$\require{mhchem}$

The conductivities (and therefore the degrees of dissociation) of solutions of weak and strong acids 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 is often used when the concept of weak acids and bases is introduced, but it can be used earlier when discussing equilibrium in general. Allow about 15 minutes for this demo.

One day of lead time is required for this project.

Discussion 

Both sodium hydroxide and hydrochloric acid dissociate almost completely into ions when in water solution. Therefore solutions of these substances conduct electricity very well. But acetic acid (K_a = 1.76 x 10^-5)¹ and ammonia (K_b = 1.78 x 10^-5)² only dissociate slightly in water. Therefore their solutions only conduct electricity weakly. 

Ammonia dissociates according to the equation:

$\ce{NH3(aq) + H2O(l) <-> NH4^{+}(aq) + OH^{-}(aq) }$

Materials 

• light bulb conductivity tester
• switched power strip
• wash bottle containing deionized water
• large beaker for catching rinse water
• 1 600 mL beaker containing deionized water
• 4 600 mL beakers containing 1M solutions of hydrochloric acid, sodium hydroxide, ammonia, and acetic acid

Procedure 

Make sure that the power to the conductivity tester is turned off. Turn on the conductivity tester. Immerse the leads of the conductivity tester in the beaker containing deionized water. Point out that the light bulb does not light up. Turn off the conductivity tester. Rinse the leads with deionized water. Repeat this procedure using the remaining solutions. The bulb should light up brightly for the hydrochloric acid and sodium hydroxide solutions and dimly for the ammonia and acetic acid solutions.

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. 1M hydrochloric acid, ammonia solution, sodium hydroxide solution, and acetic acid are all caustic. If they get on your skin wash with soap and water. If they get in your eyes flush with water for 15 minutes and get immediate medical attention.

Footnotes 

¹"Dissociation Constants of Inorganic Acids in Aqueous Solutions." Handbook of Chemistry and Physics. Weast, Robert C., ed. 67^th ed. Boca Raton, FL: CRC Press, Inc., 1986. p.D-163.²"Dissociation Constants of Inorganic Bases in Aqueous Solutions at 298K." ibid.. p.D-161.