Oceanography Lecture Notes Outline
Physical and Chemical Properties of Water
I. Contents - Topics
Covered
The Chemical
Structure of Water
The Three
Physical States of Water
The Heat
Capacity of Water
The Surface
Tension of Water
The Viscosity
of Water
Compressive
Nature of Water
Water Density
Water – the
Universal Solvent
Water
Transparency
Water and Sound
II. The Molecular Structure of Water
A. The Special
Properties of Water are Due to its
Chemical Structure
1. A water molecule is
composed of two hydrogen atoms
covalently bonded to one central oxygen
atom
·
Water molecules have a bent
“L” shape, with
the hydrogen atoms bound to one side of
the
oxygen atom – this produces special
properties
2. The hydrogen-oxygen
bond of water is polar, which
means that the hydrogen
has a positive electrical
charge and the oxygen
has a negative charge
3. The positive and negative
charges attract, forming a
hydrogen
bond between water molecules that are close
together
·
The hydrogen
bond is like an ionic bond
4. Hydrogen bonds are weak
compared to the covalent
bonds that connect the
hydrogens and oxygen of the
water molecule. However,
they are much stronger than
the bonds that form
between most other molecules.
B. Water
Exists in Three Physical States on the Earth:
Solid (Ice), Liquid, and Gas (Water Vapor)
1. These changes
of state represent changes in the relations
between water molecules and the motion of the
molecules
·
In water vapor, the molecules are moving rapidly
and are not associated with one another.
·
In liquid
water, molecular motion is slower and
transient 3-dimensional associations of
molecules,
connected by hydrogen bonds, occur.
·
In solid
ice, molecular motion is slower still, and
each molecule has a fixed place within a
crystal
structure, which is held together by
hydrogen bonds.
·
Because of the hydrogen
bonds between water
molecules, it takes an unusual
amount of energy
to melt ice or evaporate liquid water.
C. Latent Heat of
Vaporization
1. When water is
heated to 100oC, additional heat must be applied in order to cause
the water to evaporate (form steam). This added heat is called the latent heat of vaporization.
· This latent heat must also be removed when water vapor
condenses.
2. The latent heat of vaporization is very
important in
moderating Earth's climate.
· There is a global pattern of greater evaporation of water
form the oceans at low latitudes, transport of water vapor toward the poles,
and greater precipitation at high latitudes.
· This results in removal of heat (latent heat of
vaporization) from low latitudes and release of this heat at high latitudes as
the water vapor condenses
D. Latent
Heat of Fusion
1. When water is cooled to 0°C, an additional amount of heat
must be removed in order to form ice. This additional heat is called the latent heat of fusion.
·
This latent heat must also
be added when ice melts.
2. The latent heat of fusion is very important in
moderating
Earth's climate.
·
During periods of global
warming, melting of the ice caps acts as a large thermal “heat sink” for excess
heat coming from the equator
·
This results in a global
thermal buffer effect.
F. Dissolved
Substances Change Water’s Boiling and Freezing Points
1. The salt in
seawater raises its boiling point and
decreases
its freezing point, to about -2oC. The
change increases with the amount of salt
in water.
2. Note that any
dissolved substance affects freezing and boiling point (antifreeze, ethylene
glycol, works on this principle). This is because the dissolved substance
affects the association between
water molecules.
III. The Heat Capacity of Water
A. Water Has an
Unusually High Heat Capacity
1. Defined as the amount of heat required to raise
the
temperature on water one degree centigrade
2. An increase
in temperature equals an increase in the
motion of water molecules. It takes lots of
heat to make
water molecules move faster, because of
hydrogen bonds
3. The high heat capacity of water
is also important in
moderating climate.
Coastal areas have less variable
temperatures than inland
areas, because water
temperature changes slowly
in response to changes in the
amount of heat from the
sun.
IV. The Surface Tension of Water
A. Water Has High Surface
Tension
1. Defined:
surface tension is resistance to penetration or
stretching of the surface
2. Surface tension is high because of the cohesion
of the
water molecules due to hydrogen bonds.
3. Water surfaces can support small objects that are denser
than water and would otherwise sink.
·
These include some aquatic
animals. (Water striders, freshwater insects, are the most familiar example.)
4. Surface tension damps
(tends to decrease) capillary
waves, the small
waves that are the first to form as
wind blows over a water surface.
V. The Viscocity of Water
A. Water Has a High Viscocity
1. Defined: viscosity is the resistance of a liquid
to motion.
2. The viscosity of water is high
compared to that of chemically similar liquids (for example, alcohol)
3. The viscosity of water is low compared to that of chemically
dissimilar liquids (for example, oil)
4. The viscosity of water is
important to marine organisms.
·
It helps to prevent tiny
plants and animals (plankton)
from sinking
·
It also causes resistance
to the motion of larger,
swimming animals.
VI. The Compressive Nature of Water
A. Water is Not Very
Compressive
1. Even at the very high pressures of the deep sea
· About 500 times the pressure at the sea surface
2. This property is also
important to marine organisms.
·
If they lack air-filled cavities (like lungs or
swim bladders), they can often tolerate large changes in depth and pressure
without major ill effects.
VII. Water Density
A. Water Density Decreases with Increasing Temperature
1. Warm water is less dense than cold water
B. Water Density Increases
with Increasing Salt Content
1. Saltwater is denser than
freshwater
C.
Water Density Profoundly Affects Ocean Circulation
1. Increases in seawater
density with decreasing temperature (especially) and with increasing salt
content cause the vertical circulation of ocean water.
·
This is essential to the ventilation, or supply of oxygen,
to the deeper parts of the ocean.
2. Seawater (in contrast to pure or freshwater) has
its greatest density at its freezing point.
D. Water Density Profoundly
Affects Sea Life
1. Decreased seawater density with increasing
temperature creates a warm surface layer which suspends ocean plant life
(phytoplankton) near the surface where they can get enough sunlight to grow.
F. Ice is Less Dense than Water, so Ice
Floats
1. Ice and its snow
cover insulates bodies of water and
helps keep the underlying
water from freezing.
VIII. Water as a Solvent
A. Water is a Very Good Solvent
1. Defined: a solvent is fluid that can dissolve substances
2. Water
dissolves rocks (very slowly)
· The dissolved minerals from rocks is a primary
source of many salts found in seawater
3. Water also dissolves
and makes available substances
essential to marine organisms
· like fertilizers and carbon dioxide (for plants)
and oxygen (for animals).
IX. The Transparency of Water
A. Water is Transparent to Visible Light
1. This
allows plant growth to substantial depths.
2. Water does gradually absorb
light.
·
This prevents
plant growth below about 200 m depth even in the clearest ocean water.
B. Water is NOT Transparent to Ultraviolet light or to Infrared
1. Absorption of UV light
protects sensitive organisms.
2. Absorption of UV and
Infrared acts to heat up water.
X. Water and Sound
A. Water Transmits Sound Well
1. Many organisms rely on sound for echolocation or for
communication.
2.
Many oceanographic instruments rely on sound transmission by seawater – for
example:
·
Echo sounders
to measure depth
·
Bioacoustics to
measure animal abundance and location