The Crystal Demonstration


Concepts Shown:

grains, grain orientation, solubility limit with nucleation, and growth


1 vial of salicylic acid dissolved in alcohol. Alcohol must be used because it evaporates quickly due to its high vapor pressure. As the alcohol evaporates, the solution becomes concentrated, making the acid exceed its solubility limit and grow crystals at a fairly rapid rate. The chemical name for salicylic acid is o-hydroxybenzoic acid. Its chemical formula is shown below. 1 vial of urea dissolved in alcohol. Its chemical formula is also shown below. [eq]. 2 blank microscopic slides, 1 dropper pipette, polarizing filters (optional), a stereographic microscope that can magnify 40X or less so that the crystals can be clearly viewed.


The lecturer or recitation instructor can use a projecting microscope in class. First, of course, the microscope must be focused are around 40X magnification or less. Then, simply place a drop of either solution on a slide and spread the solution around the area of the slide with the end of the pipette. Then just wait around thirty seconds for the crystals to begin forming as the alcohol evaporates. The salicylic acid will form many small crystals at a very rapid rate once they begin. The urea will grow large, long crystals at a rate that is slower than the acid, but it will still take only a couple of minutes at the most for the whole solution to crystallize. As the demonstration is being done, the instructor may explain about how, as the alcohol evaporates, the solution reaches its solubility limit and the dissolved chemicals have to nucleate. The different orientations in the crystals should be visible through the microscope. The urea solution tends to show this the best because of its elongated crystals. Using polarizing filters may help to visualize these various orientations, because each crystal diffracts light in directions that correspond to its orientation of the molecules. The recitation instructor could also take the students to the lab for this demonstration if it is not desirable to bring the microscope to the classroom itself. Another possibility is to use this demonstration in the lab tours that are frequently done in the department.


The most important point illustrated in this demonstration is that grains in a metal, like steel or copper, represent different orientations of the arrangement of molecules. In this demonstration, light diffracts through these crystals in such a way that the various orientations are easily observed. Another principle shown is that the solvent of as solution evaporates, the concentration of the solute increases until it reaches its solubility limit and begins to nucleate crystals. As more and more solvent evaporates, the crystals grow. As shows in the diagrams below, many small crystals nucleate around the edges of the solution. Eventually, as the crystals grow, they impinge to make a polycrystalline structure, and each single crystal, which is now a grain within the polycrystal, has a particular orientation. I was, however, unable to show the various orientations in the diagrams. [eq].



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