Water
Water
• Unique properties – important for understanding
interaction between ocean & atmosphere
– Climate
• Dissolved constituents and how they affect
water’s properties
• Phenomena in the ocean that are important –
light and sound in water
– Why is the ocean blue?
• Then ocean & atmospheric circulation – waves &
tides
What is Water
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H2O
Covalent & hydrogen bonding
Water as a solvent
Density
Heat Capacity and specific heat
Salinity (next time)
A covalent bond
exists when two
electrons are shared
by two non-metallic
atoms. Dipolar.
**Electron density higher around oxygen, giving that end a partial
negative charge.
•Oxygen has 6 electrons, and wants 2 more
electrons for stability.
•Hydrogen has 1 electron and wants 1 more electron
for stability.
Electrons and “charge” not
distributed evenly so molecule
becomes polar.
104.5°
Hydrogen bond is a weak bond formed when a charged part of a molecule
having polar covalent bonds forms an electrostatic (charge, as in positive
attracted to negative) interaction with a substance of opposite charge. The weak
bonds are easily and rapidly formed and broken under normal biological
conditions. They are extremely important in biological systems and Their
presence explains many of the properties of water.
H-bonding imparts
adhesion and cohesion of
water.
Creates surface tension
and wetting ability
Water cohesion (imparted by the H-bonding between water
molecules) causes surface tension & capillary action.
Adhesion allows water molecules to “stick” to other things
or get wet.
Water is a powerful solvent
Ionic molecules dissolve well in water.
Ethanol dissolves
well in water because
it is polar.
On the other
hand, oil is
repelled by
water because it
is non-polar, and
thus does not
dissolve well in
water.
Water & heat
• Temperature – how rapidly molecules are
vibrating
– Response to addition and removal of heat
• Heat capacity
– Amount of heat required to change the temperature of
a substance (raise or lower)
– Calorie is the amount of heat required to raise
temperature of 1 gram water by 1 oC
– Heat capacity of water is among the highest of all
known substances
– This high heat capacity is a result of water’s structure
(H-bonding)
Many metals have
low heat capacities
(change temperature
with small changes
in heat content)
Significance of heat capacity
• Water is a good buffer for heat
• Land temperatures change more easily
and rapidly than water
– Compare daily changes on land and in the
sea
– Compare land near the coast with inland
4 J ≈ 1 calories (kg)
Sensible heat versus latent heat
Latent heat
• Latent heat does not cause temperature
change
• Much higher for vaporization than for
fusion
• Must break H bonds for vaporization
• Evaporation has a cooling effect because
heat is removed from water
• Evaporation is a huge source of heat flow
between the atmosphere and the ocean
Water and density
• Density is mass per unit volume
• Density curve is NOT linear
– Normally, as you heat something, molecules move
faster and then density decreases and vice versa
• Density and state (liquid, gas, solid)
• Solid ice is less dense than liquid water
– Changes bond angle between oxygen and hydrogen
atoms in water (from 104.5 to 109o) allowing lattice
structure
Maximum density of water is about 4oC
Normal substances
3-D
Water
Implications of density
• Ice floats
• Thermohaline circulation
• Salt increases density of water (fresh
water should float on salty water)
• Water column stable when density
stratified (less dense water rises and
denser water sinks)
Salt and properties of water
• Salt water is 96.5% water and 3.5%
dissolved solids and gases
• Salt changes water’s density
• Salt changes water’s freezing behavior
• Maximum density of seawater is about –
2oC
• Why does ice that forms from seawater
still float? (salts don’t freeze!)
• We’ll talk more about salinity next time.
Now let’s think about the ocean
• Distribution of temperature and heat with
latitude
• Distribution of temperature and heat with
depth
Thermostatic effects
• Ice and vapor
• Water and air movement
– Because of the higher latent heat of
vaporization, atmosphere transfers more heat
per unit mass than liquid water.
Annual changes in air temperature depending on air movement
Ocean breezes buffer heat by causing evaporation
Annual changes in sea ice in Antarctica
Over 18,000 km3 of polar ice thaws and refreezes each year
Ocean and air currents
• Ocean and air currents result from unequal solar
heating (more on these later)
• Transport heats from equator to poles
• Gulf stream is about 10oC warmer than water
moving south to replace it
• Half the solar radiation entering the water results
in evaporation
• Condensation is usually removed from where
evaporation is occurring
• Air moves about 2/3 of heat and ocean currents
the other 1/3.
Polar
Temperate
Tropical
Warming Cycle:
1. March
2. May
3. June
4. August
Cooling Cycle:
1. August
2. September
3. October
4. November
5. January
Temp. sea water
Temperature degree C
Land shows greater extremes in temperature near poles
Global climates
Water Vapor
Take home points
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Unique properties of water
Hydrogen bonding
Heat capacity
Latent heat versus sensible heat
High latent heat of fusion and vaporization
Relative density of states of water
Effect of salt on properties of water
Transport of heat by ocean and atmosphere