Alaska Sea Grant

Investigation 4: Temperature and Salinity

Class Time Required Activity 4A: 1 ½ -2 class periods
Activity 4B: 3-4 class periods
Activity 4C: 2-3 class periods
Materials Needed
Teacher Preparation Read through all of the lesson and background materials, and watch the video clips. Gather materials, try out the lab activities, revise student handouts if needed, and make copies.
Prior Student Knowledge Students should have experience with phase changes of matter and the movement of molecules during phase changes. They should also have some experience with the concepts of solubility and solutions.
Vocabulary concentration, currents, density, diffusion, gravity, thermohaline,
Science GLEs Addressed

6th Grade: SA1.1, SA1.2, SG2.1

7th Grade: SA1.1, SA1.2, SB1.1, SG2.1 

8th Grade: SA1.1, SA1.2 

Investigation 4Overview: Overview: In this 7-9 day investigation, students are engaged in a variety of hands-on demonstrations and experiments that will help them to understand thermohaline circulation in the ocean. They begin with an introductory activity that helps them to review or arrive at a definition of density, then go on to demonstrate the effects of temperature and salinity on density and design their own experiment to simulate ocean mixing. They experiment further with hot and cold water as they consider the effects of tropical and polar climates on ocean currents, and end by writing conclusions to summarize their learning.


4A: Density (1.5 - 2 class periods)

Focus Question:

  • How do the densities of fluids vary, and how does that affect their behavior?  

Engagement: (15 minutes)

Demonstrate a layered “fruit juice cocktail” in a test tube using cranberry juice, orange juice, and seltzer water, and observe. Pictures and a video are also available. Discuss the demonstration. Allow students to offer explanations and ask questions about why the liquids separated and formed layers, why the seltzer water stayed on top instead of on the bottom, etc.

Explain to the class (or remind them) that density is mass per unit volume. Show 3 or 4 plastic soda bottles filled with different materials: cotton balls, water, sand, iron filings, etc. Discuss the idea that the size and volume of the bottles is the same, the space taken up by the materials is the same, but the mass is very different.

Exploration: (45 minutes)

Demonstrate how to calculate the density of a fluid by finding the mass of 500 ml of fluid in a beaker, subtracting the mass of the beaker, and dividing the mass by 500 to get grams per ml.

Divide students into small groups, distribute materials, and have them measure and calculate the densities of following fluids:

  1. Cranberry juice
  2. Orange juice
  3. Seltzer water
  4. Plain tap water

To save time, have each group test just one fluid. All of the liquids should be at the same temperature.

Share and compare the calculated results and revisit the students’ explanations of why the layers formed as they did in the test tube. Then, have students experiment to find out how a substance dissolved in a fluid will affect its density.

Provide lab instructions to students, or allow them to design the experiment themselves if they have experience in doing so.

Explanation: (15 minutes)

Share and compare the results of the density calculations as a class.
Discuss, and/or ask students to write in their science notebooks:
When salt water and fresh water come together in the ocean, what do you think happens?
Which do you think is more dense, warm water or cold water? Why? How could you test that? (Discuss molecular motion as water warms up, and demonstrate hot and cold (colored) water in a test tube if time permits).
How would you explain what density is to a younger student?
Lead the class in the development of a definition of density.

Elaboration (5-20 minutes)

Have students apply their knowledge by making predictions about one or both of the following demonstrations:
A: Fill a tub with water and show students two cans: one of diet soda and one of regular soda. Ask students to predict what will happen when you put the cans in the water, and to explain their predictions. Put the cans in the water and observe. Have students compare the results to their predictions, and develop new explanations if necessary.

B: Marsili’s demonstration. Follow the instructions in the “lesson plan” to build your own model of Marsili’s device, OR use the online demonstration with a projector.

Ancient mariners knew that the salty Black Sea flowed into the Mediterranean Sea, and that the Atlantic also flowed into it at the other end through the Strait of Gibraltar. There were strong currents flowing into the Mediterranean, but the water level never seemed to rise. This phenomenon mystified sailors. In 1679, Count Luigi Marsili set up a model to show the current flow of the Mediterranean using several different salinities of water. He theorized that the waters had different densities. What was denser would push aside the less dense fluid. Perhaps this process would cause a flow at depth, which had not been discovered. What do you think was going on?”

