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Lesson 6: Ocean Layers I
Overview
Lesson 6 presents the importance of ocean chemistry with
an activity about salinity, a measure of salt content in
ocean water. Both salinity and temperature vary
throughout the ocean. Because both salinity and
temperature affect the density of water, this variance
results in layers of water within the ocean. These layers in
part determine the formation of ocean currents, which will
be discussed in future lessons.
Lesson Objectives
Students will:
1. Define salinity and its common units
2. Predict the relationship between salinity and
temperature and density of ocean water
3. Describe how differing salinity and temperature
throughout the ocean results in layering
Lesson Contents
1. Teaching Lesson 6
a. Introduction
b. Lecture Notes
c. Additional Resources
2. Teacher’s Edition: Layers in the Ocean
3. Student Activity: Layers in the Ocean
4. Student Handout
5. Mock Bowl Quiz
Standards
Addressed
National Science
Education Standards,
Grades 9-12
Unifying concepts and
processes
Science as inquiry
Physical science
Science in personal and
social perspectives
Ocean Literacy
Principles
The Earth has one big
ocean with many
features
DCPS, High School
Chemistry
C.2.1. Investigate and classify
properties of matter,
including density, melting
point, boiling point, and
solubility
Environmental Science
E.2.1. Understand and
explain that human beings
are part of Earth’s
ecosystems, and that human
activities can, deliberately or
inadvertently, alter
ecosystems
E.1.6. Plan and conduct
scientific investigations to
explore new phenomena, to
check on previous results, to
verify or falsify the
prediction of a theory, and to
use a crucial experiment to
discriminate between
competing theories
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Teaching Lesson 6 Lesson 6
Lesson Outline
1
I. Introduction
Use the demonstration below to introduce the lesson:
1. Fill a clear water bottle or beaker halfway with cool tap water. Add salt until it no
longer dissolves. Fill another bottle or beaker with a smaller amount of tap water
and a small amount of salt. Put a few drops of food coloring into this solution.
2. Gently pour the colored solution into the clear solution so that it forms a less dense
layer over the clear, saltier, denser solution.
3. Tell the students that the clear layer has much more salt than the colored layer. Ask
the students why they think the layers don’t mix together. Introduce the concept
that the denser layer is at the bottom and the top layer is less dense.
4. Tell them that salinity is a measure of the total concentration of dissolved
(inorganic) solids (e.g., salts) in water, and they will do a lab activity to determine
how salinity and temperature affect density of water layers.
II. Lecture Notes
Use the PowerPoint for Lesson 6 (File: Lesson 6 – Ocean Layers I.ppt) to present the
following information. Distribute the Student Handout before you begin for students to
take notes on key information.
Seawater is a mixture of pure water and chemical compounds (slides 4-6)
1. Seawater can be thought of as a solution. The dissolving agent or solvent is liquid
water (H
2
O) and the materials dissolved in water, such as salt, are the solutes.
2. The majority of seawater is pure water (H
2
O), typically over 90%.
3. The major seawater constituents are seven dissolved salts that account for over
90% of all dissolved substances in seawater.
4. In a typical seawater sample of 1000 grams (1 kilogram), the seven primary salts
that would be present, in order from largest amount to smallest amount would be:
i. Chloride (Cl
-
): 18.98g (54% of solution weight)
ii. Sodium (Na
+
): 10.56g (30% of solution weight)
iii. Sulfate (SO
4
2-
): 2.65g (8% of solution by weight)
iv. Magnesium (Mg
2+
): 1.28g (4% of solution by weight)
v. Calcium (Ca
2+
): 0.40g (1% of solution by weight)
vi. Potassium (K
+
): 0.38g (1% of solution by weight)
1
Unless otherwise indicated, all websites provided or referenced in this guide were last accessed in
November 2010.
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Teaching Lesson 6 Lesson 6
vii. Bicarbonate (HCO
-
): 0.14g (< 1% by weight
How do scientists figure out how much salt is in the water? (slides 7 and 8)
1. Salinity is a measure of the total concentration of dissolved (inorganic) solids (e.g.,
salts) in water.
2. The major constituents or salts of seawater display an interesting feature. These
salts are always present in any of the world’s ocean basins and in the same
proportions to each other even if totally salinity varies. This is the Principle of
Constant Proportions and helps scientists measure salinity. This principle states
that the ratio of any two major constituents dissolved in seawater is constant. If a
scientist can measure one of the major constituents in seawater, he/she can
calculate the amounts of all other major constituents and the salinity of the sample.
