This demonstration provides experimental evidence on the nature of ionic and molecular substances in solution and as a fused (molten) ionic solid. The major purpose is to show that ionic solids conduct the electric current both in solution and when fused, whereas molecular solids do not.Materials
If the conductivity tester used is powered by a 120-V source, then use caution to prevent electric shock and DO NOT ALLOW STUDENTS TO USE THE APPARATUS. The substances used are innocuous and do not require special handling except for the silver nitrate. This material is both toxic and caustic. Handle it with care, particularly when it is fused since it is also an oxidizer. Solutions may be safely disposed by flushing down the drain with water. The solid NaCl and sugar may be used for all demonstrations and then disposed in the trash. Keep the crucible with silver nitrate in a dark bottle for later use or for next year. It may be reused many times.Procedure
Half-fill a 50-mL beaker with solid NaCl and a second 50-mL beaker with solid sucrose. Test the electrical conductivity of the solid NaCl with the tester (light bulb remains dark). Clean the electrodes and then test solid sucrose (light bulb remains dark). Have students tell what's observed in each case.
Half-fill two 50-mL beakers with the two solutions and a third with distilled water. First test the electrical conductivity of the distilled water (light bulb remains dark). Then test the electrical conductivity of each of the two solutions in turn (NaCl causes light bulb to glow, while bulb remains dark in the sucrose solution). Have students report what is observed in each case.
Half-fill a small crucible with silver nitrate. Place the crucible on a triangle supported on an iron ring mounted on a ring stand. Heat the crucible with a lab burner until the silver nitrate is melted. Carefully test the electrical conductivity of the melted silver nitrate (light bulb glows brightly). Have students report what they observe. Note: Sodium chloride is used in solution because all students are familiar with it. However, silver nitrate is used as a melt because it melts at a considerably lower temperature than does sodium chloride.
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