1. Auburn University Science in Motion

2. “Microdensity of Plastics”

3. Introduction: Density is used to identify substances.
Density is found by measuring mass by volume.
Density =

4. Density can be determined by comparison.
A low density object will float on top of high density liquid.
A high density object will sink to the bottom of a low density liquid.
If the density of the object is equal to the density of the liquid, the object will be suspended; neither sinking nor floating.
The density of the object can be determined indirectly by measuring the density of liquid.

5. Recycling Code
Abbreviation
Type of Plastic
Density (g/mL) 1
PETE
PolyEthylene Terephtalate
1.39 2
HDPE
High Density PolyEthylene 3
PVC
PolyVinyl Chloride 4
LDPE
Low Density PolyEthylene 5 PP
PolyPropylene 6 PS
PolyStyrene 7
Other
varies

6. Purpose:
Determine the density of a small piece of plastic and used the density to identify the type of plastic.

7. Equipment/Materials:
4 inch test tube
Known plastic
Unknown plastic
Micropipet
Micropipet tips
Parafilm
Dropper bottle of saturated KI
Dropper bottle of methanol
Dropper bottle of distilled water
Weigh boats
Analytical balance

8. Safety: Always wear goggles in the chemistry lab
Never eat or drink in the chemistry lab.
Avoid any contact with chemicals.
Wash hands after conducting lab.

9. Procedure
Obtain dropper bottle of methanol, potassium iodide, and distilled water.
Fill the test tube one-third full of distilled water.
Obtain a small piece of a known plastic.
Record its recycling code on the data table.

10. Carefully place the plastic in the water, trying not to create air bubbles on the surface of the plastic.
If bubbles form, gently tap the side of the test tube with your finger to knock the air bubbles off the surface.
If the plastic floats, add a few drops of methanol.
If the plastic sinks, add a few drops of KI solution.

11. Cover the test tube with Parafilm, and hold it in place with your finger.
Carefully (to avoid forming bubbles) invert it several times until the liquid is thoroughly mixed so that it is homogeneous.
Keep the open end of the test tube covered with the Parafilm to prevent evaporation of the methanol.

12. Repeat these steps until the plastic is suspended in the solution.
Once the plastic is suspended in the solution, the plastic's density is equal to the density of the solution.

13. To find the density of the solution:
Place a weigh boat on the balance and tare (rezero) the balance.

14. Put a clean tip on the micropipette and remove 1
Put a clean tip on the micropipette and remove mL of the solution from the test tube; dispense the liquid into the weigh boat.

15. Quickly record the mass of the solution on the data table.
Known Plastic:
Recycling code of your plastic
Mass of solution (g)
Volume of solution (mL)
Density of solution (g/mL)
Density of plastic (experimental value) (g/mL)
Density of plastic (accepted value) (g/mL)

16. Repeat the procedure for a piece of plastic with an unknown density
Repeat the procedure for a piece of plastic with an unknown density. Record the data for the unknown plastic on the data table.
Unknown plastic:
Letter code of unknown plastic (from label)
Mass of solution (g)
Volume of solution (mL)
Density of solution (g/mL)
Density of plastic (experimental value) (g/mL)
Accepted value of density of plastic (g/mL)
Recycling code of unknown plastic

17. Rinse the used micropipette tips, test tubes and weigh boats with water. Return all unused samples and solutions to the appropriate reagent bag.