Lecture 4 (Ch. 5 of text)
Properties of Seawater (Part I)
Biology is wet and dynamic
-
Random walks in biology 1983, by H. C. Berg
The most common molecule on the Earth - water
Soup as salty as the seawater
Learning Objectives
1. Understand the nature of the water molecule and its
unique properties and how these are altered by the
presence of salt in solution.
Review of following terminologies:
Atoms
Temperature
Heat
Specific Heat
Thermodynamics
Internal Energy and Entropy
2. Know the types of materials that are dissolved in sea
water, their importance and how they vary with time.
3. Explain variations in salinity, temperature, and
pressure within the sea and how they alter the
chemical and physical properties of the ocean.
Atomic Structure (Ch.5.1)
• The smallest unit of a
substance that retains all of
its chemical properties
• Subatomic particles include
protons, neutrons, and
electrons
• A stable atom of an element
is electrically neutral
• An ion is an atom with either
positive- (excess protons) or
negative- (excess electrons)
charged
• Atoms that chemically
bonded to another (either
same type or different one)
comprise a molecule
• An element has isotopes due
to its varied no. of neutrons
in nucleus
Periodic Table
稀土元素
錒系元素
More likely to loose electrons
and be positively charged
More likely to gain electrons
and be negatively charged
The charge of an ion is the single most important
reason for its ability to bond with other elements
Basic physical notions (Ch. 5.2)
H2O: the only substance that can co-exist naturally as a
gas, liquid, and solid states on the Earth’s surface
聚合體
States of Matter (e.g. water)
Thermodynamics: the kinetic theory of heat
Phase diagram for water
a mercury thermometer
Precisely, what does the
temperature reading mean?
Why does the hot-air
balloon move upward?
State variables
What is temperature?
it is a measure of the average translational kinetic
energy associated with the disordered microscopic
motion of atoms and molecules.
It is measured in (Celsius, Kelvin, and Fahrenheit).
using  Kinetic temperature definition
Concept of Entropy (熵 )
Thermal Equilibrium = Maximum Entropy
dS = dQrev = 0; dQrev : heat being reversibly
qqqqqqqqqadded or removed
T
However, the concept of temperature is
complicated by internal degrees of freedom
Box is solid
(Constant volume)
Heat:
1) Represents the
transfer of energy from
high to low temperature.
Therefore, heat has units
of Energy
Q
2) An object does not
possess "heat"; the
appropriate term for the
microscopic energy in an
object is internal energy.
Heat may be defined as energy in transit from a
high temperature object to a lower temperature
object.
State variables
What is Internal Energy?
Internal energy = Measure of the Kinetic
energy and potential energy on atomic and
molecular scales.
There is a difference between
Temperature and Internal Energy
k = 1.38x10-23 J/◦K
Change of Internal Energy
First Law of Thermodynamics
U 
U
S
S 
V
Temperature
U
V
V
S
Pressure
Specific Heat Capacity?
Amount of energy needed to raise
temperature of unit volume of water by one
degree C. Units: energy/(mass * degree)
Flow chart of thermodynamics
http://hyperphysics.phy-astr.gsu.edu/hbase/heacon.html#heacon
Flow chart of thermodynamics
http://hyperphysics.phy-astr.gsu.edu/hbase/heacon.html#heacon
Facts:
– Water has one of highest heat capacities
ccknown, makes water excellent heat
cctransfer material
– Allows ocean currents to modulate global
ccclimate
The amazing water molecule (Ch. 5.3)
Liquid/gas state only
Plus
high heat
capacity
(thermal
inertial) and
solvent power
(Covalent bond)
共價鍵結
(in electricity)
(hydrogen bonding)
Water is a powerful solvent – for example
Sodium Chloride
Rock SALT
N aC l
Na
 Ionic bond
Cation
陽離子
Cl

