Anticipated Student Results

The following table presents sample data for this activity. Student groups will likely produce some variations, but data should resemble the sample data.

With comparable data, students should have little difficulty recognizing basic differences in behavior among different classes of solids. For example, NaCl has a characteristically high melting point, is hard but brittle, is water soluble, and its water solution conducts electricity. By contrast, lauric acid has almost diametrically opposite behavior. It's thus fairly simple to relate properties to the type of bonding during the post-lab discussion.

Answers to Implications and Applications

1. Molecules held together by weak van der Waals forces, as in lauric acid, can easily escape from the solid. This is why this substance has an easily-detected odor, and also why it deforms easily. The weak intermolecular bonding force explains the low melting point. Some highly-volatile substances such as hydrogen and oxygen gas are odorless. The smaller the amount of substance that can be detected by the human nose, the smaller the amount that needs to be present to be detected.

2. Molecules of lauric acid are held in the solid state by weak van der Waals forces, hence it has a low melting point (< 100 °C). The other three substances have strong ionic or covalent bonding forces as solids. NaCl is an ionic solid (m.p. about 800 °C), SiO2 a covalent network solid (m.p. > 2000 °C), and Fe a metal (m.p. > 2000 °C).

3. No, not really. With lauric acid, weak intermolecular forces are overcome easily without affecting the intramolecular covalent bonds. On the other hand, when NaCl melts, strong ionic interactions must be overcome to separate the ions.

4. The intramolecular forces are strongest in lauric acid. These consist of covalent bonds holding the carbon, hydrogen and oxygen together in molecules. The intermolecular forces are moderately weak van der Waals forces, consequently the molecules are rather loosely held within the crystal and the crystal lattice is easily disrupted.

5. NaCl dissolves in water but not in cyclohexane, while lauric acid dissolves in cyclohexane but not in water. Neither silica nor iron dissolve. Water consists of polar molecules; their polar nature allows them to overcome the attractive forces in NaCl (and many other polar solutes) sufficiently for the substance to dissolve. Since lauric acid is nonpolar, it does not interact with water strongly enough to dissolve. Cyclohexane is a nonpolar solvent and can dissolve lauric acid. The very strong attractive forces within the other two solids prevent their solution.

6. Student answers will vary depending upon the identity of the unknown. In any case, their answers should be based on the data collected for the unknown and should justify the classification as one of the four classes of substances.

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