The Water Molecule

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Previous Lecture

• The Water Molecule
• chemistry and properties
• Heat
• phases of matter, transitions, problems
• Global Implications
• temperature and global glaciations
• Properties of Water

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Lecture 3 Seawater Chemistry
Sodium and chloride ions in NaCl are held
together by ionic bonds, electrostatic attraction
that exists between ions of opposite charge.
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Look For the Following Key Ideas
Water is a polar molecule with the remarkable
ability to dissolve more substances than any
other natural solvent. Salinity is the
measure of dissolved inorganic solids in water.
The most abundant ions dissolved in
seawater are chloride, sodium and sulfate.
The ocean is in Steady State.
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Overview

• Water as a Solvent
• Dissolving power
• Salinity
• Definition, components, determination
• Steady State
• Salts, residence time, problems

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The Dissolving Power of Water
What are solutions and mixtures? A solution is
made of two components, with uniform molecular
properties throughout The solvent, which is
usually a liquid, and is the more abundant
component. The solute, often a solid or gas, is
the less abundant component. A mixture is
different from a solution. In a mixture the
components retain separate identities, so it is
NOT uniform throughout.
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The Dissolving Power of Water
A water molecule is composed of two hydrogen
atoms and one oxygen atom. Water is a polar
molecule, having a positive and a negative side
(from lecture 2). The polarity of water
molecules causes ionic compounds to dissolve.
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The Dissolving Power of Water
As solid sodium chloride dissolves, the positive
and negative ions are attracted to the positive
and negative ends of the polar water molecules.
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Salinity
What is salinity? Salinity is the total quantity
of dissolved inorganic solids in water. Waters
colligative properties are ? The heat
capacity of water decreases with increasing
salinity ? As salinity increases, freezing
point decreases ? As salinity increases,
evaporation slows ? Osmotic pressure
increases as salinity increases
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The Components of Salinity
The most abundant components of a kilogram of
seawater.
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The Principle of Constant Proportions
Forchhammers principle states that although the
salinity of various samples of seawater may vary,
the ratio of major salts is constant. Forchhammer
s principle is also known as the principle of
constant proportions.
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Determining Salinity
How do scientists determine the salinity of
seawater? Salinity can be determined by
measuring the chlorinity of the sample. Since the
chlorinity is easy to measure, and the principle
of constant proportions applies to all seawater,
scientists can use the following formula to
determine salinity Salinity (in parts per
thousand) 1.80655 ? chlorinity (in parts per
thousand)
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Determining Salinity
How do scientists determine the salinity of
seawater? There are a variety of other methods,
including one my lab is working on
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Steady State
Is the ocean becoming progressively saltier with
age? No, the proportion and amounts of dissolved
solids remain constant. This concept is known as
the steady state ocean. Ions are being added
to and removed from the ocean at the same rate.
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Steady State Addition and Removal
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Steady State

• The steady-state concept is VERY important and is
  applicable to many different problems.
• Keys to steady state
• The overall quantity of the compound (or )
  remains constant.
• 2) The input rate equals the removal rate
  (sources sinks)

