THE PHYSICAL
PROPERTIES OF WATER
Major Concept
• The many unique properties of water are a
result of the two elements involved,
hydrogen and oxygen
– their ratios,
– the bond type, and
– the physical architecture of the polar water
molecule
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Polar MoleculeCovalant Bond
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WATER
• In 1783 the English scientist Henry Cavendish
determined that water was made up of hydrogen and
oxygen.
• Hydrogen and oxygen are bound together by sharing
electrons, a type of bond called a covalent bond.
• Because of the shared electrons and the asymmetry of
the bond, water molecules have a definite shape, a
slightly negative end, and a slightly positive end. This is
called a polar molecule.
• Due to its polarity, water can make weak bonds with
itself (called hydrogen bonds) that means that even
liquid water is slightly structured or ordered.
• This bonding also creates surface tension and enables
water to be somewhat of a universal solvent.
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Major Concept
• Water exists in three forms or states in the
range of temperature and pressure we find
at Earth’s surface.
• It exists as a solid (the mineral ice), as a
liquid, and as a gas (water vapor).
• To change temperatures in any one state
requires heat energy input or export, and
changing states from one form to another
often requires large energy exchanges
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Water Properties
• Tremendous amounts of heat (energy) are removed and
liberated in different atmospheric processes around the
globe.
• In some cases it is possible to:
– cool water below 0°C and remain liquid,
– boil water at temperatures below 100°C, and
– change the state of water from solid directly to gas in a process
called sublimation.
• Sublimation occurs when snow or ice evaporates under
very cold, dry conditions.
• The properties of water (its behavior) are altered with the
addition of trace amounts of other compounds. These
usually serve to lower the freezing temperature and elevate
the evaporation or boiling point of water
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Water Properties- Heat Capacity
• heat capacity of water is very high compared to
the land or the atmosphere
• Heat capacity is a measure of how much heat a
substance can absorb or release for a given
change in temperature.
• Substances with a high heat capacity require the
transfer of large amounts of heat energy for small
changes in temperature.
• Water has a very high heat capacity compared to
other naturally occurring earth materials. The heat
capacity of water is much higher than most other
liquids because of the hydrogen bonding between
water molecules.
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Heat Capacity
• Temperature variations on land range from a high of about
50°C in the Libyan desert in summer to about -50°C in the
Antarctic for a range of 100°C.
• Temperature variations in the ocean range from a high of
about 28°C in equatorial regions to a low of about -2°C in
Antarctic waters for a total range of only 30°C.
• People who live by large bodies of water have a good
understanding of the effect of different heat capacities of
land and water.
• In Milwaukee it is common to hear weather predictions of
daytime high temperatures for the city with the additional
comment “cooler by the lake” in the summer and “warmer
by the lake” in the winter.
• The high heat capacity of water and its ability to hold heat
makes it possible for surface currents to redistribute
excess heat in equatorial regions to higher latitudes as
they flow away from the equator on the western sides of
ocean basins.
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What does it mean?
• When ice forms heat is lost to a heat-deficient
atmosphere
• When ice melts the energy is gained by water
• This prevents the higher latitudes from
becoming much colder than -1.8oC, the freezing
temperature of ocean water
• This prevents colder regions from getting colder
and equatorial regions from getting warmer
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Major Concept
• The density of water is extremely sensitive
to changes in temperature, salinity, and
pressure.
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Density
• The density of seawater is greater than the density of
freshwater because seawater contains dissolved salts.
• The density of pure water at 3.98°C, or approximately
4°C, is 1.0 g/cm3. The densest it can be.
• The density of seawater of average salinity is about
1.0278 g/cm3. Because of this, fresh water will float on
ocean water.
• Pressure has relatively little effect on density compared
to salinity and temperature, however, the density will
increase with increasing pressure (or depth) and
becomes important in the deep sea where salinity and
temperature remain nearly constant.
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Pressure
• Pressure increases with depth in the
oceans. Pressure increases by about 1
atmosphere, for each increase of 10 m (33
ft) in depth.
• The pressure in the deepest ocean trench
at 11,000 m (36,000 ft) below sea level is
about 1100 atmospheres.
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Density increases as salinity
increases
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Universal Solvent
• Water has been called the universal
solvent because of its exceptional ability to
dissolve a wide variety of materials.