3. There are several tools available to measure ion concentration and to approximate
salinity. One example is an instrument known as a CTD, short for conductivity,
temperature and depth. The CTD measures how easily electric currents pass
through a sample of seawater (conductivity). Because salt content directly affects a
water sample’s conductivity, the measurement of conductivity can be translated to a
measurement of salinity. In addition, the NASA Aquarius satellite is a relatively new
technology that will be used to measure salinity on a global scale from space.
Salinity is variable across the ocean (slides 10 and 11)
1. Salinity is not homogenous throughout the ocean.
2. Geographically, salinity varies as a function of evaporation, precipitation and
freshwater input (e.g., river outflow, ice melt).
3. In general salinity increases with decreasing latitude from the poles to the tropics
and then decreases from the tropics to the equator. At the mid-latitudes, high
salinity levels can result from warm, dry conditions and increased evaporation. Near
the equator, greater rainfall occurs resulting in lowered salinity. At high latitudes,
melting ice contributes to lower salinity levels.
Why is salinity important? (slide 12)
1. Salinity is one variable that affects the density of water. When water has different
densities, it forms layers as was demonstrated at the beginning of the lesson.
2. Layers in the ocean are part of the reason the ocean has currents, which we will
learn about in future lessons.
III. Additional Web Resources
1. Background information:
http://www.classzone.com/books/earth_science/terc/navigation/home.cfm
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Teacher’s Edition Lesson 6
Layers in the Ocean
Overview
In this activity, students mix saltwater solutions to determine how temperature and
salinity affect water density. Students observe how density differentials lead to layer
formation in saltwater. This is an important foundation for students to understand currents
and weather patterns, which are presented in subsequent lessons.
This lesson was adapted from the “Density: Sea Water Mixing & Sinking” activity from the
NASA Aquarius project, on the web at: http://aquarius.nasa.gov/seawater_mix_sink.html.
Alternative lesson ideas can be found at the NOAA site below. For example, “A funny taste”
and “Salt’n’Lighter” may be interesting to your students:
http://www.srh.noaa.gov/jetstream/ocean/seawater.htm.
Background
Students have learned that seawater is a solution made of water mixed with many different
types of salt. The ocean is not a homogeneous solution where all of the water is the exact
same temperature and exact same salinity. There are parts of the ocean that have cooler
water and some parts with warmer water. Likewise, certain parts of the ocean have higher
salinity than others. What effect do you think this might have on the ocean? In this activity,
students conduct a test to see how temperature and salinity affect the density of ocean
water.
Materials
Triple–beam balances or electronic scales (If you don’t have scales, you can make
seawater without measuring by pouring salt into a sample of water just to the point
at which salt no longer dissolves.)
Food coloring or dye
Clear Tupperware or other plastic containers large enough to hold 1L of liquid (two
per group)
Two 500mL beakers for mixing and pouring per group (Note: If you do not have
beakers for measuring, use plastic cups and tell students to fill the cups halfway
rather than measuring.)
If available, ice can be used to make the cool solutions and hot plates to make the
warm solutions
Procedure
1. Break your students into groups and distribute the Layers in the Ocean activity.
2. Assist your students with the activity, but let them mix the solutions if possible. If
you are short on supplies, you may want to complete this activity as a class, using
student volunteers to do the mixing.
3. As the activity progresses, ask students to describe the following concepts:
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Teacher’s Edition Lesson 6
Cool water is denser than warm water and will sink below the warmer water.
Water that is more saline is denser than water with lower salinity and thus
will sink below the water of lower salinity.
Water temperature has more of an effect on water movement than salinity.
Stress to your students that the layering they observe in the lab occurs on a large scale in
the oceans, and that these layers lead to ocean currents
Making the saltwater stock solution
The normal saltwater solution is made by dissolving 35g (about 3.5 tablespoons) of table
salt (NaCl) into a liter of room-temperature water. This roughly approximates the salinity
of seawater at 3.5%. If your plastic containers are too small to hold 1L of liquid, you can
make your saltwater solutions by placing 3.5g (a little less than half a tablespoon) of salt
into 100mL of water. If you don’t have a scale, gently pour salt into water while stirring just
until the salt stops dissolving.
Temperature test
1. The first test involves having students create 500mL of hot and 500mL of cold
seawater. The cold water can be made by chilling a beaker of the “seawater” in an
ice bath for a few minutes.
2. The warm seawater can be made by warming the solution on a hot plate or using
hot tap water to make the solution. Add food coloring to one of the solutions.
3. Ask the students to predict which sample will rise or sink when gently poured into
the room-temperature solution and record their prediction in their lab notebook.
Caution: If the beakers are poured into the Tupperware too quickly the solutions will
mix. Otherwise, your students should note that the hot solution floats on relatively
cooler water and the cool solution sinks below relatively warmer water.
