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This simple and portable device lets you test the conductivity of liquids anywhere, any time.
Test if a solution conducts electrical currents:
Fill a cup with salty water or other ionic solution, and then rest the device on the rim of the cup (make sure the aluminum leads are immersed in the solution). If the light bulb glows brightly, then the solution is very conductive. If it barely glows—or not at all—then your solution is not very conductive.
Test a series of solutions (try solutions with varying concentrations of electrolytes):
Fill each cup to the same height with solutions containing varying amounts of salt. Rest the device on the rim of each cup, one after the other (make sure the leads are immersed in the solutions at the same depth—this will keep the contact area the same for each solution tested). Compare how the light bulb glows in each of the solutions.
Test to see if a chemical reaction is occurring:
Fill a cup with a salt solution and then place your device on the rim of the cup. Watch the aluminum leads and see what develops. Do you notice a color change or gas at one of the aluminum leads? Do you see your solution getting cloudy or detect an odor emanating from the cup? These and other subtler features may indicate that a chemical change or reaction is happening.
If your solution lights the bulb, it contains electrolytes. Electrolytes are charged particles, usually ions in solution. These ions can have positive or negative charges. Ions are attracted to their corresponding oppositely charged lead: Positively charged particles move and are attracted to the negative aluminum lead and negatively charged particles move and are attracted to the positive aluminum lead. This flow of charged particles completes the circuit, causing the light bulb to glow.
Often, these moving ions can cause interesting chemical phenomena to result. Ions may come out of solution, interact with the aluminum metal, cause gas bubbles to form, decompose the foil, and change the pH of the solution. You may observe several chemical changes and reactions in your cup of solution.
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Attribution: Exploratorium Teacher Institute