Remarks

Approximately 30 min are needed to prepare the demonstration and to "dry-run" it. From 10 to 20 min (depending on teacher input and student interaction) are needed to perform the demonstration.

The demonstration may be used either in the pre-lab discussion of either of the laboratory activities, during the post-lab discussion, or it may be used to illustrate properties of ionic solids and molecular solids. In any case it illustrates an important property of ionic solids-electrical conductivity-and is worth doing. An expanded form of this demonstration may also be used in the Condensed States module.

A suitable home-made electrical conductivity tester can be simple or fancy*. A simple one consists of two porcelain light bulb receptacles mounted base-to-base on a ping pong paddle. These are wired in series with an electrical cord fitted with an on-off switch. Place the base with intact heavy wires of a 150-W light bulb that has the glass envelope removed in one receptacle and a 25-W clear glass light bulb in the other. The two wires in the 150-W base act as the test electrodes. To avoid electrical shock, always turn off the switch or pull the plug while cleaning or rinsing the electrodes or changing the solutions.

Materials may be safely disposed by flushing solutions down the drain with water, and, after using the solids for all classes, they may be placed with solid waste. Due to the expense involved, keep the crucible with silver nitrate in a dark bottle for use later or next year.

The results can be conveniently used to define an electrolyte and a nonelectrolyte (the former exists as ions in water solution while the latter does not). The solids do not conduct since the ions in sodium chloride are not mobile and cannot move toward the electrodes and act as charge carriers. Sucrose is a molecular solid and does not have ions present in either the solid or solution. The melted silver nitrate conducts because the ions are able to move to the electrodes and act as charge carriers. This emphasizes that an ionic solid consists of ions and not molecules. Don't get involved in a discussion of electrical conductivity now, since the students' backgrounds are probable insufficient for understanding. The point is to distinguish between electrolytes and nonelectrolytes and to show that ionic solids exist as ions in the solid state. If students ask about the lack of conductivity of distilled water, you can test the conductivity of tap water. Tap water conducts because there are ions present in it but not in the distilled water.

McClellan, A.L. (Ed.). (1963). Teacher's Guide for Chemistry: An Experimental Science (157-158). San Francisco: W.H. Freeman and Company.

*References for making a low-cost, safe apparatus for conductivity testing may be found in the Acids and Bases module in the Instrumentation section.

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