Anion
陰離子
Cation: in greek, downward, descend; anode 陽極
Anion: in greek, go up, ascend; cathode 陰極
外層覆蓋物
Hydration: Water (as solvent) dissolves salts (as
solute) by surrounding the atoms in the salt molecule
and neutralizing the ionic bond holding the molecule
together.
All solid-state substances sink in their
own liquids, except the water !
Mickey’s ears
loosen when
entering the
ice house
(109.50 )
Properties of Seawater
•
•
•
•
Salinity
Temperature
Density
Pressure
Salinity:
the total weight in grams of dissolved salt in
1 KG of seawater expressed as ‰ (part per
thousand PPT)
Another factor contributing to the
dissolution of salts is acidity

pH   log 10 [ H ]
Sources for acidity in the ocean
Carbonic Acid

 H C O 

H
H 2 O  C O 2 


2
3


 H C O3
Bicarbonate Ion
(重碳酸鹽離子)
Example:
Orthoclase (正長石)
2 K A lS i3 O 8  2 H

 2K


 H 2O  C O 2  

 2 A l 2 S i 2 O 5 ( O H ) 4  4 S iO 2
Kaolinite
Dissolved Silica
Major constituent (conservative ions)
of Seawater (c.f. Table 5.1)
Seawater
Rivers
(鉀)
35 PPT
0.088 PPT
Sources of Salt in Seawater
Weathering
and erosion of
rocks on land
陽離子交換
落塵
Rate of
supply of
salts from
rivers: 1015
grams/year !
Surprisingly,
The Ocean DOES NOT get saltier at
least during the past 1.5 billion years
The "steady state" results from the removal rate of
salts from the ocean being equal to the input rate.
This balance holds because the removal rate of
salts is related to their concentration, and
increases when their concentration increases
Sinks of Salt in Seawater
Removal of salt from
seawater occurs by
(1) inorganic process, such as
a. evaporation-precipitation
loop through the formation of
supersaturated solution, b.
wind, c. adsorption, and
(2) organic process such as
diatoms having silica shells
and forams with carbonate
shells that are precipitated
from the uptake of Si4+ and
Ca2+ from seawater (Ch. 4)
石膏
The equilibrium state of salinity of
seawater gives a hint toward the grand
sedimentary cycle in geologic time frame
If rivers are the primary supply of salts.
Why is CALCIUM and BICARBONATE so
little in seawater?
Seawater
Rivers
35 PPT
0.088 PPT
… different processes removing
different elements
HCO3 and Ca are used biologically very
rapidly (non-conservative)
Na and Cl are removed very slowly, by
absorption on clay particles (conservative)
Elements have different residence times,
decided by reactive speed and demanding
by ocean biota (see Table 5.8)
Residence time
the average time a component spends in
a system (such as monthly stipend in your
account).
RT 
C
r
C = total amount
r = the removal rate (units of [C]/time)
slow
absorption
removal
Rapid
biological
removal
The ocean gets thoroughly stirred, by
currents, every ~1,600 years,.
This is called the mixing time of the
ocean: the time it takes the ocean to mix
thoroughly
Salts stay in seawater much longer
than 1,600 years
Salt well mixed  Constant proportion of
relative constituents salts of seawater
The Hydrological Cycle
Surface Salinity
(which season?)
Subsidence regions
Evaporation − Precipitation
(c.f. Fig.5-12 of text)
Spread out of
Mediterranean
seawater
Effects of salinity on the properties of water
Chemical/physical structure of the oceans
(Ch.5.5)
January
Winter
July
Summer
July-January
Summer-Winter
Why is the deep ocean cold?
Transfer of Heat to the Ocean (heat flux)
Absorption of solar radiation
decreases rapidly with depth
Vertical Structure of Temperature
Thermocline
Vertical Structure of Temperature
Outstanding question:
what sets the depth of the thermocline?
Salinity
How do the water masses move?
c.f. Fig.5.13b
Temperature
Vertical profiles
DENSITY: controls the
movement and stability of
the ocean water masses
Next time we will talk more about
DENSITY and PRESSURE
in the OCEAN