Density

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​

5-10 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.

Explain:
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.

Divide the students into pairs or groups. They will role-play the captains of a cargo ship about to leave from Seattle, Washington, headed to the port of Anchorage, Alaska.

Background:
More than 98% of cargo shipped to and from the United States is transported by water. Students should realize that despite the prevalence of air travel and advances in aerospace technology, the earth’s oceans are still vital to freight transportation, energy production, and recreation (NOAA 2008). The port of Anchorage is the most active port in Alaska through which more than 95% of all cargo entering and leaving Alaska passes, acting as a distribution center of goods to the rest of Alaska. It serves 80% of Alaska’s population and 90% of the consumer goods entering Alaska.

The scenario:

• The goal is get to the port of Anchorage as quickly as possible; each captain is paid by how quickly they get their cargo to the port of Anchorage.
• There are currently two weather systems impacting the route, a high pressure system over the Gulf of Alaska, and a low pressure system over Bristol Bay.

Assignment: Develop the quickest route to Anchorage.

Distribute the map to have students plot their ship’s course.

Things to keep in mind:

• Students should assume that a straight line is the quickest path (disregard rhumb lines and the Great Circle methods) to get between two points with still water.
• Students should use their knowledge of the average circulation in the Gulf of Alaska (students should use their maps in their science notebooks) in this exercise.
• Students might consider how the “normal” current might be affected with the storm.
• Seattle, Washington, and Anchorage, Alaska, are 2,327 kilometers apart.

The following feedback can be offered to the students and proposed as questions to answer in their science notebooks:
Most students will direct their ship between the low and the high pressure systems where there is a direct current north into Anchorage. Probe the students to see if they understand that this route is directly opposed to the “normal” current that the Gulf of Alaska gyre produces.

Possible discussion/analysis questions could revolve around:

How much can a single weather system affect the Gulf of Alaska gyre?
How long would the weather system have to be there to direct the water against the normal current?
Which side of Kodiak Island might be quicker?
How do the magnitudes of the winds associated with the weather systems compare to the speeds of the current in the gyre?
Based on the students’ reasons for their proposed ship route, do they think it would take longer to complete the northbound trip or the southbound trip?
There isn’t necessarily a right or wrong answer to this scenario. Evaluation of this exercise hinges on the extent to which the students can justify their route.

Evaluation

35-50 Minutes

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

Extensions

Local Expert.
Bring science into your classroom by inviting a local meteorologist to talk to the students about their job of forecasting, weather patterns in Alaska, and how they use technology to do their job. In Alaska there are National Weather Service offices with meteorologists in Anchorage, Fairbanks, Juneau, Annette, Barrow, Bethel, Cold Bay, King Salmon, Kodiak, Kotzebue, McGrath, Nome, St. Paul, Valdez, and Yakutat. If a visit is not possible, it might be possible to bring the scientist into your classroom with Skype software. Skype allows you to “call” another person computer-to-computer and have a conversation replete with audio and video free over the Internet. It is available as a free download on the Internet and is used to network with anyone else who also has Skype free of charge.

Quikscat Images.
A possible sidebar activity with a foray into technology would be to explore QUIKSCAT data, showing current satellite-derived ocean surface winds. QUIKSCAT images are derived from an instrument called a scatterometer, mounted on polar orbiting satellites that calculate wind speed and directions based on ocean surface roughness algorithms. You can use an archived image if you don’t have Internet access.
Do the winds from the QuikScat image match well with the ocean currents the students sketched in their science notebooks? If so, why? If not, why not?

Coriolis Effect.
Examine the Coriolis Effect, which is responsible for the “spin” of a weather system. You might also want to use a lesson plan for a hands-on activity.