The Properties of Sea Water
What makes water so special?
Why is the ocean salty?
Where’s the Water?
Reservoir
Volume (106 km3)
Ocean
Ice (polar)
Groundwater
Lakes
Atmosphere
Rivers
1370
29
5
0.1
0.01
0.001
Percent
97.3
2.1
0.4
0.01
0.001
0.0001
Where did the water in the
Oceans come from?
Outgassing (H2O, CO2) of the Earth
from volcanoes, early in its history,
but continuing today
Sedimentary rocks as old as 3.8
billion years!
A much smaller amount from comets
that pass by
The Water Molecule
Water is a “Polar” Molecule
Weak electrical attraction makes for
“sticky” molecules
This explains its unique properties:
heat capacity
surface tension
dissolving power
Temperature vs Heat
Temperature is a measure of how
fast the molecules in a substance are
moving
Heat is a measure of how much
energy has to be put into (or gotten
out of) a substance to change its
temperature, or “state” (solid, liquid,
gas)
Sensible Heat vs Latent Heat
 Sensible heat is what we sense from
different temperatures; energy needed to
raise T (or released to decrease T)
 Latent heat is the energy needed to
change state (ice to water, water to
vapor)
Exists in three states on the planet surface
water vapor
ice
liquid water
Changes of state
Water co-exists on the Earth in 3 physical states: Habitable Planet
solid
liquid
gas
Changes of state always occur
at constant temperature
The heat needed for a change of state
is called latent heat
Heat and the three Physical
States of Water:
Evaporation from lakes, oceans,
rivers, etc. occurs for
temperatures lower than 100 oC
But it
requires
more energy
to do so
Major source of energy to power
the Earth’s weather systems
Energy liberated into the
atmosphere (condensation)
Energy removed from surface (evaporation)
Density of Pure Water
Consequences…
Consequences
Bottom temperature of deep, cold lakes is always
4 oC.
Ice floats on the water surface, so fish survive.
Pipes (or beer bottles) can freeze and burst.
Surface tension - measure of how
difficult it is to stretch or break the
surface of a liquid
Water has the highest surface tension of all common liquids
Special Properties of Water
Density of solid is less than liquid
Melting and boiling points are very
high
Highest heat capacity
High heat of fusion and vaporization
Tremendous dissolving power
Dissolving Power of Water
Why is the Ocean Salty?
Total dissolved solids (called “salinity”)
About 3.5% by weight (average
seawater)
Usually expressed as 35 0/00 (parts per
thousand, ppt)
Varies geographically according to
Evaporation, Precipitation, and Rivers
The Most Abundant Ions
Chloride (Cl-)
Sodium (Na+)
Sulfate (SO42-)
Magnesium (Mg2+)
Calcium (Ca2+)
Potassium (K+)
19.0 g/kg
10.6 “
2.6 “
1.2 “
0.4 “
0.4 “
35.2 g/kg
Ions in Sea Water
Anions are negatively charged
examples: Cl-, SO4Cations are positively charged
examples: Na+, K+, Ca++, Mg++
Measuring Salinity
Principle of Constant Proportions
e.g., SO42-/Cl- is a constant,
independent of salinity
This means we need measure only one
ion to get salinity; i.e., ClToday salinity is measured quickly by
electrical conductivity of sea water
Where does Salinity come from?
Terrigenous input (rivers, dust, ash)
Hydrothermal vents
Dissolving old sediments (evaporites)
Steady State: Inputs equal Outputs
Weathering of Rocks
H2O + CO2 ---> H2CO3 (carbonic acid)
“acid” rain, pH ~4-5; environmental
concerns (HCl, HF, H2SO4)
Dissolves rock minerals into ions,
which travel down rivers to the ocean
Residence Time
How long do the various dissolved ions stay
in the ocean? Depends on how “reactive”.
Residence Time: The average time spent by
a substance in the Ocean =
Amount in Sea
Rate entering or exiting
Residence Time
For water entering through rivers, the
residence time is about
Volume = 1370x106 km3 (oceans)
Flux 0.037x106 km3/yr (rivers)
= 35,000 years.
For Cl-, the residence time is 100 Ma!
For Fe2+, the residence time is 200 yr
Which is likely to obey Constant Proportions?
Adding salt lowers the
freezing temperature:
Seawater freezes at about -2 oC
Freezing removes fresh water, leaves salt
The Hydrologic Cycle
Evaporation - Precipitation
Over the oceans, evaporation exceeds
precipitation
The balance is restored by rain over
the continents, returning water via
rivers
Salt in the Ocean
increases its density
Ocean Surface salinities
Evaporation vs Precipitation
Which processes change the
surface salinities ?
saltier
fresher
 evaporation
 precipitation
 sea ice formation
 sea ice melting
 freshwater
runoff from land
Which ocean is the saltiest?
The Atlantic!
In spite of the fact that many BIG
rivers empty into the Atlantic than
the Pacific, the Atlantic is actually
significantly saltier because of the
evaporation-precipitation cycle and
the Isthmus of Panama plus input
from the Mediterranean Sea
Surface salinities
Evaporation, precipitation, and wind patterns
explain high/low salinity of Atlantic/Pacific.
Summary:
Water is a polar molecule -- unique
properties (melting pt, heat capacity,
dissolving power, water denser than ice)
Salinity is the total dissolved solids
Salinity in the surface ocean varies by
Evaporation - Precipitation
Principle of Constant Proportions
Residence Time in the Oceans