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OVERVIEW: Using Salt Solutions to Understand Chemistry Concepts


by Ted R. van Daalen Wetters, Ph.D. and Rachel Kalish, B.A.
Oceana High School: Pacifica, California


Summary:
Students carry out a series of 4 labs in which they measure the current passing through various concentrations of salt solutions. For each lab the teacher provides a conceptual framework, the experimental question (stated below) and procedure. Because a "discovery" element is an intent of this particular curriculum, some key concepts are withheld from students until after completion of certain labs to allow them room for finding explanations on their own. It is also intended that the labs and teaching content be flexible and adaptable to the context and ambitions of any particular teaching situation. The lab sequence and their underlying questions are summarized as follows:

LAB 1-NaCl
  • Q. How does the amount of electrical current passing through a solution depend on the concentration of salt?

LAB 2-NaCl vs. CsCl
  • Q. How do different salts compare in their ability to carry an electric current?

LAB 3-NaCl vs. KCl
  • Q. How well can two different salts be compared by measuring current, if their molecular sizes are very similar?

LAB 4-NaCl vs. CaCl2
  • Q. What is the effect on electric current of using a salt with atoms that have two positive charges instead of one?

Objectives:
  1. To introduce students to fundamental concepts of chemistry such as -
              ions             periodic table
              charges             moles
              atomic masses             volume
              atomic structure             molarity
              nomenclature             solutions
              molecular structure            


  2. To demonstrate that substances can be measured in various ways depending on their inherent chemical properties, i.e.,
              by mass triple beam balance
              by volume graduated cylinder
              by charge amp meter on power supply


  3. To provide students "authentic" science experience through a series of labs that build upon each other and from which progressively more complex information is learned.

  4. To reinforce lab write-up skills as a habit of the practice of science.

  5. To encourage cooperative skill building in group work through the lab series.

Teaching and Lab Sequence:

This unit was carried out over 2 weeks and one final exam period (for lab 4). Since Oceana H.S. is on a block schedule, teaching and labs were combined in 100-minute periods. Once the students are acquainted with lab procedures, they move quite quickly (20' - 30') through each lab. This allows time to be built into each period for write-ups, concept reviews, collection and review of class data, problem-solving discussions or other activities deemed appropriate to each stage of the unit. Since my intent was for students to be challenged by (apparent) observational contradictions, I intentionally withheld conceptual material pertinent to particular labs until after the labs had been carried out. For example, in LAB 2 the students discover that the same mass concentrations of NaCl and CsCl yield different linear relationships between current and salt concentration. The ideas for relative atomic size and molarity are not taught until after the lab to allow students a chance to see if they derive their own explanations for the discrepancy.

The explanation above should serve as a caveat for the sequence of the unit described. If some ideas seem out of order, then it was done so by design.

Note:
The salt labs in the form described here grew out of an original concept by Rachel Kalish to have students use the electrical properties of salt solutions to understand basic concepts of chemistry.



Day 1     Day 2     Day 3     Day 4     Day 5     Student Info



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