4. Once the students have finished observing the layering and made notations in their
notebooks regarding their observations, have them discard the solutions in the
drain and rinse their Tupperware for the next test.
Salinity test
1. For the next test, students will make a 500mL sample of regular seawater by adding
17g (a little less than two tablespoons) of salt to 500mL of room temperature tap
water in their containers.
2. They will make another solution that is FOUR times as salty in a beaker by adding
60g of salt to 500mL of room temperature water.
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Teacher’s Edition Lesson 6
3. Students should gently pour the saltier solution into the stock. Make sure to add dye
to one of the solutions. They should see that the more saline solution sinks to the
bottom.
Temperature vs. salinity
1. Finally the students should predict which is more important to the layering:
temperature or salinity.
2. Ask the students to design an experiment to test their hypothesis. If you have time,
you can even let them go ahead and try their experiment.
Answer key
1. Write your predictions for the hot and cold water test.
Answers vary. Make sure students write the reasoning behind their
predictions (e.g., the hot water will float BECAUSE it is less dense than cool
water).
2. Did your prediction match the observed results of the test?
Answers vary.
3. Describe your results in terms of the density of the solutions. Which solution was
denser? How does temperature affect density?
As temperature increases, density decreases.
4. Record your predictions for the salinity water test.
Answers vary.
5. Did your prediction match the observed results of the test?
Answers vary.
6. Describe your results in terms of the density of the solutions. Which solution was
denser? How does salinity affect density?
As salinity increases, density increases.
7. What is your hypothesis about which factor, salinity or temperature, affects density
more? Write down how you would test this hypothesis.
Answers vary.
8. If you ran your test, did the results of your final experiment support your
hypothesis, or were they different from your hypothesis?
Answers vary.
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Student Handout Lesson 6
Student Activity Lesson 6
Layers in the Ocean
You know that the ocean is a solution made of water mixed with many different types of
salt. Do you think that the ocean is one homogeneous solution where all of the water is the
exact same temperature and exact same salinity?
As you might have guessed, there are parts of the ocean that have cooler water and some
parts with warmer water. Likewise, certain parts of the ocean have higher salinity than
others. What effect do you think this might have on the ocean? In this activity, we will
conduct a test to see how temperature and salinity affect the density of ocean water.
Materials
Scale
Food coloring or dye
Tupperware container
Table Salt
Two 500mL beakers
Ice
Making your seawater (stock solution)
1. Dissolve 35g of table salt (NaCl) into a 1L of room-temperature water in one plastic
container. This roughly approximates the salinity of seawater at 3.5%.
2. Using a 500mL beaker, transfer HALF of this solution (500mL) into your second
plastic container. You will use these solutions for your temperature test.
Temperature test
For this test, we will create hot and cool seawater solutions and mix them with room
temperature seawater. Do you think the hot water will mix evenly with the room
temperature water? If not, do you think it will float on top of or sink beneath the room
temperature water? Write down your hypothesis on the lab worksheet. Also write a similar
hypothesis for your cold water test.
1. Place 500mL of tap water into a beaker. Add 17g of table salt. Place this solution on
ice for a few minutes.
2. Add a few drops of colored dye to your cold seawater.
3. Gently pour the cold solution into one of your plastic containers of stock seawater.
Record your observations.
4. Place 500mL of HOT tap water into another beaker with 17g of salt. Your teacher
will tell you whether to use water from the hot tap or to use a hot plate.
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Student Activity Lesson 6
5. Add a few drops of dye to the hot seawater.
6. Gently pour your hot solution into your second plastic container of room
temperature stock seawater. Record your observations.
Salinity test
For this test, we will make a solution that has high salinity (much more salty) compared to
our stock salinity. Which layer do you think will be on top – the high or low salinity water?
Record your hypothesis on the lab worksheet.
1. Rinse out your plastic containers and beakers from the previous test.
2. Make a new stock solution in one plastic container by mixing 17g of salt into 500mL
of room temperature water.
3. In one of your beakers, mix 500mL of room temperature water with 60g of salt. This
solution is four times as salty as the regular stock seawater.
4. Add a colored dye to the salty solution.
5. Gently pour your salty solution into your plastic container of stock seawater. Record
your observations.
Salinity vs. temperature
What do you think has the greater effect on density: temperature or salinity? How would
you test your hypothesis? Write out the steps for your experiment. If you have time, your
teacher may let you run your test!
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Student Activity Lesson 6
Lab Worksheet
1. Write your predictions for the hot and cold water test.
2. Did your prediction match the observed results of the test?
3. Describe your results in terms of the density of the solutions. Which solution was
denser? How does temperature affect density?
