This lab goes along with Chapter 13 on ionic aqueous solutions in Modern Chemistry.  This chapter discusses how ionic compounds dissociate in water into their different ions and therefore the solutions should conduct electricity since it contains freely moving charged particles.  Molecular compounds, like sugar, do NOT dissociate into their components so there are no charged particles floating around to conduct electricity.  The lab, II-2: Conductance of Ionic & Molecular solutes,  is in the home scientist chemistry kit manual CK01A.  img_8049The students populate a 24 well reaction plate with 6 different chemicals at 4 different concentrations, 1.0M, 0.5M, 0.25M and 0.125M.   Its pretty easy to mess this up if you’re not paying attention so it helps to have lab partner reading out the instructions and double checking where everything is going.  Once the reaction plate is populated the students used a digital multimeter to measure the resistance of each solution. img_8050

For the solutions with ions the resistance should decrease with higher concentration.  For molecular solutions the resistance should be high for all concentrations.  Measuring all the resistances took quite a while because you had to dip the probes in tap water, dry them off, dip them in distilled water, and dry them off again, between every well!  It also took a while for the resistance to settle down.  Once they have all their data, students plot the conductance (1 over the resistance) versus concentration for the 6 chemicals.  Hopefully they’l see that some compounds had a resistance that depended on concentration, while some (molecular) did not.

Here are the videos I suggested the students watch before class:

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