This exercise for a mineralogy or geochemistry course, published in the Journal of Geoscience Education, uses optical microscopy to measure the grain sizes of equant halite crystals and the time for complete dissolution of each grain. Using hot plates, the procedure can be repeated at a second, elevated temperature, allowing determination of the temperature dependence (Arrhenius activation energy) of the reaction. The data generated are then used in homework exercises to calculate (1) a simple rate law for the dissolution of halite in pure water; (2) the rate of coefficient in that rate law, at two different temperatures; and (3) the activation energy of the halite dissolution reaction. The lab and homework illustrate several basic principles of chemical kinetics, including diffusion, the temperature dependence of reaction rates, and the relationship between rate-determining mechanisms and crystal-surface morphology.
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