Introducing Ionic Bonds: How does the string hold together?

Paul Bang, Southwest Star Concept School, Okabena, MN based on an activity in "Bet You Can" by Vicki Cobb and Kathy Darling p. 31.
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Initial Publication Date: September 26, 2007

Summary

In this guided inquiry, students hang a pop-can top opener from a ring stand by a prepared cotton string. The string is soaked in a salt solution (1 Tablespoon salt dissolved in 1/2 of cup of water) and dried. The students burn the string and observe the results. If the students are gentle, the "ashes" will be holding up the pop-top. The lab group should record hypotheses on what is holding it together. Share them with the class; discuss and eliminate some. They should then begin to investigate the "ashes" with tweezers, microscopes, fingers, water, and anything else that they may want to test their hypotheses. They should record their observations and any new questions and hypotheses. Discuss observations and results. If time, conduct more experiments to test new hypotheses. Introduce ionic compound characteristics, ionic bonds, and crystals.

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Learning Goals

This activity is designed for students to investigate an unknown situation, make observations and share data orally.
This activity can introduce chemical bonds and their characteristics.
Vocabulary words: ionic and covalent bonds, crystal structure, and flammability.

Context for Use

This activity can be used in Middle School Physical Science as an inquiry lab and practicing making hypothesis and testing. It should take from 30- 45 minutes. It can also be used in a high school chemistry course in the same manner, or as a discrepant event to introduce ionic bonds.
You need to soak and dry the cotton string in advance.
It is easy to adapt.

Description and Teaching Materials

I usually give minimal instructions and the materials list. I emphasize that they should record observations and make hypotheses about what they observe. I sometimes mention that they should limit the amount of bending of the string.

Each lab group needs: 6 inches of pre-soaked cotton string (I make 2-3 strings/group in case they want to make multiple attempts), ring stand and ring, pop top, and matches (fire source).

The string should be tied to the pop top and then to the ring. Light the string on fire, and don't disturb it with heavy breathing. The cotton string should be soaked for 1 hour in a saturated salt solution (1 TBSP in 1/2 cup water -heated)
and then dried.

Adapted from "Bet You Can" Cobb and Darling, Avon/Camelot Books,New York, 1983.

Teaching Notes and Tips

The students should limit the amount of bending of the string. You can demonstrate the lab first and then let them attempt it. Goggles should be worn. You could also do this at the end of the period so that they go home and research possible explanations.

Assessment

Each group should record observations and hypotheses. These are shared with the class, and turned in to instructor. If a hypotheses is refuted it should have a line through it. Once the concepts of bonds have been discussed, the student should turn in a written explanation of the activity and how the cotton burns but the crystals keep the ashes together.

Standards

Grade 9-12 II. Physical Science A.6 - forming compounds through bonding

References and Resources