vocalizations can travel over entire
ocean basins!!!