HandOuts19_Density.docx.pdf - Determining the Density of Some Recyclable Plastics Specific Safety Concerns​ Today you will be working with cut pieces

HandOuts19_Density.docx.pdf - Determining the Density of...

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Determining the Density of Some Recyclable Plastics: Specific Safety Concerns : Today you will be working with cut pieces of commercial plastics, water, and NaCl solutions. They pose no serious chemical hazards, but the plastic pieces can be sharp and are consequently cutting hazards. The NaCl solutions can irritate the skin, especially if it gets in a cut. Additionally, the devices used to measure the densities, the pycnometers, are fragile and, if parts break, their broken surfaces can be cutting hazards. So, wear you lab coats and safety goggles, and be careful about touching sharp surfaces. If you cut yourself, rinse the wound thoroughly and ask your TA to get you a bandage. Outline of this Document - Background Material - Watch out for Mistakes - Important Lab Techniques – mass and volume measurements - General procedure for today including calculations - Specific Procedure - Your completed ELN should include - Exercise before lab Background Material: Density is the ratio of mass to volume for a given material. It is an intensive property of a material – that is, it is independent of the amount of material present. Indeed, a material’s density is the same all the way down to the molecular level, if the composition remains the same. Knowledge of density is useful in a variety of situations: - identifying an unknown material, - verifying a material’s purity, - converting a knowledge of volume to mass and vice versa, - understanding how atoms are packed together. - when separating materials of different densities. In this lab we’ll determine the densities of several types of plastics and figure out how to separate them from each other. The stuff you put in your recycling bin may be separated by a group of folks sitting by a conveyor belt looking for the little recycling triangles and sorting the plastic bottles as they move past. They also may be separated in a slightly more sophisticated manner, such as separation using “sink-float tanks”. In such devices a liquid is chosen such that one kind of plastic will float in it but the others will sink (Andrady, Anthony L. Plastics and the Environment . New York: John Wiley & Sons, 2003). You should think about what sort of properties a liquid should have if it is to allow one plastic to sink and another to float . [Much more sophisticated techniques have been developed that can use spectroscopic analysis. (See for instance, Wahab, D.A., Hussain, A., Scavino, E., Mustafa, M.M., and 1
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Basri, H. “Development of a prototype automated sorting system for plastic recycling”, American Journal of Applied Sciences , 3(7): 1924-1928 , 2006.)] A liquid chosen to separate two materials will have density smaller than the more dense material and larger than the less dense material. The more dense material will sink and the less dense material will float. To make a liquid with a density in a narrow range, one commonly makes a salt solution. The densities of such solutions are fairly sensitive to concentration, and are easy to make. Today you will determine the concentration range of NaCl solutions that will separate several specific plastics.
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