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Jargon Alert
Sources and Sinks
Source The rate at which a compound is
added to the ocean.
Sink The rate at which a compound is
removed from the ocean.
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Residence Time
Residence time is the average length of time an
element spends in the ocean. The residence time
of any element depends on its chemical activity.
Amount of element in the ocean
Residence Time
The rate at which the element is added to or
removed from the ocean
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Residence Time
AKA Turnover time, Lifetime, Natural
lifetime, e-fold time t (Tau)
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Calculating Residence Time
You will be expected to understand the
relationship between residence time, quantity,
and input/removal rate.
Furthermore, you will be expected to solve
complex questions based on this relationship.
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Calculating Residence Time
The major source of Calcium (Ca2) to the oceans
is river input. What is the rate at which
calcium is added to the ocean? What is the rate
at which calcium is removed from the ocean?
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Calculating Residence Time
The major source of Calcium (Ca2) to the oceans
is river input. What is the rate at which
calcium is added to the ocean? What is the rate
at which calcium is removed from the ocean?
What we need to know Residence Time? Quantity
of Ca2 in Ocean?
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Calculating Residence Time
The major source of Calcium (Ca2) to the oceans
is river input. What is the rate at which
calcium is added to the ocean? What is the rate
at which calcium is removed from the ocean?
What we need to know Residence Time? 1,000,000
years (table 7.3) Quantity of Ca2 in Ocean? 0.4
g/kg (table 7.1)
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Calculating Residence Time
The major source of Calcium (Ca2) to the oceans
is river input. What is the rate at which
calcium is added to the ocean? What is the rate
at which calcium is removed from the ocean?
What we need to know Residence Time? 1,000,000
years (table 7.3) Quantity of Ca2 in Ocean? 0.4
g/kg (table 7.1)
Convert Ca2 concentration (0.4 g/kg) to quantity
in entire ocean.
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Calculating Residence Time
The major source of Calcium (Ca2) to the oceans
is river input. What is the rate at which
calcium is added to the ocean? What is the rate
at which calcium is removed from the ocean?
What we need to know Residence Time? 1,000,000
years (table 7.3) Quantity of Ca2 in Ocean? 0.4
g/kg (table 7.1)
Convert Ca2 concentration (0.4 g/kg) to quantity
in entire ocean.
0.4g (Ca2)/kg (seawater) 1.371021kg (seawater)
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Calculating Residence Time
The major source of Calcium (Ca2) to the oceans
is river input. What is the rate at which
calcium is added to the ocean? What is the rate
at which calcium is removed from the ocean?
What we need to know Residence Time? 1,000,000
years (table 7.3) Quantity of Ca2 in Ocean? 0.4
g/kg (table 7.1)
Convert Ca2 concentration (0.4 g/kg) to quantity
in entire ocean.
0.4g (Ca2)/kg (seawater) 1.371021kg
(seawater) 5.51020 g (Ca2)
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Calculating Residence Time
The major source of Calcium (Ca2) to the oceans
is river input. What is the rate at which
calcium is added to the ocean? What is the rate
at which calcium is removed from the ocean?
What we need to know Residence Time? 1,000,000
years (table 7.3) Quantity of Ca2 in Ocean?
5.51020 g (Ca2)
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Calculating Residence Time
The major source of Calcium (Ca2) to the oceans
is river input. What is the rate at which
calcium is added to the ocean? What is the rate
at which calcium is removed from the ocean?
What we need to know Residence Time? 1,000,000
years (table 7.3) Quantity of Ca2 in Ocean?
5.51020 g (Ca2)
Amount of element in the ocean
Residence Time
The rate at which the element is added to or
removed from the ocean
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Calculating Residence Time
The major source of Calcium (Ca2) to the oceans
is river input. What is the rate at which
calcium is added to the ocean? What is the rate
at which calcium is removed from the ocean?
What we need to know Residence Time? 1,000,000
years (table 7.3) Quantity of Ca2 in Ocean?
5.51020 g (Ca2)
Amount of element in the ocean
Residence Time
The rate at which the element is added to or
removed from the ocean
Amount
Rate
Residence Time
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Calculating Residence Time
The major source of Calcium (Ca2) to the oceans
is river input. What is the rate at which
calcium is added to the ocean? What is the rate
at which calcium is removed from the ocean?
What we need to know Residence Time? 1,000,000
years (table 7.3) Quantity of Ca2 in Ocean?
5.51020 g (Ca2)
Amount
Rate
Residence Time
5.51020 g (Ca2)
Rate
5.51014 g/year
106 years
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FYI
Something like this will be on the
Final Something like this will also be on the
1st Midterm
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Conservative and Nonconservative Constituents
Conservative constituents of seawater are those
constituents that occur in constant proportions.
Conservative elements have long residence times
