Procedure

1. Place 1-g samples of sodium chloride, NaCl; lauric acid, CH3(CH2)10CO2H; silicon dioxide, SiO2 (sand); and iron filings in separate evaporating dishes. Test hardness by rubbing each solid with the bottom of a plastic pen or stirring rod.

2. Cautiously smell each substance by fanning any vapors with your hand to your nose. An odor implies some volatility.

3. Place pea-sized samples of NaCl, lauric acid, silicon dioxide, and iron filings in separate small test tubes. Test the melting point of each in a boiling water bath. Then place any of the substances that did not melt in a clean, small evaporating dish. In turn, place each evaporating dish on a wire gauze or wire triangle and heat directly with a burner flame. As soon as the solid melts, remove the flame. Do not heat any substance longer than five min. Remember that the heated substances require time to cool before disposing. Handle hot evaporating dishes with tongs and place on a ceramic pad to cool.

4. Arrange a test tube rack with four pairs of test tubes. Add 0.5 g of NaCl to each test tube in the first pair. Do likewise with the other solids in the remaining three pairs of test tubes. Add 5 mL of distilled water to the first of each pair of test tubes. Add 5 mL of cyclohexane to the second of each pair of test tubes. Stopper and shake the test tubes noting any evidence of dissolving of the solid.

   Note: Steps 5 and 6, at the option of your teacher, can be done as a demonstration.

5. Test the conductivity of solid NaCl, lauric acid, SiO2, and iron by placing a sample in an evaporating dish and touching the solid to the wires of a low-voltage conductivity tester.

6. Test the conductivity of distilled water and cyclohexane, distilled water solutions, cyclohexane solutions, and any solvent/solid mixtures of the four substances. Place the solutions and/or mixtures in a 50-mL beaker and immerse the electrodes. Rinse the electrodes with the solvent before and after each test.

7. Obtain an unknown solid and attempt to identify its bond type by repeating the tests suggested above.

8. Dispose of materials as earlier directed in the "general safety considerations" and/or as directed by your teacher. Thoroughly wash your hands and clean all equipment prior to leaving the laboratory.

Data Analysis and Concept Development

The properties of the four classes of substances studied showed marked differences. These differences are explained in terms of the bonding that holds the substances together in the solid state. Study the data table and list typical properties expected for the four kinds of substances, e.g., order of melting, hardness, solubility. Remember that water is a polar solvent while cyclohexane is a nonpolar solvent. Identify the unknown as being either ionic, covalent molecular, covalent network solid, or metal. Then answer the Implications and Applications questions that follow.

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