A conductivity testing light bulb apparatus is immersed in solid ammonium acetate. It does not light up. But when the ammonium acetate is melted, the ions are free to move and the apparatus lights up. Illustrates the structure of ionic compounds. 

Curriculum Notes 

This demo is usually presented when the structure of ionic compounds or the nature of the ionic bond is being introduced. Allow about 20 minutes for this demo, but some lecture can be done during this time while you are waiting for the ammonium acetate to melt.

One day of lead time is required for this project.

Discussion 

Mobile charges must exist in a material for it to conduct electricity. In metals current is conducted by loosely held valence electrons. The ions in an ionic compound are capable of conducting electricity. However, in the solid state these ions are tightly bound in the crystal lattice and are not mobile. When an ionic solid is melted the ions are free to move and the substance will conduct electricity. Ammonium acetate is used in this demonstration because it has a relatively low melting point (114 degrees Celsius) for an ionic solid.

Materials 

• hot plate
• crystallization dish containing ammonium acetate
• conductivity tester
• power strip

Procedure 

* Turn on the hot plate and allow it to heat up about 10 minutes before you intend to start the demo. 

* Turn on the power supply to the conductivity tester. 

* Hold the crystallization dish containing the solid ammonium acetate under the conductivity tester so that the probes of the tester are in contact with the ammonium acetate. The lightbulb on the tester will not light up. 

* Turn off the power supply to the conductivity tester. 

* Place the crystallization dish containing the ammonium acetate on the hot plate and allow it to heat until all of the ammonium acetate has melted. 

* Adjust the ring stand and the conductivity tester so that the probes of the tester are immersed in the ammonium acetate melt. 

* Turn on the power supply to the conductivity tester. The lightbulb on the tester will light up. 

*Turn off the power supply to the conductivity tester.

Safety Precautions 

* When the power supply to the conductivity tester is on, there is a potential difference of 120 V AC across the probes. Be careful not to touch the probes when the power is on. It will shock you! 

* Be careful not to burn yourself on the hot plate. Remember that you are heating it before anything is placed on it. 

*The ammonium acetate may splatter while being heated. In addition to the burn hazard, ammonium acetate is a skin irritant. Watch it off immediately with cool water followed by soap and water and treat the burn. 

*Ammonium acetate fumes can irritate the respiratory tract. Perform demo in a well-ventilated area. Avoid breathing the fumes given off during this demonstration. Work with lecture demonstration coordinator to remove the hot ammonium acetate as soon as the demo is finished.