and are the most abundant dissolved material in
the ocean. Nonconservative constituents have
short residence times, and are usually associated
with seasonal, biological or short geological
cycles.
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Conservative and Nonconservative Constituents
Conservative constituents of seawater include
major salts such as sodium, chloride, and
sulfate. Nonconservative constituents include
nutrients such as phosphate and nitrate.
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Conservative and Nonconservative Constituents
The mixing time of the ocean is 1600 years.
Conservative constituents are well mixed in the
ocean, therefore have residence times gt1600 years.
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Calculating Residence Time
In the 1960s scientists were confused about
magnesium (Mg2) in the ocean. The quantity of
Mg2 was measured to be 1.91021g, and hadnt
changed for millions of years (indicating steady
state). The input of Mg2 from rivers was
calculated to be 1.31014 g/yr. The removal of
Mg2 as sediment-bound minerals was found to be
11013 g/yr. In the late 1960s scientists
discovered hydrothermal vents, and found they
precipitated Mg2. How much Mg2 is lost at
hydrothermal vents? What is the residence time
of Mg2 in the ocean? Based on this residence
time, do you think Mg2 is conservative?
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Calculating Residence Time
In the 1960s scientists were confused about
magnesium (Mg2) in the ocean. The quantity of
Mg2 was measured to be 1.91021g, and hadnt
changed for millions of years (indicating steady
state). The input of Mg2 from rivers was
calculated to be 1.31014 g/yr. The removal of
Mg2 as sediment-bound minerals was found to be
11013 g/yr. In the late 1960s scientists
discovered hydrothermal vents, and found they
precipitated Mg2. HOW MUCH Mg2 IS LOST AT
HYDROTHERMAL VENTS? What is the residence time of
Mg2 in the ocean? Based on this residence time,
do you think Mg2 is conservative?
Assume Steady State Sum of Sources Sum of Sinks
36
Calculating Residence Time
In the 1960s scientists were confused about
magnesium (Mg2) in the ocean. The quantity of
Mg2 was measured to be 1.91021g, and hadnt
changed for millions of years (indicating steady
state). The input of Mg2 from rivers was
calculated to be 1.31014 g/yr. The removal of
Mg2 as sediment-bound minerals was found to be
11013 g/yr. In the late 1960s scientists
discovered hydrothermal vents, and found they
precipitated Mg2. HOW MUCH Mg2 IS LOST AT
HYDROTHERMAL VENTS? What is the residence time
of Mg2 in the ocean? Based on this residence
time, do you think Mg2 is conservative?
Sum of Sources Sum of Sinks 1.31014 g/yr
11013 g/yr Vent Removal Vent Removal
1.21014 g/yr
37
Calculating Residence Time
In the 1960s scientists were confused about
magnesium (Mg2) in the ocean. The quantity of
Mg2 was measured to be 1.91021g, and hadnt
changed for millions of years (indicating steady
state). The input of Mg2 from rivers was
calculated to be 1.31014 g/yr. The removal of
Mg2 as sediment-bound minerals was found to be
11013 g/yr. In the late 1960s scientists
discovered hydrothermal vents, and found they
precipitated Mg2. How much Mg2 is lost at
hydrothermal vents? WHAT IS THE RESIDENCE TIME
OF Mg2 IN THE OCEAN? Based on this residence
time, do you think Mg2 is conservative?
Assume Steady State
Amount
Residence Time
Rate
Rate is the total rate of addition or removal
38
Calculating Residence Time
In the 1960s scientists were confused about
magnesium (Mg2) in the ocean. The quantity of
Mg2 was measured to be 1.91021g, and hadnt
changed for millions of years (indicating steady
state). The input of Mg2 from rivers was
calculated to be 1.31014 g/yr. The removal of
Mg2 as sediment-bound minerals was found to be
11013 g/yr. In the late 1960s scientists
discovered hydrothermal vents, and found they
precipitated Mg2. How much Mg2 is lost at
hydrothermal vents? WHAT IS THE RESIDENCE TIME
OF Mg2 IN THE OCEAN? Based on this residence
time, do you think Mg2 is conservative?
Amount
Residence Time
Rate
1.91021g
Residence Time
1.21014 0.11014 g/yr
Residence Time 1.46 107 yr
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Calculating Residence Time
In the 1960s scientists were confused about
magnesium (Mg2) in the ocean. The quantity of
Mg2 was measured to be 1.91021g, and hadnt
changed for millions of years (indicating steady
state). The input of Mg2 from rivers was
calculated to be 1.31014 g/yr. The removal of
Mg2 as sediment-bound minerals was found to be
11013 g/yr. In the late 1960s scientists
discovered hydrothermal vents, and found they
precipitated Mg2. How much Mg2 is lost at
hydrothermal vents? What is the residence time
of Mg2 in the ocean? Based on this residence
time, do you think Mg2 is conservative?
Mg2 Residence Time 1.46 107 yr
Mixing time of the ocean is 1600 years
14,600,000 yr is much greater than 1600 yr
Conservative
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Review

• Water as a Solvent
• Dissolving power
• Salinity
• Definition, components, determination
• Steady State
• Salts, residence time, problems

41
Lecture 3 Summary
Many processes regulate the composition of
seawater. The concept of a steady state ocean
summarizes the idea that ions are added to and
removed from the ocean at the same rate.
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Next Lecture pages 178-182

• Dissolved Gases
• Needed for life
• Acid and Base
• pH scale, seawater pH
• CO2 System
• Gas to solid, buffer
• Atmospheric Composition