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Determining a Number of Unknown
Plastics from Their Physical and Chemical Characteristics |
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In the organic chemistry and
biochemistry standard, students are expected to know that the bonding
characteristics of carbon allow the formation of many different organic
molecules of varied sizes, shapes, and chemical properties and that large
molecules (polymers), such as proteins, nucleic acids, and starch, are formed
by repetitive combinations of simple subunits. Also, that the bonding characteristics of
carbon result in the formation of a large variety of structures ranging from
simple hydrocarbons to complex polymers and biological molecules. This
experiment will acquaint students with a variety of common plastic polymers
that are associated with various recycle numbers and use physical and
chemical characteristics to identify these polymers. |
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Introduction Recycling has become an important issue over
the last several years. There is, however, a large misconception that plastic
containers are all alike, except perhaps for their different colors and
thicknesses. This lab is intended to point out the inherent differences among
polymers most commonly used today and to illustrate why not all plastics can
be recycled in the same manner. The common recycled plastics and their
corresponding numbers are: (1)
polyethylene terephthalate (PET); (2) high-density
polyethylene (HDPE); (3) polyvinyl chloride (PVC); (4) low-density
polyethylene (LDPE); (5) polypropylene (PP); and (6) polystyrene (PS). The various uses of these plastics can be
found on a web site of the American Plastics Council
(www.americanplasticscouncil.org) |
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Materials and
Methods
Student Instructions: 1. Obtain the different samples of
plastic from the assigned location. Record in your lab notebook the color and
shape of each, as well as the corresponding letter. 2. Before conducting the tests,
construct a flow chart of the tests and then use your flow chart to identify
each unknown plastic. In your results show your flow chart with the
appropriate decision points. 3. Complete each of the tests described
below on each plastic. In your results
section construct a chart showing the letter of the unknown (A, B, C, etc.)
and its corresponding identification. 4. In your discussion, comment on any
irregularities or ambiguities in your testing. |
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Tests H2O Test: Place the sample in a beaker of water, poke the pieces with a stirring rod to knock off
any adhering bubbles. Note whether the sample floats or sinks. Use the
Density Table to classify the plastic. Copper Wire Flame Test: Using the forceps, hold the copper wire in
the flame of a Bunsen burner until red hot Remove from the flame and
carefully push the hot-wire through the sample. Place the wire back in the
flame, observing the color at same time that comes from the wire. Note: The
halogens (fluorine, chlorine. bromine. and iodine) will react with copper
metal to create copper(II) ions which will give off
a green flame. The absence of copper(II) ions will
result in a yellow flame. Use the polymer names to explain the color of the
flame and narrow your polymer choices accordingly. Acetone Test:
Place the sample in the beaker of acetone for about 5-10 sec. Remove
the sample and press firmly between fingers. The polymer chains made from styrene .,will "loosen up" in acetone (this is
known as swelling), allowing the surface to become somewhat soft and
impressionable. The other polymer chains will not do this. Heat Test:
Using tongs, plunge the sample into boiling water. PET (1) has a relatively
low softening point and should show some reaction to the boiling water. The
other polymers. having a higher softening point will
not react. 2-Propanol Test: Place the sample in the solution: poke the
sample with the stirring rod to release any adhering bubbles. Note whether
the sample floats or sinks. Use the Density Table to classify the
plastic. The density of 2-propanol is
0.786 g/mL.
The solution is 70% 2-propanol in water. You will need to compute the density of the
solution. Salt Water Test: A solution of salt water has been made so
that it has a density of 1.10 g/mL. Test whether or not the respective plastics
sink or float in the solution. |
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Substance Densities (g/mL) (1) PET
1.38 to 1 39~ (2) HDPE
0.95 to 0.97 (3) PVC
1.16 to 1.35 (4) LDPE
0.92 to 0.94 (5) PP
0.90 to 0.91 (6) PS
1 05 to 1 07 |
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Questions 1. Your boat is sinking
about two miles off shore and you're not a good swimmer. You notice six large
solid plastic blocks labeiled 1, 2, 3, 4, 5, 6.
Having only two arms, which two should you grab? Why? 2. You are on a camping
trip and have run out of wood (but there is lots of discarded plastic to
bum). Which type of polymer would you use to cook your food? Which type would you use to read by? 3. You are in charge of a
recycle center. Describe a process or
series of processes by which you could automatically separate different types
of plastic. 4. You decide to jazz up
your bathroom cabinet by transferring your fingernail polish into a more
stylish container. The next day, reaching for the bottle, you find a messy
bloc. What was the stylish bottle probably made of? What is the active ingredient in the
fingernail polish? 5. Why is it important
to dislodge any adhering bubbles in the density tests? |
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References Kollman, C.S., Qualitative Polymer Analysis
Lab. Science Content Standards for
California Public Schools (SCSCPS), |
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