The Nature of Salt

1½ - 2 Class Periods

Overview

In this module students will identify the physical and chemical properties of salt.

Background

Chemically, salt consists of two elements, sodium (Na) and chlorine (Cl). Elemental chlorine is the chlorine atom (Cl), which pairs to form chlorine gas (Cl2). Neither element separately occurs free in nature, but each is found in abundance as the useful compound, sodium chloride (NaCl), or salt. It occurs naturally in many parts of the world as the mineral halite and as mixed evaporites in salt lakes. Seawater contains an average of 2.6% (by weight) NaCl, or 26 million metric tons per cubic kilometer (120 million short tons per cubic mile), an inexhaustible supply.

Sodium chloride in solution consists of a positively charged sodium ion and a negatively charged chloride ion. In solid form the atoms arrange themselves in a cubic crystal lattice. Table salt, when viewed under a magnifying glass, can be seen to consist of tiny cube flakes. Salt varies in color from colorless, when pure, to white, gray or brownish, typical of rock salt (halite). Chemically, it is 60.66% chlorine (Cl) and 39.34% sodium (Na). The atomic weight of chlorine is 35.4527 and that of sodium is 22.989768.

Getting Ready

Time: 1 1/2 to 2 class periods

Materials

Per Student Group: 1/2 cup table salt, magnifying glass, Student Page: Properties of Sodium Chloride, large halite crystal (sodium chloride), hammer, periodic table, graph paper, Styrofoam balls (14 large and 13 small balls), and straws.

Instructions

Review the instructions below for each exercise. Photocopy and distribute the Student Page. Distribute materials to each student group. Halite crystals can be ordered from a scientific supply company.

Activities

Activity A - Physical Properties of Salt

1. Provide each student group with a small amount of table salt, a magnifying glass and the Student Page: Properties of Sodium Chloride. Instruct them to record their observations of the compound.
2. After students have recorded their observations, provide them with a large cubic halite crystal. If you do not have enough crystals for each student group, then set up a Crystal Station in a corner of the room which students can visit. Students should see if their observations of table salt differ from the large halite crystal.
3. With a hammer strike the halite crystal, breaking it in small pieces. Once again, have students see if their observations of the compounds differ.
4. Ask students what they believe gives salt its definite cubic, or isometric crystalline form. Explain that the cubic structure of salt is due to the way the atoms are arranged via ionic bonds.

Activity B - Salt: The Molecule

1. Explain to the students that salt is a compound that is made up of two elements - sodium and chlorine. Review the definition of a compound.
2. Have students look up the elements sodium and chorine in the periodic table and have them record the atomic weight, atomic number, and the valence on the student page. Also note the position of Na and Cl in the periodic table.
3. Students should then find in the periodic table or another source, the electron configuration for each element (Na[K] 1s²[L]2s²2p⁶[M]3s¹ and Cl[K] 1s²[L]2s²2p⁶[M]3s²3p⁵)   . Note - K, L, and M refer to orbiting shell structures.
4. Review the Octet Rule with students and ask them how Na and Cl can be bonded to form a compound which will fill both their outer shells.
5. Based on this information, have students create a Bohr Model of NaCl at the bottom of the Student Page: Properties of Sodium Chloride.

Activity C - Ionic Bonds and Salt

1. Explain that salt is formed by an electrostatic bond or ionic bond. A sodium atom which has one electron in its outer shell donates it to the chloride atom which is lacking one electron in its outer orbit.
2. Ask students which element received an electron and which element donated an electron in creating this ionic bond. Have them decide which element is an anion and which is a cation. Have students write Na and Cl with the appropriate + and - signs indicating their ionic status.
3. Tell students that the strong electrostatic force of attraction in ionic compounds reaches out in all directions. Each ion in a solid ionic crystalline substance is surrounded by other ions of opposite charge. To illustrate this concept have students fill in the ions (circles) with the correct + or - sign on the Student Page. Color code Na and Cl so they can be easily distinguished. Also explain that the Cl ion is actually much larger than the Na ion.
4. As we already discovered, salt has a cubic structure. Discuss with students that this is related to its crystalline structure. Have students create a 3 dimensional diagram of a salt crystal using two different symbols for sodium and chloride. Different size circles should work fine connected by straight lines. The diagram should be 2 cells high by 2 cells wide by 2 cells in length. They should keep in mind that + and - combine, or in other words an Na ion must combine with a Cl ion.
5. If time permits, students can actually create a truly 3-D molecular model of salt using straws and different size Styrofoam balls, or similar materials. Larger balls should be used to represent chloride ions than those used for sodium ions.

Assessment

Student Page: Properties of Sodium Chloride can serve as an assessment tool.

Conclusion

Complete the chart on the Student Page: Properties of Sodium Chloride and discuss the markedly different properties of NaCl as compared to its individual elements.

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