Rock Cycle - Chemistry (4)
 Lab 

   
OBJECTIVES:
  • Exploring two elements that form a compound.
  • Investigating the cubic shape of halite.
VOCABULARY:
  • atom
  • compound
  • element
  • halite
  • molecule
MATERIALS:

Students explore the cubic nature of salt.

BACKGROUND:

A compound consists of two or more different types of atoms that are chemically bonded. Halite, composed of sodium and chlorine, is an example. Electrons move around the nucleus of an element in specific and set orbitals. There are a finite number of electrons in each of these orbitals.

If an atom does not have the full number of electrons in each orbital, it seeks a partner that can "loan" one or more electrons to "fill" its molecular orbital. This is the essential cause of chemical bonding. For example, a sodium ion, which has a positive charge wants to give up an electron whereas a chlorine ion, which has a negative charge wants to accept an electron. The two elements combine to form an ionic bond (bond formed by the attraction of unlike charges) and thus form the compound, halite. There are several additional types of bonding of molecular orbitals  which students will learn in high school.

The type of bonding between atoms and the characteristics of those atoms determines to a large degree how a compound will "appear" when the atoms combine. In halite, the chlorine atom is twice the size of the sodium atom. When the chlorine atoms "nestle" into a "packed" position, the sodium atoms fill in the gaps. This packed position has a cubic structure, which is reflected in the cubic nature of halite. You can demonstrate this by placing small and large plastic beads in a small, cubic, clear, plastic box. Shake the box. If one bead is twice the size of the other, they will pack in a cubic pattern. The large beads represent chlorine (Cl) atoms and the smaller beads represent sodium (Na) atoms.

PROCEDURE:
  1. In this lab, the students will look at different specimens of commercial salt from Cargill Salt Company in Newark, California. These can include: mill feed - kiln dried, used for animal feed; blending - vacuum dried, used in food processing; granulated - vacuum dried, used in food processing and table salt; pellets - kiln dried, used as water conditioner; medium - kiln dried, used as water conditioner; bakers - vacuum dried, used in making butter. Kiln and vacuum dried refer to the type of process used to make the salt.
      
  2. Summarize the composition and bonding behavior of halite. You might tell your students that the atoms are "holding hands" and are brought together by an "attractive" force. Draw the following diagram on the board for the students to see this "bonding" , or use the electronic presentation. Explain that since chlorine is twice the size of sodium, when they combine sodium fills in the spaces between the chlorine, forming cubes.
      
  3. The students will try to determine if all types of salt are "cubic." They should use a magnifying glass or a microscope to see the specimens in detail. Have them examine the specimens without taking them out of the plastic bags.
      
  4. Ask students to think about why each type is different. Do not give them too many hints, but have them "guess"  the use of each specimen. Have them record their guesses on the lab sheet.
      
  5. As they look at the samples, ask the students to draw  the salt crystals. Monitor their progress as they work their way through the samples. Remind them to draw accurate pictures (pencils work best for this exercise).
      
  6. Review their answers, then answer the conclusion together. While all of these samples are composed of halite, only the blending, granulated, and baking (all vacuum dried) specimens are cubic. The mill feed and pellets are not cubic, and the medium is only roughly cubic. This is because the cubic structure can be broken if the process of making the salt crushes the crystals (mill feed) or combines the crystals (pellets).

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