OS1 the Oceans
Fall 2007
Name: _______________________
Section/ TA: ____________________
Seawater
TA Initials:
for finished Activity. 1 & 2
Or lose 10% of credit!
Seawater is an unusual substance. It is pure water mixed with various salts, trace
elements, and gases. The physical and chemical properties of seawater allow it to
store and transport heat, help keep atmospheric concentrations of CO2 low, and
support a remarkable variety of life forms.
Activity 1: Seawater Density
MUST BE DONE IN CLASS
The density of a substance relative to the substance around it control which
substance will sink or float. For example, when fudge syrup is added to milk it
sinks to the bottom of the glass. This is because the fudge syrup is denser than
the milk: its mass to volume ratio is higher than the mass to volume ratio of milk.
The separation of the substances of different densities is called stratification. In
the ocean we observe the stratification of water masses of different densities
similar to the stratification of syrup and milk in a glass.
Density in the ocean is controlled primarily by temperature and salinity differences.
These properties drive the thermohaline circulation of water in the ocean.
“Thermo” refers to the temperature component of density while “haline” refers to
the salinity component.
Temperature effect on density: There is an inverse or negative relationship
between temperature and density. If temperature increases, density decreases; if
temperature decreases, density increases.
Increasing temperatures cause substances to expand and become less dense. This
is because as a substance acquires heat, the molecules vibrate more and demand
more space. The addition of heat does not change the mass of the substance. An
increase in the volume without an increase in mass decreases the mass to volume
ratio; thus the density decreases. The maximum density of pure water occurs at
4˚C. As pure water cools below 4˚C, it becomes less dense until it freezes at 0˚C.
This is because as liquid water freezes, hydrogen bonds form an open structure of
ice. Because the structure of ice is more open than liquid water it occupies a
greater volume and is less dense. That is why ice floats on water.
Salinity effect on density: There is a direct positive relationship between salinity
and density. If salinity increases, density increases; if salinity decreases, density
decreases.
Before we can understand this relationship, we must first define salinity. Salinity is
the total amount of dissolved salts per unit of water. The salinity of seawater is
commonly measured in g/kg or ‰ (parts per thousand or ppt). These two units
are equal because there are 1000g/kg. The typical salinity of seawater is about 35
ppt = 35‰, but there is a range of salinities seen in the ocean, depending on the
water depth, temperature, and other factors.
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OS1 the Oceans
Fall 2007
Water masses in the ocean are composed of volumes of seawater having the same
density (mass/volume). Water that is colder or more saline is denser. These two
primary controls on density (temperature and salinity) are illustrated in Figure 1.
Figure 1: The relationship between temperature, salinity and density of
seawater. The lines indicate combinations of temperature and salinity that
result in the same density. Note that many combinations of temperature and
salinity can lead to the same water density.
1. Using Figure 1, estimate the densities of three seawater masses:
A: salinity = 34.5‰, temperature = 3.5°C, density = ______________
B: salinity = 34.5‰, temperature = 13.5°C, density = ______________
C: salinity = 36.5‰, temperature = 20°C, density = ______________
2. Deep ocean water tends to be very dense. Which of the three water masses
would be most likely to form deep ocean water?
3. Imagine that the salinity of water mass B decreases, but the water mass
maintains the same density. How must the temperature change to allow this to
occur? Look at Figure 1 and explain your reasoning.
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OS1 the Oceans
Fall 2007
4. Consider a water mass with a salinity of 35‰ and a temperature of 5°C.
What is the density of this water mass after it warms to 15°C, if salinity stays
constant?
5. Consider a water mass with a salinity of 35‰ and a temperature of 10°C. If
the salinity increases by 1‰, by how much must the temperature change in
order for the density to remain the same?
