Grade 7 Teacher Background Information

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Formative Assessment Probe

Background information on formative assessment probe and 7th grade probe.

Purpose

The purpose of this assessment probe is to elicit students’ ideas about density and how salinity affects the density of water.

Explanation

The correct answer is 2. The top will be primarily blue (distilled water). The weight of salt water is the sum of the weight of the water and the weight of the salt in the water. For equal volumes of fresh and salt water, the density (weight/unit volume) of the salt water is greater, so the fresh water will be layered on top of the salty layer.

Administering the Probe

You may wish to use props such as different colored liquids or pictures of containers with different-colored liquids with labels. If you wish to turn this into a demonstration or hands-on experiment, it is likely that some mixing of the layers will occur, but this would be minimized if the water was poured slowly and carefully. This works best with large differences in salinity.

Grade Level Curricular and Instructional Considerations

At ages 9 and 10 students begin to relate the density of one material to that of another. For example, children say that a material floats because it is ‘lighter than water’. (Making Sense of Secondary Science, p. 78)

At this age, the students’ understanding of weight may be rudimentary. The probe uses directly observable attributes which is the best approach for this age. Molecular models of density involving atoms and molecules should not be introduced until high school.

National Science Education Standards

5-8 Properties and Changes of Properties in Matter

A substance has characteristic properties, such as density, a boiling point, and solubility, all of which are independent of the amount of the sample. A mixture of substances often can be separated into the original substances using one or more of the characteristic properties.

Related American Association for the Advancement of Science Benchmarks

6-8 Structure of Matter

Equal volumes of different substances usually have different weights.

Where Did the Rubber Bath Toys Go?

A great book to use for this entire unit is Tracking Trash: Flotsam, Jetsam, and the Science of Ocean Motion by Loree Burns. Chapters 1 and 3 are a perfect fit with Investigation One. Chapter 3 is about the rubber bath toy spill and Chapter 1 is about the Nike shoe spill.

Story about the Rubber Duckies online version PDF version

Longer version at higher reading level.

Weather and Circulation Systems

Chapter 2 of Tracking Trash works well with this investigation. Its title is “The Science of Ocean Motion.”

If you are unfamiliar with using the Socratic Seminar method for group discussion, take a look at these sites:
How to prepare for a Socratic seminar
Socratic Seminar Strategy Guide
Socratic seminar guidelines for students

Ocean Current Information
A good basic approach to determining currents is to estimate the contribution of each of the forcing mechanisms. For nearshore currents tides generally dominate, whereas on the open shelf winds dominate, tides are involved, and density plays a small role. For long-term drift applications, winds and density are more significant than tides.

Weather and wind influence currents only where weather is occurring (this may seem intuitive), whether it is a large weather system encompassing a thousand miles of ocean or just a local sea breeze in a small inlet. The influence of wind on currents gets more complex when wind-generated waves and swells travel far from their source area and affect other regions of the ocean. In this way it can be seen how the effect of wind and weather on ocean currents ranges from small-scale geographic regions to larger ones, just as it ranges from short time scales (hours) to large time scales (seasons) in duration.

While tide effects are not significant in influencing currents in large open swaths of water like ocean basins, tides are very influential on currents close to land, especially in areas where the topography exaggerates tidal effects. Turnagain Arm south of Anchorage, Kachemak Bay near Homer, Whittier’s Passage Canal, Seward’s Resurrection Bay, and Cook Inlet all experience significant tidal impacts. Tidal cycles are roughly 12 hours long with about 6 hours between a high and low tide.

The direction of tides doesn’t always dictate the direction of the current. A good investigation for this section is to have students pit tidal influence against an opposing wind; the students’ arguments on which one has more influence will help clarify their understanding of the conceptual underpinnings of currents.

The influence of water density on currents, driven by both thermal and salinity gradients, is less in shallow water near land masses, and more significant at increased depths. In fact, density gradients are the main driver in the exchange of surface and deep water.

Currents Tutorial from NOAA
Surface Ocean Currents
Ocean Odyssey – Density Current video from NASASciFiles (covers density, salinity, currents)
Ocean Odyssey – Surface Currents video from NASASciFiles
Global current image
Reading global isobaric patterns – Video or Reading
Global ocean current circulation animation:
Global wind speed Jan and July plus an animation
NOAA National Data Buoy Center
Ocean Currents and The Distribution of Life
Ocean in Motion: Ekman Transport background
Wind Driven Surface Currents, Gyres Background:
Coriolis effect. Examine the Coriolis effect, which is responsible for the “spin” of a weather system. A lesson plan for a hands-on balloon activity.

Waves and Tides

If you and your students don’t have much experience with tides, it is not hard to find stories about tides in Alaska on the Internet.
Examples:
Story about a cruise ship stuck at low tide
Description of the hazards on the Turnagain mudflats
Story of a fisherman in Anchorage getting trapped at low tide
NOAA Ocean Service Education

Tides and Water Levels lesson plans

If students need more learning activities centered around tides to better understand the concepts, NOAA has lesson plans:

Tides: Ups and Downs
Tides and Moon with time-lapse and marigrams
Kachemak Bay tidal cycle: the movie (time-lapse of Kachemak Bay extreme tidal cycle)
Ocean Odyssey – Tides and Waves video from NASASciFiles
Ocean Tides at the Bay of Fundy video From: Britannica Online
Waves

Temperature and Salinity Effects on Deep Ocean Currents

Density background: Density is defined as mass per unit volume, or grams per cubic centimeter in the metric system. In fluid systems, one fluid floats on top of another if it has a density that is less than the other. The downward gravitational force of the upper layer is less than the upward buoyant force of the underlying fluid. Density differences can be caused by temperature, compositional, or pressure differences. In this experiment, the differences are based primarily on composition of the fluid. Even though all layers are fluids, they do not mix rapidly if handled gently, and will stay separate for a class period or more. Masses of subtly distinctive (having different temperatures or salinities) ocean water can persist for months and over distances of hundreds of kilometers. Given these properties, scientists can identify and track water masses, and learn about the speed and path of various water masses around the world.

Online sources for Tornado Tubes
Tornadotube.net
How to build a tornado tube