In the Laboratory
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Floating Plastics An Initial Chemistry Laboratory Experience Enrique A. Hughes, Helena M. Ceretti, and Anita Zalts* Instituto de Ciencias, Área Química, Universidad Nacional de General Sarmiento, Roca 850, 1663 San Miguel, Prov. Bs. As., Argentina; *
[email protected] The investigation of density and other properties of solutions as an initial experiment (1, 2) and experiments for separation and identification of plastics (3–6 ) have been proposed previously. Yet all of these are separate experiments, which don’t exploit the advantages of integrating them in one exercise. We have designed this experiment as an initial chemistry laboratory experience (college or high school), integrating both ideas. The experience illustrates a fundamental concept, density, and gives students practice in some important skills: weighing, measuring volumes, and comparing experimental and expected results. All of us use daily a great variety of liquids, most of which are mixtures. For example, when we drink a glass of “pure water” we are really ingesting a solution of gases (O2, CO2, N2) and various ions (Na+, K+, CO32᎑, H3O+, OH᎑, etc.) in water. Chemists in particular routinely use a great variety of solutions, many of which they prepare themselves. Often, the only information is the concentration (e.g. 2% sodium chloride), which the user must be able to interpret so as to carry out the necessary preparation. Sometimes it is necessary to confirm that the intended product was obtained. This may be done by different means, some of which involve measuring one or more physical properties of the solution: color (absorbance of light), electrical conductivity, melting point, boiling point, osmotic pressure, and density. In particular, density (d ) is a property of importance in many situations. As d = m/V, it can be used to predict whether an object will float or sink in a liquid. All that is necessary is to compare their densities, because any solid or immiscible liquid will float on a liquid of higher density as a consequence of Archimedes’ principle (7). Density also has applications in everyday situations of environmental importance. We can consider, for instance, the issue of plastic wastes: the benefits of recycling used plastics are widely accepted, but there are practical difficulties that must be solved. One of these is the identification of the different kinds of plastic in the waste mixture. For example, we can confidently expect an object made of Plexiglas (d = 1.24 g/mL) to sink in water (d = 1.00 g/mL), whereas one made of polypropylene (d = 0.91 g/mL) will float. The reverse application is also useful: if a piece of plastic floats on pure water we know it can’t be Plexiglas. Many manufacturers mark their products with the SPI (Society of the Plastics Industry) code (8, 9), but these marks are useful only in a manual separation system. Moreover, most waste is broken or unidentifiable, which means that some other method of identifying plastics is necessary. For achieving this, density is a key property. We can design a method based on flotation, using a series of solutions with gradually increas-
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ing densities. Each plastic fragment will sink in those liquids of lower density than its own and will float on the first solution whose density is higher (and on the others too, of course). We have used this experiment as an initial laboratory experience in our introductory chemistry classes for nonchemistry majors, and also in special classes for high-school teachers. The essential concept of density as the relationship between the mass of a body and the volume it occupies is clearly shown. The concept of density is further developed by its application to flotation in a simple, easily understood experience, and the concept of concentration can easily be incorporated, including definitions and use of molarity, volume percent, etc. We have found this experiment very useful as an introduction to lab work, as it provides a clear and simple application of using balances and volumetric apparatus, handling powders and liquids, and dissolving solids. All the equipment necessary is common in a university or high-school lab (balances of 0.1 g sensitivity, volumetric flasks, etc). The chemicals are common and not particularly dangerous (10, 11): ethanol, sodium chloride, potassium carbonate, sucrose, water, pieces of different plastics. Hazards This experiment presents no significant hazards. W
Supplemental Material
Details of the experiment and a student handout are available in this issue of JCE Online. Literature Cited 1. Olmstead, J. J. Chem. Educ. 1986, 63, 538–540. 2. Richardson, W. S.; Teggins, W. E. J. Chem. Educ. 1988, 65, 1013–1014. 3. Kolb, K. E.; Kolb, D. K. J. Chem. Educ. 1991, 68, 348. 4. Seymour, R. B.; Stahl, G. A. J. Chem. Educ. 1976, 53, 653. 5. Blumberg, A. A. J. Chem. Educ. 1993, 70, 399–403. 6. Anderson, G. E. J. Chem. Educ. 1996, 73, A173. 7. Dickson, T. R. Introduction to Chemistry, 7th ed.; Wiley: New York, 1995; pp 52–59. 8. Levinson, A. S. J. Chem. Educ. 1993, 70, 174. 9. Kolb, K. E.; Kolb, D. K J. Chem. Educ. 1993, 70, 174. 10. Material Safety Data Sheets; http://www.phys.ksu.edu/area/jrm/ Safety/msds.html (accessed Feb 2001). 11. National Toxicology Program, Chemical Health and Safety Data; http://ehis.niehs.nih.gov/ntp/docs/chem_hs.html (accessed Feb 2001).
Journal of Chemical Education • Vol. 78 No. 4 April 2001 • JChemEd.chem.wisc.edu