4. Record your predictions for the salinity water test.
5. Did your prediction match the observed results of the test?
6. Describe your results in terms of the density of the solutions. Which solution was
denser? How does salinity affect density?
7. What is your hypothesis about which factor, salinity or temperature, affects density
more? Write down how you would test this hypothesis.
8. If you ran your test, did the results of your final experiment support your
hypothesis, or were they different from your hypothesis?
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Student Handout Lesson 6
Tips for the Bowl - Salinity
Definitions and facts
Write definitions and key concepts for these terms during your teacher’s presentation. You
may see them again!
Salinity:
Typical unit of salinity:
Rule of Constant Proportions:
Facts:
The equator and high latitudes (near the poles) have low salinity and the mid-
latitudes have high salinity
The Red Sea and Mediterranean Sea have the highest salinities on Earth.
Make the connections!
The relationships between variables and concepts are very important for success at the
Bowl. Make sure you understand the following relationships from this section.
1. We know the proportion of elements in seawater (see second page). Therefore if we
measure one component of a sample of seawater, we can calculate all the other
elements using the Rule of Constant Proportions.
2. As temperature increases, the density of water decreases, so colder layers of water
sink below warmer layers.
3. As salinity increases, the density of water increases so saltier water layers sink below
less salty layers.
4. Water temperature is the more important of the two factors in determining ocean
layers.
Why does it matter?
The concepts you learn are almost always related to human interaction with the oceans.
Here are just a few of the human-ocean interactions from this lesson. Can you think of
others?
Different water densities form currents in the oceans, which impacts our climate
and weather. We will learn more about this in other lessons.
Human activities that impact water temperature and salinity have the capacity to
impact water density and currents. Can you think of any human activities that might
make the water warmer?
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Student Handout Lesson 6
Major components of ocean water
Hydrogen and oxygen are the most abundant elements in seawater, followed by the major
components listed below. You should remember that trace elements are those elements
that occur at less than 0.95parts per million (ppm).
2
2
Photo courtesy of the Chesapeake Bay Program. .2006. EPA 903-R-06-003, CBP/TRS 281/06, Chesapeake
Bay: Introduction to an Ecosystem (p 11).
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Student Handout Lesson 6
Salinity
1. The “Rule of Constant Proportions”
w. Allows scientists to determine salinity from the concentration of a
single ion in seawater
x. Means that salinity is constant throughout the oceans
y. Means that salinity and temperature have the same effects on seawater
density
z. Means that density of seawater is constant
2. Seawater is
w. A mixture
x. An ion
y. A compound
z. An element
3. Reminder question: When carbon dioxide combines with ocean water, what is
produced?
w. Calcium carbonate
x. Bicarbonate
y. Carbonic acid
z. Carbonate
4. Which of the following has the highest average salinity?
w. Mediterranean Sea
x. Atlantic Ocean
y. Arctic Ocean
z. Gulf of Mexico
5. Traditionally, salinity has been expressed as...?
w. grams per cubic centimeter
x. parts per million
y. grams
z. parts per thousand
6. Which of the following is the most abundant ion in sea water?
w. Chloride
x. Calcium
y. Potassium
z. Sulfate
7. Reminder question: This term refers to the capture and storage of carbon:
Answer: Carbon sequestration
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Mock Bowl Quiz Lesson 6
8. What percentage of average seawater is pure water?
w. 3.5
x. 50
y. 75.5
z. 96.5
9. Short answer: As salinity increases, what happens to water density?
Answer: It increases
10. Team challenge question
1. In the image above, which region do you expect to have higher salinity, Region A
or Region B? Why? (3pt)
2. In what two general parts of the globe is salinity typically lowest? (2pt)
3. Name one piece of technology used to measure salinity. (1pt)
4. What property of water is determined by salinity and temperature? (1pt)
3
Photo: NOAA, http://www.ngdc.noaa.gov/mgg/global/global.html
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Mock Bowl Quiz Lesson 6
Mock Bowl Quiz Lesson 6
ANSWER
1. In the image above, which region do you expect to have higher salinity, Region A
or Region B? Why? (3pt)
Region A (1pt). Region A is mid-latitude with higher evaporation (2pt).
2. In what two general parts of the globe is salinity typically lowest? (2pt)
At (1) the equator and (2) high latitudes (1pt each).
3. Name one piece of technology used to measure salinity.(1pt)
Satellite, CTD, salinometer are all acceptable responses.
4. What property of water is determined by salinity and temperature? (1pt)
Density
4
Photo: NOAA, http://www.ngdc.noaa.gov/mgg/global/global.html