Show students the actual model or the picture of the model on the video demonstration. (Do not play it yet.) Explain that you have some dense Mediterranean seawater (cranberry juice) and some less dense Atlantic water (soda water). Explain that there are two corks in the barrier, one at about one inch below the surface and one at about 3 inches below the surface. Ask students to predict what will happen when the corks are removed, in their science notebooks. Pull the corks or show the video (you don’t need the sound).

Discuss how the demonstration compared to the students’ predictions and how it relates to what actually happens in the Mediterranean Sea.

This demonstration’s geometry relates equally well to an example closer to home. The cold, dense summer water of the northern Gulf of Alaska builds up onto the shallower continental shelf and eventually flows north at depth though Hinchinbrook Entrance into the deeper Prince William Sound basin. Surface currents don’t match what we think is happening at depth.
Watch the animation that illustrates this concept.
More info and instructions for setting up this demonstration can be found at Count Marsili and the Mediterranean Current, includes additional information and instructions for the demonstration.


Formative assessment should be taking place during class discussions, observation of student work, and examination of science notebooks.

4B: Hot and Salty, Cool and Fresh (3-4 class periods)

Focus Question:

  • How do salinity and temperature affect ocean currents?  

Engagement: (25 minutes)

Begin a ladder of learning about ocean currents, by posing the question: “What do you know about ocean currents?” and ask students to record the date then write silently for 2-3 minutes everything that comes to mind about ocean currents. After 2-3 minutes ask for volunteers or select students to share what they know. Have students draw a line under what they just recorded and reserve the rest of the notebook page for further entries. This is the first rung of the ladder. Tell students that they will revisit the ladder as they progress through the unit and will be able to see what they’ve learned.

Once again, refer back to the maps that the students made in Investigation 1 showing the locations where the rubber bath toys were found. Give students another chance to revise their original hypothesis about the movement of the toys. Remind them that they have been considering things that happen at the surface of the ocean during the last two investigations: wind, weather systems, waves, and tides. Then explain to them that this investigation will focus on another type of ocean motion: deep currents within the oceans.

Exploration: (50-60 minutes)

Pose the question: How do salinity and temperature affect ocean currents? Ask students to look back at their density calculations and think about how water would move in the ocean if there were an area of hot water and an area of cold water. What if the water in one part of the ocean was saltier than another? Ask if they can think of places or circumstances that would produce those conditions. Students can share their responses orally and/or write in their science notebooks.

Students in groups of 2-4 will conduct two experiments that use two clear plastic water bottles connected together. They may be connected with a “tornado tube” or “vortex tube” available at science stores or online. If these tubes are not available, duct tape can be used to secure the bottles together.

Demonstrate the use of the bottles, using ordinary tap water: fill one bottle to the top with water, and screw the tornado tube onto the top. Fill the other bottle with water. Cut the rim off of a plastic yogurt lid so that you have a rigid, flat disk. Hold this tightly over the second bottle and carefully invert it onto the first bottle. Carefully pull out the plastic disk, and tighten the tornado tube onto the top bottle. Or, join the two bottles by holding them carefully and tightly wrapping duct tape around them. Very carefully, over a spill-proof surface, lay the bottles on their sides. Be careful not to move them any more than necessary. Tighten the tornado tube or tape if you can, and hold a blank piece of white paper behind the tubes so that it will be easier to see what’s happening.

Then, have students rotate through two stations where they observe what happens when saline water mixes with fresh water, and what happens when cold and hot water mix.

Explanation: (30 minutes)

Ask students to gather in groups of four (different from lab partners) and come up with a list, in their notebooks, of what they have learned from their investigation. Each group must have data to share with the entire class, backed up with evidence that they collected. Establish a master list of student findings on a white board, overhead, or other.