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Universal Solvent
• The polar nature of the water molecule
and its hydrogen bonding allow water to
dissolve salts and many other compounds
easily.
• Polar water molecules are able to attract
and surround ions in seawater.
• Salts are supplied to the oceans by
underwater volcanism and runoff from
land.
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Energy
• The water on, in, and around the Earth
transmits energy in many forms, including
light, heat, and sound.
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Color
• Coastal water varies in color. It may be green, yellow,
brown, or red depending on the nature of the material in
the water; silt from rivers, microscopic organisms, or
even dissolved organic substances.
• Open-ocean water is often clear and blue.
• The color of an object is due to the particular wavelength
of light that reflects off of the object and can be seen by
our eyes. Objects in deep water often appear dark
because only the blue light that penetrates to the
greatest depth is illuminating them.
– A lobster in shallow water is red but black in deep water
• Particles suspended and compounds dissolved in the
water will absorb and scatter light, thus reducing its
penetration below the surface. The depth of penetration
will decrease as the amount of suspended and dissolved
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material in the water increases.
Turbidity
• The attenuation of light with depth can be
measured with a simple device called a Secchi
disk. A Secchi disk is a white disk that is
lowered into the water until it disappears from
view.
• In water with large amounts of suspended
sediment or marine organisms a Secchi disk
may disappear at depths of 1–3 m (3-10 ft). In
the open ocean it may still be visible at depths of
20–30 m (65-100 ft) and in one measurement in
Antarctica's Weddell Sea a Secchi disk didn't
disappear until it reached a depth of 79 m (260
20
ft).
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Sound
• Sound is transmitted very efficiently in seawater.
The velocity of propagation of sound in seawater
is roughly five times as great as in air due to its
higher density.
• Sound propagating in the oceans can be used to
locate organisms and objects either by listening
for the sounds that they produce or by sending
pulses of sound into the water to be reflected
back from the target of interest.
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Traveling at an
average velocity of
1500m/sec, a sound
pulse leaves the
ship, strikes the
bottom and returns.
In water 4500m
deep sound requires
3 seconds to reach
the bottom and 3
seconds to return
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Sound
• Sound is also used to locate objects underwater using a
system called sonar. Sonar is an acronym that stands
for “sound navigation and ranging.”
• Sound is also used extensively as a simple and accurate
means of determining the depth of seawater. Precision
depth recorders (PDR’s) transmit pulses of sound that
reflect from the bottom and return to the surface. By
knowing the average velocity of sound in water and the
time required for the sound to travel downward and up
again, the depth can be easily calculated.
• It is also possible to send communications large
distances through the oceans with extremely low
frequency (ELF) sound antennas.
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A chart recorder
displaying a seismic
reflection line
Sub-bottom
bottom
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Sound waves change
velocity and refract as
they travel at an angle
through layers of
different densities
Sound beams bend
toward regions in
which sound travels
more slowly and
away from faster
regions
The degree of refraction and change in velocity with depth must
be known in order to determine the actual position of the target
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Sound Applications
• In the 1950s and 1960s the United States
Navy installed large arrays of hydrophones
on the seafloor to monitor submarines.
• These arrays were known as sound
surveillance systems (SOSUS).
• SOSUS arrays have been used by
scientists to monitor sounds made by
marine organisms and underwater
volcanoes.
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Sea Ice and Fog
• Two states of water, solid and water vapor (or
ice and fog), can produce particularly hazardous
conditions.
• Sea ice forms at polar latitudes. As the water
freezes it excludes the salt molecules from its
crystalline structure lowering its density and
allowing it to float. The salts that are left behind
concentrate to increase the salinity of the nearsurface water and slightly lower its freezing
point.
• Sequences of freezing and thawing continue to
exclude more and more salt and the salinity of
the ice will decrease each time.
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ARCTIC
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ANTARCTIC
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Sea Ice
• With the first freezing of a thin layer of ice on the
sea, waves and wind break it apart into small
pieces called pancakes.
• Pancakes gradually freeze together to form
larger pieces called floes, some of which float on
the water and some of which are attached to
land are called fast ice.
• Freshwater ice that enters the sea (icebergs)
comes from glacial ice. These can be very
hazardous to navigation as most of the iceberg
is below the surface.
