Conductivity of Electrolytes
This simple demonstration illustrates the conductivity properties of a strong electrolyte, a weak electrolyte, and a non-electrolyte. A light bulb conductivity tester does not light up at all when the electrodes are inserted into either solid granular sodium chloride or solid granular sucrose. Deionized water also fails to light the light bulb. However, when these substances are dissolved in water, the sodium chloride solution lights the bulb brightly while the sucrose solution still fails to light the bulb. 1M acetic acid makes the bulb glow dimly.
This demonstration can be used to illustrate the nature of chemical bonds or the solution process. Allow about 10 minutes for this demo.
Substances only conduct electricity when they contain mobile charged particles. The loosely held outer shell electrons of metals are sufficiently mobile to conduct electricity, so the screwdriver tests positive for conductivity. Sodium chloride contains both sodium and chloride ions, but in the solid state they are locked in place in the crystal lattice and are therefore unavailable to conduct electricity. But when sodium chloride is dissolved in water, the crystal lattice is disrupted and the solvated ions are free to move and conduct electricity. Substances like sodium chloride which strongly conduct electricity in aqueous solution are called strong electrolytes.
All of the bonds in the sucrose molecule are strong covalent bonds.Therefore there are no charged particles present to conduct electricity either in the solid state or in solution. Substances like sucrose which do not conduct electricity in aqueous solution are called non-electrolytes.
One of the hydrogen atoms of acetic acid is loosely bonded to the rest of the molecule. Therefore the polar water molecules occasionally detach a positively charged hydrogen ion from the rest of the molecule creating a pair of ions. Since only a small percentage of the acetic acid molecules exist in the dissociated state at any given time, acetic acid solutions only conduct electricity weakly. Substances like acetic acid which weakly conduct electricity in aqueous solution are called weak electrolytes.
If you would like to show your students that pure ionic substances in the liquid state (melts) conduct electricity, check out the "Conductivity Of Ammonium Acetate" demo.
- 1 150 mL beaker containing about 20g of NaCl
- 1 150 mL beaker containing about 20g of sucrose
- 500 mL of deionized water
- 200 mL of 1M acetic acid
- 3 400 mL beakers
- 2 stirring rods
- conductivity tester mounted on ring stand
- power strip
- wash bottle containing deionized water for rinsing electrodes
- large crystallization dish to catch rinse water
- screwdriver with insulated handle
- paper towels
- Plug in power strip to electric outlet and plug in conductivity tester to power strip. Make sure that the power strip is turned off!
- Turn on the power strip. Short the electrodes with the blade of the screwdriver to show the students what a positive conductivity result looks like. The bulb lights up. Turn off the power strip.
- Turn on the power strip and insert the electrodes into the NaCl crystals. The bulb does not light. Turn off the power strip. Rinse and dry the electrodes.
- Turn on the power strip and insert the electrodes into the sucrose crystals. The bulb does not light. Turn off the power strip. Rinse the electrodes.
- Pour about 200 mL of deionized water into one of the 400 mL beakers. Turn on the power strip and insert the electrodes into the water. The bulb does not light. Turn off the power strip.
- Add the NaCl to the water in the beaker and stir. Turn on the power strip and insert the electrodes into the NaCl solution. The bulb lights up. Turn off the power strip. Rinse the electrodes.
- Pour about 200 mL of deionized water into one the other 400 mL beakers and add the sucrose. Stir. Turn on the power strip and insert the electrodes into the sucrose solution. The bulb does not light. Turn off the power strip. Rinse the electrodes.
- Pour about 200 mL of 1M acetic acid into the remaining 400 mL beaker. Turn on the power strip and insert the electrodes into the acetic acid solution. The bulb glows dimly. Turn off the power strip.
None of the chemicals used in this demo are hazardous The main hazard is the conductivity apparatus. The electrodes are connected straight to 120 volts of AC current. Severe shock can result if you touch them when the circuit is "live." Be sure to turn the power strip off when there is any chance that you might touch the electrodes.