the ions are mobile, until they find each other and form a solid. What is the significance of this demonstration? The only way the solid lead(II) iodide precipitate can form in the middle of the petri dish is for 1) the Pb(NO 3) 2 and the KI to dissociate into ions in the water and 2) the Pb 2 + ions and I - ions must migrate in the aqueous solution, i.e. However, if the camera is of low quality students may not see the vivid yellow color of the precipitate. In a large lecture hall a document camera is necessary to project the image of the reaction.
This demonstration works best if the instructor uses this demonstration as the basis of a presentation and takes about 12-15 minutes to have students do the activities that accompany this demonstration. The instructor should be prepared to lead a discussion of writing the 'molecular equation", "full ionic equation" and the net ionic equation. The line of precipitate does not begin to appear until a two or three minutes time have elapsed and it takes a few more minutes for the line to reach across the dish. Presenting the Demonstration This demonstration takes about 8 minutes to perform if you rush through it and do it is as a teacher presenting information to students. Reinforce the concept that soluble ionic salts are strong electrolytes and the ions are mobile ions in solution, i.e. This demonstration, with the pedagogy of having students "do the work" of predicting products, writing equations, and drawing particle level diagrams works best if it is presented AFTER the precipitation of silver chromate demonstration - which is done in a teacher directed manner.ġ. A set of lecture presentation slides to accompany this demonstration is available.
This demo illustrates these rules.Ī student activity sheet to accompany this demonstration is available. 1 1 http ://The solubility rules tell us that salts of group 1 (IA) metals are soluble, salts containing the nitrate anion are soluble, and salts containing the iodide anion are insoluble except for those containing the lead(II) cation (among others). The net ionic reaction is Pb 2 + ( aq ) + 2 I - ( aq ) -> PbI 2 ( s ) Eventually the lead ions begin to react with the iodide ions and precipitate out where they "meet" which is near the center of the Petri dish. The solid potassium iodide ane solid lead nitrate dissolve and their constituent ions begin to diffuse through the water. It could also be used when the mechanism of dissolution is presented to illustrate that dissolved substance are still present, even though they may become "invisible." This demonstration is usually performed when solubility rules, precipitation reactions, and net ionic reactions are being discussed. This demonstration, when presented as a guided-inquiry active student learning approach, combined with particle diagrams can be highly effective because it addresses all three areas of Johnstone's triangle: macroscopic, symbolic, and microscopic (particle level).
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