Alaska Sea Grant

Will They Mix? Teacher Notes

Formative Assessment Probes
The brief, formative assessment resources included with these units are called "assessment probes." They are called "probes" beakersbecause they are designed to probe and uncover student thinking. Teacher and researcher Page Keeley has written extensively about the probes as part of the Curriculum Topic Study approach to analyzing science and mathematics topics. See the Curriculum Topic Study website for more information. 
These probes are designed to be used diagnostically and formatively. They are intended to help you to tap into students' thinking about particular science topics -- topics that are identified by the National Science Education Standards as significant and developmentally appropriate for the target age level of the unit. While they are intended to sample students' thinking (and to probe for common misconceptions), they are NOT intended to measure what students have learned as a result of the unit content. We encourage you to use these tools--and to develop your own--to better understand each student's development as a learner, and to modify your teaching accordingly.

Ongoing assessment throughout the investigations is important for several reasons. It can reveal when students are confused or have misunderstandings, need more time to investigate, or need more explanation. You can tailor the investigations to meet the needs of your students, and change direction whenever necessary. Frequent assessment does not have to be time consuming or tedious. A quick assessment can give you a lot of information about student comprehension and understanding.

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.

Related 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 Benchmarks for Science Literacy
6-8 Structure of Matter
  • Equal volumes of different substances usually have different weights.

Related Probes in Uncovering Student Ideas in Science by Page Keeley
Floating Logs V2, p.27-32.
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