Activity 2: Heat Capacity
MUST BE DONE IN SECTION
(Get TA Initials on pg 1 or lose 10%)
Heat capacity is defined as the amount of heat (calories) required to raise the
temperature of 1 gram of a substance by 1oC. Heat capacity values indicate the
extent to which a substance can absorb or release heat energy without a change
in temperature. Substances with high heat capacities absorb (or release) more
heat before changing temperature than do substances with low heat capacities.
The heat capacity of liquid water is 1 cal/goC, higher than all other common
substances. The high heat capacity of water is due to hydrogen bonding
between water molecules. The unequal sharing of electrons in the covalent
bonds between hydrogen and oxygen atoms result in bipolar water molecules
(the bonds forming the water molecule are described as polar covalent). A
hydrogen bond forms between the slightly negatively charged oxygen atom of
one water molecule and the slightly positively charged hydrogen atom of
another water molecule. Each water molecule can form up to four hydrogen
bonds. As a result, energy must be expended in breaking these hydrogen bonds
before the temperature of the water will rise. This is particularly important as
water changes state between solid, liquid, and vapor (gas).
When a substance changes state, there may be no increase of temperature at
the point where state change occurs even though heat is continuously applied.
The heat energy is used entirely to break the bonds necessary to complete the
state change. For water, 80 calories of heat are required to convert 1 gram of
ice to 1 gram of liquid; this is the latent heat of melting. The amount of heat
required to convert liquid water to vapor, the latent heat of vaporization, is 540
calories.
1. How much heat would be required to raise the temperature of 10 grams of
pure water from 25oC to 35oC?
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OS1 the Oceans
Fall 2007
2. How much heat would be required to raise the temperature of 10 grams of
pure water from 95oC to 110oC?
3. How much heat is released when reducing the temperature of pure water
from 57oC to 50oC? You can provide the answer as heat per unit volume, and
assume that we are talking about 1 cm3.
4. How much heat is released when reducing the temperature of pure water
from 8oC to -2oC?
5. Is heat absorbed or released from/to the atmosphere when it snows?
Explain.
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OS1 the Oceans
Activity 3: World Ocean Salt
Fall 2007
(can take home !)
1. Calculate the amount of salt in the oceans today (in grams) using the following
information and steps:
Assume that the oceans of the world can be approximated by a basin with totally
vertical sides – like a bathtub whose bottom has a constant area.
Water density = 1000kg/m3
Average ocean salinity = 35 g/kg
Average depth of the ocean = 3800 m
Radius of the earth (r) = 6.37 X 106 m
Equation for surface area of a sphere (A): A = 4πr2
Area = side X side, or side2
Volume = side X side X side, or side3
a. In order to calculate the total volume of the ocean, you’ll need to first
determine the surface area of the Earth. Show your work.
b. The global ocean covers 70% of earth’s surface area, so take 70% of the value
you calculated for Earth’s surface area to determine the surface area of the
ocean.
c. The average depth of the ocean is 3800m. You have two components of
volume in the value you just calculated for ocean surface so multiply your
ocean surface area value by 3800m to get ocean volume.
d. Use water density to convert volume to mass so that you have the total mass
(in kg) of water in the ocean.
e. Use the salinity to calculate the total amount of salt in the ocean.
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OS1 the Oceans
Fall 2007
2. Determine how thick a layer covering the ocean floor this salt would form
using the following steps:
a. Convert your answer from question 1 part e from grams of salt to
volume using the density of salt (2.165 g/cm3).
b. Distribute that volume on the ocean floor by dividing the volume of salt
by the area of the ocean to get thickness of salt covering ocean floor.
Extra Credit: There are about 30 million cubic kilometers of ice on the planet
(3x107 km3). If all of it melted and ended up in the oceans, what would the
new salinity of the oceans be? Don’t forget to correct for the change in
volume between ice and water! Pure water has a density of 1 gram/mL, ice
has a density of 0.92 gram/mL. How much heat would be required to melt all
of the ice if we assume that it starts at -2°C and we heated it to 4°C?
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