PHYSICAL PROPERTIES OF
SEAWATER
How Unique is Water?
• Water is one of only 3 naturally occurring
liquids (mercury and ammonia)
• Only substance occurring naturally that
exists in all 3 states – solid, liquid, and gas
– on Earth’s surface
• Extremely large liquid range (0oC - 100oC)
• Expands, becomes less dense as a solid
The Nature of Pure Water
• Water made of 2 hydrogen atoms and 1
oxygen atom
• Water is a polar molecule (+ end & – end)
• Water’s unusual structure causes them to
“stick together”
• Water molecules form hydrogen bonds
• Hydrogen bonds not very strong, but make
water different from any other substance
on Earth
Figure 3.1
Water has…
• Cohesion – sticks to
itself
• Adhesion – sticks to
others
• Surface Tension – a
measure of how
difficult it is to stretch
or break the surface
of water
– No waves without
surface tension
Water has…
• Low Viscosity – little resistance to flow
• Good for Earth’s marine organisms –
WHY?
The Three States of Water
• Water is the only substance that naturally
occurs as a solid, liquid and gas on Earth
• Evaporation absorbs heat
• Condensation releases heat
Water is Weird
• Density – the mass of a certain volume of
a substance
• Solid water is less dense than liquid water
• Water becomes more dense as it cools
– Water is most dense @ 4º C
– Becomes less dense as it nears 0º C
• Good for the planet – WHY?
Heat Capacity
• The amount of heat required to raise the
temperature of 1 gram of a substance 1º C
– Water has one of the highest
– 1 calorie raises 1 gram of water 1º C
– It absorbs large amounts of energy before the
temperature changes
• Good for earth’s climate – WHY?
Changes of State
• Latent Heat of Fusion / Melting –
– The amount of heat required to melt a
substance without change in temperature
– 80 calories per gram
Changes of State
• Latent Heat of Vaporization / Condensation–
– The amount of heat required to change a
substance from a liquid to a gas without change in
temperature
– 540 calories per gram
Why is Water Important?
• Water is the universal solvent
• Water can dissolve more than any
other natural substance
• Water can dissolve many hydrophilic
substances
– Ionic compounds
– Other polar compounds
• Form “spheres of hydration”
• http://programs.northlandcollege.edu/biology
/Biology1111/animations/dissolve.html
Figure 3.5
Seawater has dissolved solids…
• Source –
– Chemical weathering of crustal rocks
– Hydrothermal vents
– From volcanic eruptions
Water …
• Density of pure water is 1 g/mL @ 4º C
• Density of seawater is 1.0278g/mL @ 4ºC
• Density is determined by temperature and
salinity
– Seawater gets denser as it gets saltier, colder
or both
• Because temperature varies more than
salinity, density is controlled by
temperature
Density…
•
•
•
•
Changes with depth
Densest water sinks
Ocean becomes layered, stratified
Seen in profiles of salinity, temperature,
and density
• The greater the difference in density
between surface and deep water, the
more stable the water column and the
harder it is to mix vertically
Figure 3.31
Pressure…
• Water is noncompressible – does not
change volume with increasing pressure
• So is seawater
• Pressure increases with increasing depth
– Has small effect on volume
– 1 atm (14.7 lbs/in2) for every 10 m (33 ft)
– Pressure in deepest trench ~1100 atm
– As pressure increases, gases are
compressed
Figure 3.13
Buoyancy
• The ability of an object to float by
displacing a volume of water equal to its
own weight
Water…
• Transmits energy
– Heat
– Light
– Sound
• Refraction – the bending of light and
sound waves due to density differences
that affect the speed of energy
transmission
– increases with increasing salt, decreases with
increasing temperature
Water…
• Transmits heat energy by
– Conduction – molecule to molecule
– Convection – moving fluids & density driven
– Radiation – direct from source (sun)
Water is …
• Transparent –
transmits light energy
readily
– Important for
photosynthesis
– Oceans are blue
because blue light
penetrates the
deepest
– Coastal waters
sometimes green
because blue
absorbed
Water…
• Transmits sound faster & farther than in air
– 1500 m/s in seawater (& 60 times farther)
– 334 m/s in air
• At 1000 m combination of salinity, temp &
pressure creates a zone of minimum
velocity for sound – the Sofar Layer
(sound fixing and ranging layer)
– Sound waves produced here do not escape
& travel long distances
Sofar Channel