Physical Properties of
Water
•
•
•
•
Heat Energy and Water
Density Structure of Seawater
Optical (Light) Properties
Sound in Seawater
Density & Temperature
Taking heat energy away →
Heat energy causes water
molecules to vibrate greatly and
space themselves out; also few
H-bonds that break and reform
very rapidly.
Maximum density at 4ºC when
water molecules vibrate less and
pack together the tightest.
Density decreases at < 4ºC as
more and more H-bonds form.
The dramatic drop in density
occurs when the perfect crystal
forms; complete H-bonding; ice.
This requires removal of a lot of
heat without a temperature
change.
ICE FLOATS!
Perfect water crystal
Heat Energy and Water
• Heat is energy produced by molecular vibration (number of molecules and
how rapidly they vibrate). The calorie is the unit of heat.
• Temperature is only a measure of how rapidly molecules vibrate.
• Different substances have different temperature responses for the same
amount of heat added or removed.
• Specific heat is the measure of how much heat is required to raise 1g of a
substance by 1ºC.
• Specific heat of liquid water is 1.0 calories. Relative to other substances,
water can absorb or release a lot of heat with little temperature change.
• In contrast, the specific heat of sand is about 0.1 calories. It does not
resist change in temperature with gain or release of heat. Think of hot,
mid-day, beach sand versus the cooler seawater; both received the same
solar energy.
Sensible versus Latent Heat of Water
Liquid water temperature responds in a constant manner as heat is added
or removed; we call this sensible heat as we can make sense of heat
change by measuring temperature change with a thermometer.
However, no temperature change is seen upon removing the 80cal/g from
liquid water at 0ºC to get ice at 0ºC. This is called the latent heat of fusion
(freezing). There is also a latent heat of evaporation of 585 cal/g to get
water to go to gas, i.e. energy needed to break all H-bonds.
What about Seawater?
• Physical properties discussed above are for pure water,
i.e no salts.
• Salinity will decrease freezing point (-1.9.ºC for 35 ppt).
Salts interfere with formation of perfect water crystals.
Only freshwater freezes and the salts are excluded from
the ice as dense brine solutions.
• Salinity slows evaporation, as ions of salts attract water
tighter than H-bonds of other water molecules.
• Salinity increases the density of water.
Density Structure of Seawater
• Both temperature and salinity influence the density of
seawater.
• Ocean waters will structure themselves in layers of
increasing density with depth, called strata.
• Two different water parcels can have very different
temperatures and salinity yet have the same density.
• Consider the T-S diagram below.
Temperature is the dominant
factor in controlling density in the
ocean, particularly when salinity
does not vary dramatically.
Surface water (<200 m) absorbs
the majority of solar radiation and
converts it to heat, slowly raising
surface water temperature.
A distinct layer of warmer lower
density seawater forms at the
surface zone when there is ample
sunlight and calm conditions.
The transition depths between
warm surface zone and cold
denser deep water is called the
thermocline (thermo = heat;
cline = change).
Thermocline
characteristics vary with
latitude due to
differences in solar
radiation inputs with
latitude.
Weak to no thermocline
is seen at the poles due
to cold surface water
being mixed by sinking
and mixing by storms.
When a thermocline
does exists, we refer to
the water as being
stratified (layered).
Seawater can also be stratified due to density differences
related to salinity. The depth of rapid salinity change is called
a halocline (halo = salt).
The combination of temperature and salinity determines
overall density; the depth of rapid density change is called a
pycnocline (pycno = density).
Largely due to the temperature influence on density,
pycnoclines are greatest (deepest) at the equator, and
shallow to non-existent above 60º North of South
latitude.
Winds generate waves to keep the thin surface zone
mixed to the top of the thermocline. Deep water is a
massive volume (80%) and very cold (2-5ºC)
Optical Properties of Water
• Refraction of light. When light penetrates a medium of
different density and at an angle other than 90º it will
bend.
• Refraction applies to sound as well, as both light and
sounds are waves.
• Refraction of light can create optical distortions and
influence the depth that light will travel before absorbed.
• The marching soldiers example:
Refraction:
• A fish in water looks bigger and closer than it
really is due to bending of light toward the
surface as it leaves the water to your eye.
PSYCH!
Light is absorbed and scattered as
it passes through water, so less
light reaches greater depths.
Different wavelengths of light
absorb differently in pure water.
Blues and greens penetrate
deepest and reds the least. Why
the “deep blue sea”?
Particles and colored dissolved
organic matter and further influence
how quickly different colors of light
are absorbed in natural waters.
Light greatly influences life!
Sound and Seawater
Sound is an energy wave
produced by rapid pressure
change. Like light it is lost
over distance in seawater
due to spreading, scatter,
and absorption, but sound
can travel much farther
than any light in seawater,
particularly low frequency
sound waves.
Some marine mammals use
sound to “see”, a process
called echolocation, much
like Sound Navigation and
Ranging (SONAR)
technology used by
submarines and ships.
Average speed of sound in
seawater is 5-times faster
than in air (1,500 m/s).
Increases in temperature
or pressure will increase
sound speed in seawater.
Sound traveling through
water of different
temperature or pressure
will bend; refraction.
A depth of minimum sound
velocity results from these
properties.
Sound Fixing and Ranging (SOFAR) Channel
Best depth for listening to others!