Properties of An Ionic Salt
A large cubic crystal of sodium chloride (halite) is displayed to the class. It fails to light the conductivity tester. When it is struck with a hammer, it cleaves evenly. When pulverized and dissolved in water, it lights the conductivity tester.
This demo is used to relate the nature of the ionic bond to the properties of an ionic solid.
Though ionic solids such as sodium chloride contain ions, they are unable to conduct electricity in the solid state because the ions are locked into position within the ionic crystal lattice. The strength of the ionic bonds within that lattice contributes to the hardness and rigidity of the crystal, but when the matrix is displaced by a sudden blow, all of the interactions along a plane are disrupted, resulting in even cleavage of the crystal. The ions in sodium chloride can be mobilized by dissolving it in water. Once the ions are free to move, the solution can conduct electricity.
- cubic crystal of halite (sodium chloride) about 1.5 cm on each side
- some deionized water in a beaker
- conductivity tester
- power strip
- heavy duty baggie (freezer bag)
- stir rod
- Display the halite crystal to the class. If it is a large class, use the document projector. The sample may be passed around if time allows.
- Turn on the power strip. Carefully hold the crystal against the leads of the conductivity tester. Do not touch the leads! Severe shock could result! The conductivity tester does not light up. Turn off the power strip.
- Place the crystal in the plastic bag. Set it on a hard surface and rap it sharply with the hammer. The crystal should cleave evenly in a plane. Display the pieces of the crystal to the class.
- Return the pieces of the crystal to the plastic bag and pulverize with the hammer.
- Turn on the power strip. Immerse the leads in the DI water to remind the students that DI water does not conduct electricity by itself. Turn off the power strip.
- Pour the pulverized sodium chloride into the DI water and stir.
- Turn on the power strip. Immerse the leads in the sodium chloride solution. The light bulb should light up. Turn off the power strip.
- When the conductivity tester is on it has 120 volts AC across the leads. Touching the leads could result in severe shock!
- When pulverizing the crystal the plastic bag could rupture and fragments of the crystal could be projected. Wear goggles.
Obtain a sample of halite from the geology department.