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Expect the Unexpected. Caw Kllner. Exeter Area High School. Exeter. NH 03833. As teachers of chemistry, we need to create more opportu- nities for stu...
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in~ight~ The DisappearingAct Teaching Students to Expect the Unexpected Caw Kllner Exeter Area High School Exeter. NH 03833

As teachers of chemistry, we need to create more opportunities for students to see unexpected outcomes requiring explanation, especially as their experiential base expands and they develop confidence in their predictive ability. By doing this we can help them to see the discipline as a unity of diverse principles rather than as the collection of discrete units of study which the syllabus and textbook present. A classic case of serendipity in this regard occurred when the author performed a short demonstration of qualitative analysis for his class by filling a 50-mL graduated cylinder with 0.01 M Pb(NO& to show a test for a clear solution of ions which looked identical to an adjacent graduated cylinder filled with water. When one drop of 0.1 M NaI was introduced into each, a splash of PbIz precipitate was produced in the lead nitrate solution as expected. Then, as the bright yellow cloud diffused downward into a zone of lower concentration of iodide ions, it gradually disappeared, since the K,, for Pb12 was no longer exceeded. Imagine the author's surprise when he referred back to the cylinder a minute or two later to find no precipitate! Because this was during the first quarter of school, he was not able to seize upon the

DONNABOGNER Wrchita State University Wichita. KS 87208

opportunity to expand on solubility product constants, but instead he suggested a need for chemist* ro expect the unexpected, to take continuousohservn~ions,andto betechnically prepared in order to interpret and understand anything that might arise during the course of an experiment. If this demonstration were used as a lesson, the teacher could query students as to how they wouldvary the situation in order to construct an experiment, what suppositions they would advance, how they would test them, and what they would control. The demonstration could be done initially just to see how many students (or which students) would first notice the disappearance of the precipitate. When done as a lesson inK.,,, the author would recommend doing i t on a much larger scale, using a 500- or 1000-mL graduated cylinder to heighten the drama. If lead chromate is used, illumination with a spotlight or lighted demonstration box makes a spectacular display because of the shiny crystal faces formed. The reader is encouraged to investigate further by using other salts, such as other lead salts (with& whose approximate values are shown below)

or to find other colored salts, such as CuCl (K,, = 10-7) or that could be used. CuCOa (K*, =