After students share their data, make sure the following content and vocabulary are addressed:

  1. Salinity and temperature create masses of water with different densities.
  2. Water with high salinity and cold temperature is the most dense.
  3. Gravity has a greater effect on more dense matter. Hence, more dense water will flow below less dense water.
  4. Currents are created by density differences.
  5. When water with different densities comes in contact, some mixing occurs.

Discuss different scenarios that lead to mixing in the ocean, such as:

  1. Water on the surface in a hot climate evaporates and becomes very hot and salty.
  2. Icebergs melt and introduce very cold, fresh water into the ocean.
  3. A river enters the sea.
  4. Salt water freezes to create fresh ice, leaving a higher concentration of salt in the remaining salt water.
  5. There is a hard rainstorm over the ocean.

Discussion should lead to questions such as:
What happens if the water is hot (less dense) AND salty (more dense)? What happens if it is cold AND fresh?

Elaboration (45 minutes)

Allow each group to design and conduct a new experiment using the bottles. They should decide on a question that they would like to answer, write a hypothesis, and give reasons.
For example, their question might be: “What would happen if we put salty room temperature water in one bottle and cold fresh water in the other bottle?”
Their hypothesis might be: “I think the water will mix together right away instead of separating.”
Their reason: “Cold water is more dense than lukewarm water, but fresh water is less dense than salt water. The differences might cancel each other out.

Ask the students to write out their procedures and have you look them over before they begin their test. Check for understanding and correct misconceptions as needed. As the experiments progress, roam around to all groups of students and listen to conversation, pose probing questions, and observe. Redirect if needed.

As with the previous experiments, students should record their observations with notes and drawings, and write a conclusion that compare the results with their hypothesis. They may also discuss any problems that they had.


Formative assessment can include examination of the students’ ladder of learning, lab results, and responses in the science notebooks, as well as attention to class discussion and students’ work in groups.

Activity 4C: Currents from a Cup (2-3 class periods)

Focus Question:

  • How do polar and tropical climates affect ocean currents?  

Engagement: (15 minutes)

Revisit and make entries in the ladder of learning.
Then, pose the question: How do polar and tropical climates affect ocean currents? Give students 3-5 minutes to write their hypotheses, and then have them share with the class.

Make sure students are giving cause and affect.
Use sentence stems like:
If I (independent variable) _________, then ________ will happen (dependent variable).
I think that (independent) _________ because (dependent) __________.

Exploration: (35 minutes)

Students in groups of 2-4 will do another experiment that demonstrates temperature currents, following the directions.  After coming up with a hypothesis, they will set up and observe a simulation of currents by releasing hot and cold water slowly into a tub or aquarium.

If equipment is available, give students the opportunity to use video or photos to record their observations.

Explanation: (20 minutes)

Ask students to share what they have learned and record their findings in a class list. Share and discuss any new questions that have arisen.

The following is a list of possible observations:

  1. Cold water sinks and warm water floats.
  2. Currents get established due to the difference in density between hot and cold water.
  3. When hot water cools, it sinks.
  4. When cold water warms, it rises.
  5. Currents shut down when the temperatures become equal.
  6. Water cools in polar climates and warms in the tropics.

After debriefing ask student groups to answer the following questions. Some resources from your library or the Internet will be helpful in answering these questions:

  1. Where on the surface of the earth does water get heated? Cooled?
  2. Mark these locations on the map from Investigation 1. Arctic, Antarctic, Equatorial. How do temperature differences cause ocean currents?
  3. Ask students if they think habitats could change if water temperatures became more uniform.
  4. What is a thermocline?

Ask students if there are any questions. If so, encourage classmates to answer. If time and interest permits, discuss the upwelling of nutrients and the effects on habitat.

Elaboration (20 minutes)

Have students draw a color technical drawing that includes explanations for why temperature currents establish and diminish in the ocean.


Have students write a conclusion to assess what they have learned. A well organized claims and evidence table will lead to a great conclusion.

Claims/Inferences (what you believe to be true)    Evidence/Data (should support your claims)
Students should address their hypotheses, right or wrong, things they may have had troubles with or what they think would have made their experiment go better. They should also address any questions or new hypotheses that they may have had.

