concentrated sodium silicate solutions are very basic and may cause burns. This simple, quick demonstration may he used to illustrate several points. The first is the exciting,rapid growth of the crystal itself. Secondly,the upward growth is due to the densityaf thecrystalas it forms inside a semipermeable membrane. Finally, although not directly observable, there is the flow of water through the semipermeable membrane (osmosis)to cause this upward growth.
A Colorful Demonstration To Simulate Orbital Hybridization David W. Emerson University of Nevada, Las Vegas Las Vegas, NV 89154
Many students in entry-level chemistry courses seem to have difficultv eraswine the concewt of atomic orbital hvhridization. ~ h eask i such questions as "What happened to the 2s orbital when the hyhridization occurred?" Some students also have trouble comprehending that when a carbon atom undergoes sp2 or sp hyhridization, one or two of its p orhitals, respectively, remain unhyhridized. A simole, colorful demonstration involving nothing more than several beakers of colored water can sometimes speed up student comprehension of hybrid orbitals a t the introductory level. The only materials needed are four 50-mL heakers, one 250-mL heaker, and yellow and hlue food coloring. (The small drop control vials of food color, such as Schilling Assorted Food Colors and Egg Dye, are especially convenient to use.) The four small beakers are filled (about three-fourths full) with water. One beaker is placed near the lower left corner of the overhead projector stage. The other three are lined up near the top of the projector stage a hit left of center. To the water in the lower heaker is added one drop of hlue food coloring. Then a drop of yellow food coloring is added to each of the other three beakers. It is explained that the hlue water represents the 2s orbital and the yellow water represents the three orthoeonal 20 orbitals. Their locations indicate the higher energy of the 2p orbitals (see figure). To demonstrate sw hvbridization take the emotv 250-mL beaker, and pour in& i t t h e blue water from thefiist beaker and the yellow water from one of the other three heakers. Then place the two empty beakers side by side along the middle of the projector stage, toward the right, and then distribute the green solution in the large beaker between the two empty small heakers. There are now two beakers of green water (two sp orbitals) and two of yellow (two unperturbed p orhitals). The central location of the green solutions indicates that the energy of the sp orhital is intermediate between that of the s and p atomic orhitals. T o simulate sw2 pour the areen solutions . hvbridization. . hack into the large heaker, and add the yellow solution from one of the remaining heakers. Now line up all three empty heakers and distribute the green solution among them. The three heakers of green water represent three sp2orbitals and the one remaining heaker of yellow water represents one unperturbed p orhital.
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Diagram representingthe three steps to demonstrate sp and sp2 hybridization.
To demonstrate sp3 hyhridization, simply mix all of the solutions together and distribute the green mixture among the four small beakers. The water is all green, withno hlue or yellow remaining. Shapes of the various hybrid orbitals can also he shown by using sketches on transparency film. The beakers of green solution can be arranged upon an appropriate diagram as each hybrid orbital is demonstrated, showing how electron repulsion will cause the sp orbital to be linear, the sp2 trigonal planar, and the sp3 tetrahedral. In the discussion that accompanies the demonstrations, it should be emphasized, as Bent1reminds us, that orbitals and hvhridization are onlv human efforts to model real suhsirances in order to explain their characteristics.
' Bent, H. A. J. Chem. Educ. 1984, 61, 421.