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Pancake Ice
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Pressure ridge generated by
colliding floes
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Castle Berg
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Tabular Iceberg- 500m long
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Green Iceberg
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Ice shelf, seawater rich in organic
matter freezes to underside, bergs
calved capsize to expose green ice
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Fog
• Large accumulations of condensed water vapor
(water droplets) close to the ground are called
fog. (due to changes in temperature)
• The three types of fog encountered are:
– advective fog, formed when air warmed by the
underlying water is saturated by water vapor and then
moves over colder water to condense. (Grand Banks)
– sea smoke, dry, cold air moving over the sea from the
land is warmed and picks up water vapor from the
sea and rises to cool and condense.
– radiative fog, the result of warm days and cold nights
– warm moisture laden air over the land cools at night
and the water vapor condenses.
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Advective Fog over the Golden Gate Bridge
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THE CHEMISTRY OF
SEAWATER
Buffering
• In a sequence of elegant and reversible
interactions, carbon dioxide and water
couple to form a stable buffering system
that controls the pH of the world’s oceans.
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pH
• The water molecule can break apart to form a
hydrogen ion, H+, and a hydroxide anion, OH-.
• In a pure water solution approximately one in
every ten million water molecules will break
apart. If the concentration of the two ions is
equal the solution is said to be neutral.
• In solutions that are not pure water the
concentration of the hydrogen and hydroxide
ions may not be equal. If there are more
hydrogen ions than hydroxide the solution is an
acid. If there are more hydroxide ions than
hydrogen ions then the solution is a base, or
alkaline.
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pH
• The acidity or alkalinity of a solution is
measure with the pH scale. The pH scale
ranges from a low of 0 to a high of 14. pH
less than 7 is acidic, pH of 7 is neutral,
and pH greater than 7 is alkaline.
• pH = -Log10[H+]
• pH= -Log10[10-7]= -(-7) = 7
• The normal pH of rainwater is about 5.05.6 (slightly acidic).
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Buffering
• When carbon dioxide dissolves in seawater, it
readily enters into a sequence of reversible
chemical reactions with carbon dioxide and
H2O on one end, carbonic acid (H2CO3),
bicarbonate (HCO3-) and hydrogen ions (H+),
and finally carbonate (CO32-) and two
hydrogen ions (2H+).
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Buffering
• CO2 + H2O
H2CO3
HCO3- + H+
CO32- + 2H+
• When CO2 buffers oceanic pH, it helps to maintain
oceanic pH ranges from 7.5–8.5 (i.e., slightly basic
or alkaline); the average ocean pH is about 7.8
• The pH of surface water can be as high as 8.5 if the
water is warm and high rates of photosynthesis are
extracting large amounts of CO2 from the water.
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Salinity
• The average waters of the world’s oceans
contain 3.5% dissolved salts and 96.5%
water. The content of dissolved salts in
grams of salt per kilogram of seawater is
called salinity. The average ocean salinity
is about 35 g/kg.
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Salinity in the world’s Oceans
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Salinity
• Seawater salinity is typically reported in one of
three different (but equivalent) ways:
– 1) grams of salt per kilogram of water (g/kg),
– 2) parts per thousand (ppt, or o/oo), or in
– 3) Practical Salinity Units (PSU).
• The PSU scale is defined as the ratio of the
conductivity of a seawater sample and a
standard potassium chloride solution. The PSU
scale and o/oo are numerically equal but the
PSU scale doesn’t have any units because it is a
ratio.
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Salinity
• Sea surface salinity is controlled largely by
variations in precipitation and evaporation
patterns
• These are generally a function of latitude.
• Salinity is also influenced by proximity to
land and variations in freshwater runoff
from the continents as well as seasonal
freezing and thawing at middle and high
latitudes.
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Average Sea Surface Salinities in the
Northern Hemisphere Summer in ppt
Freshwater runoff
High salinity due
to evaporation
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Salinity
• Salinity maxima occur approximately 20˚ to 30˚
north and south of the equator because of
relatively dry climatic conditions that cause more
evaporation than precipitation, thus
concentrating the salts in the water.
• The salinity minima at about 5˚ north of the
equator and about 45˚ north and south of the
equator are caused by relatively wet climates
where precipitation is greater than evaporation
and rainfall dilutes salts in the water.
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Dissolved Salts
• When salts and many other particulates
(minerals) are added to water, some
amount of the crystals or solids dissolve.
