The conductivities (and therefore the degrees of dissociation) of solutions of a non-electrolyte, a weak electrolyte and a strong electrolyte are estimated by comparing how well they complete an open circuit containing a light bulb (and therefore, how brightly the bulb lights up).
Curriculum Notes
This demo can be performed when the nature of chemical bonding is first introduced or when the process of dissolution is being discussed. It could also be used to discuss the equilibrium between the molecular and dissociated forms of acetic acid. Allow about 10 minutes for this demo.
One day of lead time is required for this project.
Discussion
This demonstration illustrates that ionic compounds, such as sodium chloride, ionize completely in water. In contrast, molecular compounds, whose atoms are held together by covalent bonds, ionize only slightly or not at all. In the acetic acid molecule, the electron-withdrawing ability of the carbonyl oxygen weakens the adjacent hydrogen-oxygen bond so that in a polar solvent such as water the molecule will ionize to some extent. Vinegar is a solution of about 5% v/v acetic acid in water. Neither water nor ethanol dissociates to any appreciable extent, therefore neither conducts enough current to light the bulb.
Materials
- light bulb conductivity tester
- switched power strip
- wash bottle containing deionized water
- large beaker for catching rinse water
- 3 600mL beakers
- 3 stir rods
- large bottle of deionized water
- table salt
- vinegar
- ethanol
Procedure
Make sure that the power to the conductivity tester is turned off. Pour deionized water into one of the 600mL beakers. Turn on the conductivity tester. Immerse the leads of the conductivity tester in the water. Point out that the light bulb does not light up. Turn off the conductivity tester. Add some table salt to the water. Turn on the conductivity tester. Immerse the leads of the conductivity tester in the solution. The light bulb should light up brightly. Turn off the conductivity tester. Hold the large beaker under the leads of the conductivity tester and rinse the leads with the water in the wash bottle. Repeat this procedure using vinegar and ethanol. Use the vinegar neat (do not dilute it with water). It is already an aqueous solution of acetic acid. The ethanol can be tested neat and in aqueous solution. The bulb should light up dimly for the vinegar and it should not light up at all for the ethanol or ethanol solution.
Safety Precautions
DANGER! When the conductivity tester is on it is an open 120V AC circuit. Touching the leads when the tester is on could result in severe electrical shock. Turn off the tester immediately after each test.
Wear goggles. Ethanol is flammable. Make sure that a fire extinguisher is close at hand.