The Rocky Road to Chemistry Lany Walker Calabasas High School, 22655 Mulholland Highway, Calabasas, CA 91303 Priscilla J. Lee Venice High School, 13000 Venice Boulevard, Los Angeles, CA 90066 Entirely too many students see chemistry in a vacuum, unrelated to the real world around them except, perhaps, as pollution. In spite of this, the chemistry of nature affords an opportunity to present our subject in a more positive light. The presentation of numerous chemical principles can be enhanced by utilizing geologic examples.' The following demonstrations and laboratories provide examples. Water of Hydratlon Heat drives water from hydrated crystals. Instead of using toxic barium salts in the lab, powdered gypsum (CaSOa.2HzO) can illustrate the 21 water-to-calcium-sulfate ratio present. Thus empirical formulas are demonstrated by using the material that composes most classroom walls! Boyle's Law What powers the explosion of violent volcanic releases? T o a great extent, this is achieved by decompressing gases, mostly steam and carbon dioxide, a la Boyle's law. Display pumice and obsidian. They are chemically similar, but gas blasts out the former while volcanic glass simply pushes and oozes out of the ground. Pumice is obsidian with gas. Energy Level T o display fluorescence, electrons are energized to higher energy levels. They then cascade down t o their ground state, emitting quanta of definite frequency as they go. Demonstrations of this phenomenon add color and excitement to a discourse on the Bohr atomor energy levels. For example, an ultraviolet lamp provides the initial stimulus. On illumination, most minerals and other compounds of uranium glow yellowish. Scheelite (CaWOa) though virtually indistinguishable from common feldspar in ambient light, fluoresces a brilliant violet-white under UV. I t is this property that allows tungsten prospectors to locate their quarry in the field (1). Magnetlsrn What makes a lodestone adhere to staples, nails, and tacks? This variety of natural iron oxide (FesOd) can lead to curiosity about magnetism. Such "ferromagnetism" requires that a d subshell be asymmetrically occupied by unpaired electrons. Crystalline Structure
How do geological materials show us their bonding patterns and internal structures? Mohs's scale mineral sets and conductivity tests can help students visualize this relationship. For example,.you can randomly tumble the standard minerals from a Mohs's hardness kit out onto lab tables and have students line them u p from softest t o hardest. (The students have just unwittingly entered the realm of structural chemistry.) Corundum (Mohs's hardness H = 9),topaz ( H = 8), quartz (H = I ) , and feldspar ( H = 6) are the hardest and possess a three-dimensional network of covalent honds in which electron pairs are shared between atoms. This is
also true of diamond ( H = lo), but the prudent instructor will withhold this item from student use. Softer gypsum ( H = 2), calcite ( H = 3), fluorite ( H = 4), and apatite (H = 5) are made of oppositely charged attracting ions and have ionic honds holding themselves together. Talc, the softest of all (H = I), is covalent (2). This "soapstone" exists as flat, two-dimensional sheets that move easily apart on scratching. The graphite in pencils does likewise. Hit pieces of calcite (CaC03) or halite (NaC1, table salt) with a hammer, and they cleave perfectly along three flat planes, another property of ionic solids (3).Now strike a brittle chunk of obsidian. This natural glass has a curved (conchoidal) fracture that annihilates the thought that i t may he arrayed of neatly organized molecules or ions. Elfecta of Heat and Color Perfect crystals of quartz beautifully display crystal imperfections. Rose quartz and amethyst owe their tints to faint traces of metal ions. Smokey quartz has been irradiated and owes its mysterious gray-brown hue to submicroscopic disruptions, atoms humped out of place. Heating should vibraie silicon and oxygen particles into their proper positions. How might this affect the color? As the crystal repairs itself, the grayness magically vanishes. Conductlvlty Electrical conductivity experiments "show",us freely mobile electrons. Several years ago coauthor Larry Walker tried to demonstrate that materials made of metals and nonmetal ions were nonconductors. A conductivity apparatus with two electrical probes and a light bulb was used. Calcite and halite typically showed no conduction, hut, when the probes touched crystalline galena (PbS) the bulb glowed brightly, stunning Walker's hypothesis. It relit on contact with several other shiny metallic sulfides and oxides, hut sphalerite (ZnS) has a pitchy luster and would not conduct. Neither would semimetallic looking hematite (FenOa). Therefore, a shiny appearance attests to loosely held electrons in compounds as well as pure metals (4). Acllvatlon Energy Iron pyrite (FeS,), one of the sulfides tested, sparks when hit with steel, releasing the s h a r ~ scent ofsulfur dioxide eas. The mineral reacts with oxygen-in the air. Striking the rock provides the energy needed to allow the reaction to occur and provides an example of activation energy (5). More hidden insights into chemistry arise from testing malachite (Cuz(0H)2CO3), a bright green ore. If you add a few drops of 6 M hydrochloric acid on its surface, carbon dioxide bubbles will appear as the crusty mineral reacts (a
Resented at the 1988 National Science Teachers Associatfon Meetina. -. St. Louis. -. MO. ' Most mineral samples are available through scientific supply houses. such as Ward's Natural Sclence Establishment. One of our best sources is BURMINCO, 128 S. Encinitas. Monrovia. CA 91016. ~
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twical reaction of carbonate-bearine rocks). If additional didrochloric acid is added, a pale yellow solution results. When a few milliliters of concentrated ammonia is carefully added to the solution, an insoluble, blue-white copper(I1) hvdroxide lines the interface. (Caution: This must be done with proper eye protection and ventilation.) Continued addition of ammonia provides an excess of ammonia molecules resulting in a deepblue complex of Cu(NHM+ permeating the solution (6). Water dilution brings back the Cu(OH)2 precipitate as ammonia becomes less available. Rock and Minerals
The use of geochemical demonstrations may stir both a greater fascination for chemistry and an appreciation for the
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subtleties of nature. Utilizing rocks and minerals in the lab can make our science courses grow in excitement and enhance our vision of the world beneath our feet. Seawater, limestone, salts from desert playas, and soils provide readily available and inex~ensiveraw materials for chemical analvsi, Literature Clted 1. Robbins, Manuel. The Collertola Book o/ Fluorescent Minwols: Vsn Naatrsnd: New
York. 1983:on 192-212.