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Sources of Salts
• The source of salts in the sea is from the
weathering, erosion, and dissolution of the rocks
that make up the earth’s crust, initially they are
primarily from the igneous rocks of the crust as
the crust and mantle degassed and
differentiated.
• volcanic eruptions and gaseous emanations.
• hydrothermal vents along mid ocean ridges.
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Major Sources and Inputs of Ions to
seawater
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Salts
• The total amount of dissolved salts in the
world oceans can be calculated at 5 x 10
22 grams (at 36 ppt).
• Each year we estimate that some 2.5 x 10
15 grams of salt are added to the oceans.
• Geological evidence and recent
calculations indicate the salt content of the
oceans has been very stable for 1.5 billion
years or more.
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Residence Times
• The average time that a substance remains in
solution in the ocean is called its residence
time.
• Aluminum and iron have very short residence
times because they react chemically with
other substances to form insoluble particles
that are added to the sediment.
• Sodium, chloride, potassium, and magnesium
have very long residence times because they
do not react chemically to form solids and
they are not extracted from the water by
biological processes.
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The salinometer
determines salinity
by measuring the
electrical
conductivity of
seawater
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Gases
• Many gases move readily across the airsea interface. T
• Three gases are particularly critical to life
on earth and the physical and chemical
balances within the ocean realm. T
• These are nitrogen (N2), oxygen (O2), and
carbon dioxide (CO2). These gases
comprise more than 99% of the gases in
the atmosphere and the oceans.
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Gases
• Dissolved gases are mixed to all depths in the oceans.
• Gases dissolve more readily in cold water, water with
low salinity, and water under pressure. Conversely,
warm waters (similar to warm sodas) can hold less
gas. Very saline water has less gas.
• Deep, cold ocean water commonly contains relatively
higher concentrations of dissolved gases.
• If we slowly add gas to a liquid we reach a point where
we cannot add more gas without some of the gas
being released from the solution simultaneously. The
concentration level where the fluid will neither gain nor
lose gas is the saturation value for that gas and is
dependent on salinity, temperature, and pressure.
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The distributionof
CO2 and O2 with
depth
Changes in O2
concentration may
exceed 400% from
the surface to depth
CO2 concentrations
change <15%
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CO2
• The annual ocean uptake of carbon (in the form
of carbon dioxide) is calculated at 2.5 billion
metric tons per year.
• Water temperature, pH, salinity, chemistry, and
geological and biological processes control the
rate at which the oceans absorb carbon dioxide.
• CO2 gets transferred from the surface to deep
ocean waters and eventually is fixed in marine
sediments as organisms remove calcium
carbonate (CaCO3) to produce skeletal material.
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O2
• The oceans have played one of the critical roles
in regulating the free oxygen concentration in the
Earth’s atmosphere for nearly 3.5 billion years.
• Photosynthesis releases oxygen to the ocean
and atmosphere.
• In excess of respiration, some 300 million metric
tons per year are released that would eventually
result in an increase in O2 concentrations in the
oceans and atmosphere.
• However, the excess oxygen is consumed
chemically during oxidation of exposed surface
sediments and animal respiration, and
decomposition of organic compounds.
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Nutrients
• The nutrients needed for plant growth in the oceans
include nitrate (NO3-), phosphate (PO43-), and silicate
(SiO4-).
• Silicate is used to form silica (SiO2), the material that
forms the hard outer wall of single-celled plant-like
organisms called diatoms as well as the skeletal parts
of some protozoans.
• These nutrients are brought to the oceans by
freshwater runoff from the continents.
• These nutrients are generally present in very low
concentrations in seawater.
• The concentration of nutrients in seawater is cyclical.
• Nutrient concentration decreases as increasing
biological productivity removes nutrients.
• Concentrations increase when populations decline
and death and decay return nutrients to the water.
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Resources
• 30% of the world’s salt is extracted from seawater.
• Salt can be concentrated by evaporation at low
latitudes, or by freezing at high latitudes.
• 60% of the world’s magnesium comes from the sea,
along with 70% of the bromine.
• Future technologies may be able to take advantage of
extremely valuable trace element concentrations in
seawater that are not currently economical to recover.
• Two examples include gold and uranium that have
estimated dissolved concentrations of 10 million tons
and 4 billion tons in seawater respectively.
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