Properties of seawater
Properties of water
1. Polarity and hydrogen bonding
cohesion
good solvent

many molecules dissolve in H2O
2. lower density as a solid

ice floats!
3. high heat capacity


water stores heat
heats & cools slowly
Salinity is an important part of ocean
chemistry
1.
2.
3.
Salinity is a measure of
the amount of dissolved
salts in water
Salinity is not
homogenous (uniform)
across the Earth’s
oceans
Both salinity and
temperature affect the
density of seawater
Seawater is a mixture of pure water and
chemical compounds
On average,

Seawater is 96.5%
pure water…


…and 3.5%
compounds
including dissolved
salts
Remember from
chemistry that when
salts dissolve they
form ions
Na+
Sodium
Chloride
(A salt)
Cl-
Water
Na+
Sodium ion
Cl-
Chloride ion
What’s in the water?

7 primary chemicals make up almost all (~99%) the
salts in seawater:
–
–
–
–
–
–
–

Chloride (Cl-): 55%
Sodium (Na+): 30.6%
Sulfate (SO42-): 7.7%
Magnesium (Mg2+): 3.7%
Calcium (Ca2+): 1.2%
Potassium (K+): 1.1%
Bicarbonate (HCO3-): 0.4%
Can you come up with an acronym to remember
them all?
How do scientists figure out how much
salt is in the water?

The Rule of Constant Proportions = the
major ions of seawater are present in a fixed
proportion to each other
–
–
This means that although salinity may vary, the
ratio of any one of the 7 primary ocean salts to
each other will not change
If we have a liter of seawater that has a 35 grams
of total salt per liter, how many grams of calcium
are there?
Salinity is a measurement of how much
salt is dissolved in the water


Common unit of salinity is Parts per thousand
(PPT)
What does ppt really mean?
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–
–
–
Imagine you have a bag of 1000 m&ms
Blue m&ms represent pure water
Red m&ms represent salts
To represent 35ppt (average salinity of seawater),
35 m&ms would be red and 965 would be blue.
The Composition of Seawater at 35ppt
Salinity
Factors that influence salinity


What factors do you think
might influence salinity?
Three primary factors
influence salinity:
1. Freshwater input - High
rates of freshwater input (river
inflow to the sea; melting ice)
will decrease salinity
2. Evaporation - High rates of
evaporation will increase
salinity
3. Precipitation - High levels of
rainfall will decrease salinity
Salinity is variable across the ocean
Photo: NASA
Why is salinity important?


Salinity is one factor that controls the density
of ocean water
What happens when water at different
depths has different densities?
–
–
Layers of water will form
Formation of layers is part of the reason we have
ocean currents
Gases in Seawater

Many gases are also dissolved in seawater including
oxygen, carbon dioxide, and nitrogen

Gases dissolve at the sea surface from the
atmosphere (gas exchange) and vice versa.

Gases dissolve better in cold water than in warm.
Transparency
 Transparency
is an
important property of
seawater. Why do
you think this is?
 Sunlight can
penetrate into water
 Different
wavelengths travel to
different depths
Conditions vary with depth

What do you think happens to each of the
following with increasing depth?
–
–
–
–
–
Oxygen
Carbon dioxide
Temperature
Light
Pressure
Pressure Increases with
Increasing Depth

Pressure is also greater
the deeper the depth.

Water at the bottom has
the weight of the water
above it pushing down
on it
1 atm = 14.7 lbs per square inch