Have students complete this performance assessment after they have learned about environmental problems and solutions. Students should be allowed to use the textbook and any other reference materials that relate to the study of this topic.

PROCESSES AND CONCEPTS

In conducting this investigation, students may use the following processes:

• Observing
• Comparing and Contrasting
• Inferring
• Predicting
• Recognizing Cause and Effect
• Communicating
• Making Tables and Diagrams
• Interpreting Data
• Making Models

In conducting this investigation, students may demonstrate knowledge of the following concepts:

• Problems associated with pesticide use
• Biological magnification
• Food chains and food webs

TIME NEEDED

The time will vary; one class period should be allowed for the planning and investigation.

PREPARATION

Assemble the materials. Caution students in their handling of the iodine crystals, which can irritate the skin; iodine fumes can be both irritating and poisonous. Instruct them to use the plastic spoons to remove the iodine crystals; only a few crystals need be placed in the bottom of the beaker. A pan balance and test tubes may be needed for the Going Further exercise.

MONITORING THE INVESTIGATION

Tell students that you want to review and approve their plan before they proceed. Be sure to include a way to record their observations over time.

After the plan is approved, monitor the work of the students and respond to their questions and procedural requests. You should not direct their work towards any fixed approach. In most performance tasks, there will be different and interesting approaches to solutions.

EXPECTED OUTCOMES

The investigation involves placing iodine crystals in the bottom of the beaker, adding distilled water, stirring, and when the water is still, adding a layer of mineral oil. The oil layer will float on top of the water, but this must happen before the iodine can reach the oil, in which it is highly soluble. Eventually the water becomes saturated with iodine and takes on a slightly yellowish color; most of the iodine crystals remain undissolved on the bottom of the beaker. Over time, the iodine from the water is absorbed by the oil; more crystals will dissolve in the water to replace the iodine absorbed by the oil, until almost all the iodine crystals are gone. The students observe that almost all of the iodine is dissolved in the oil, although hardly any was dissolved in the water at any given time. Students will conclude that the pesticide (iodine) becomes concentrated in fatty tissue (mineral oil).

For the Going Further exercise, the students' ideas might include filling separate test tubes with measured amounts of water and oil. Then they might add small, measured amounts of iodine crystals to each and stir after each addition until the color of the solution in each test tube matches the color of the solution in the beaker. The concentration can then be expressed in parts iodine per parts water or oil.

EVALUATING STUDENT OUTCOMES

The Performance Assessment Evaluation Form gives you an outline for evaluating student outcomes. In a performance task, the process is more important than the outcome. Use Part A, Planning the Investigation, to evaluate the plan. After the investigation is completed, use the other three sections of the form to evaluate the student work.