Teacher Preparation

Tips from Teachers

No tips are currently available.

Read through all of the lesson and background materials, and watch the video clips. Gather materials, try out the lab activities, revise student handouts if needed, and make copies.

Curricular Connections

Related topics for study or student research include lake stratification/turnover, chemistry of salts/ions, saturation and supersaturation, and how currents affect us.

Students may research and report (to the public) the possible effects of a change in the major ocean currents.

Collaborate with a scientist to find out about current changes in salinities (polar melting/higher temperature effects on equatorial evaporation rates).

Take a field trip to a nearby body of water to test salinity and/or temperature at surface and at depth.

Use dry ice and heat lamps to devise an experiment that will determine what happens when salt water freezes or when ice melts.

Density calculations provide a connection to math. Students can also research and analyze a temperature salinity density conversion chart.

The investigation provides a connection to language arts through opportunities for discussion, writing, and vocabulary development.

Geography and mapping can also be connected through discussion of the Mediterranean, the tropics, and the polar regions.

Ideas for adapting to different local environment or context:

Materials Needed for Investigation 4:  

Student Handouts
Items for Group Display

Fruit juice cocktail
Actual or virtual model of Marsili’s demonstration
3 or 4 plastic water bottles filled with cotton balls, water, sand, etc.

Material Items

Cranberry juice, orange juice, club soda
Sugar, salt
Per group:
Graduated cylinder
Large beaker
Spoon or stirrer
4B: Per group:
Two clear plastic water bottles
One bottle lid
Tornado tube (or duct tape)
Plastic disk
Dish towel and tray
¼ cup of salt
Food coloring
Sheet of white paper
Hot tap water (100-110 degrees F)
Ice cold (refrigerated) water
4C: Per group:
Blue food color
Red food color
Very hot water
Ice water
Insulated containers for hot and cold water
Room temperature water
2 paper or styrofoam cups
2 push pins
Marbles or rocks
Small clear plastic tub or other container, at least as tall as the cups
Sheet of white paper

Facility/Equipment Requirements 

  Computer with Internet access and projector.

Alaska Science Standards and Grade Level Expectations Addressed:

6th Grade:
The student demonstrates an understanding of the processes of science by
SA1.1 asking questions, predicting, observing, describing, measuring, classifying, making generalizations, inferring, and communicating.*
SA1.2 collaborating to design and conduct simple, repeatable investigations. (L)

The student demonstrates an understanding of the bases of the advancement of scientific knowledge by
SG2.1 recognizing differences in results of repeated experiments.

7th Grade:

The student demonstrates an understanding of the processes of science by
SA1.1 asking questions, predicting, observing, describing, measuring, classifying, making generalizations, inferring, and communicating.*
SA1.2 collaborating to design and conduct simple repeatable investigations, in order to record, analyze, interpret data, and present findings. (L)

The student demonstrates understanding of the structure and properties of matter by
SB1.1 using physical properties (i.e., density, boiling point, freezing point, conductivity) to differentiate among and/or separate materials (i.e., elements, compounds, and mixtures)

The student demonstrates an understanding of the bases of the advancement of scientific knowledge by
SG2.1 explaining differences in results of repeated experiments.

8th Grade:

The student demonstrates an understanding of the processes of science by
SA1.1 asking questions, predicting, observing, describing, measuring, classifying, making generalizations, inferring, and communicating.*
SA1.2 collaborating to design and conduct repeatable investigations, in order to record, analyze (i.e., range, mean, median, mode), interpret data, and present findings. (L)*

Essential Question:

  • What are the patterns of physical changes in aquatic environments?
  • How do they affect us?
  • What are the major weather and ocean circulation systems in Alaska?

Enduring Understandings:

  • Physical changes in the aquatic environment occur on a daily, seasonal, and long-term basis.
  • Weather systems and ocean systems have major influences on one another and the dynamics of matter and energy.
  • Science and technology can be used to detect and solve problems.
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