Microdensity of Plastics
Pre-Lab:
These questions will provide helpful hints for designing a method for the density measurement. Try to answer
all the questions on your own and then discuss your answers with your teacher before you proceed.
1. A small plastic sample is placed in a small quantity of some solution in a test tube. After stirring with a stir
rod, the plastic floats on the surface of the solution. What does this tell you about the density of the plastic
relative to the density of the solution?
2. A small plastic sample is placed in a small quantity of some solution in a test tube. After stirring with a stir
rod, the plastic sinks to the bottom of the test tube. What does this tell you about the density of the plastic
relative to the density of the solution?
3. A small plastic sample is placed in a small quantity of some solution in a test tube. After stirring with a stir
rod, the plastic is perfectly suspended (neither sinking nor floating) in the solution. What does this tell you
about the density of the plastic relative to the density of the solution?
4. A small quantity of water (density 1.00g/mL) is added to a test tube. Several drops of methanol (density
0.79g/mL) are added to the water to form a solution. How does the density of the solution compare to the
density of water?
5. A small quantity of water (density 1.00g/mL) is added to a test tube. Several drops of a concentrated NaI
solution (density 1.40g/mL) are added to the water to form a new solution. How does the density of the new
solution compare to the density of water?
6. A small plastic sample is placed in a small quantity of water (density 1.00g/mL) in a test tube. After stirring
with a stir rod, the plastic floats on the surface of the water. How might you alter the density of the liquid to
match the density of the plastic?
7. The density of aluminum is much greater than the density of water so it should sink in water. But if you
take a small piece of aluminum foil and roll it into a ball, it will float on water. How do you explain this?
8. A small plastic sample is placed in a small quantity of some solution in a test tube. After stirring with a stir
rod, it is observed that a small air bubble is attached to the surface of the plastic. How might this effect the
apparent density of the plastic sample? What can you do about the bubble?
9. A micropipette is set to deliver exactly 1000 microliters. 1000 microliters of some solution is pipetted into a
tared weigh boat on an analytical balance. The mass of the solution is 0.9455g. What is the density of the
solution?
10. A micropipette is set to deliver exactly 500 microliters. 500 microliters of some solution is pipetted into a
tared weigh boat on an analytical balance. The mass of the solution is 0.5010g. What is the density of the
solution?
11. Consider the Plastic Density table below. If a plastic sample sinks to the bottom of the test tube containing
water (density 1.00g/mL), which plastic(s) might the sample be? Which plastic(s) can it NOT be?
Recycling Code
1
2
3
4
5
6
Abbreviation
PETE
HDPE
PVC
LDPE
PP
PS
l
Type of Plastic
Density (g/mL)
polyethylene teraphthalate
high density polyethylene
polyvinyl chloride
low density polyethylene
polypropylene
polystyrene
ASIM Microdensity of Plastics 5E Student Handout revised 6/2010
1
1.39
0.95 – 0.97
1.30--1.34
0.92 – 0.94
0.90 – 0.91
1.05 – 1.07
Objective:
In this activity, you are asked to develop a method for measuring the density of a very small plastic sample.
Once your teacher has approved your method you will measure the sample’s density and identify the plastic
using the data you collect.
Safety Considerations:
 Goggles and aprons are appropriate.
 Methanol is poisonous if ingested.
 NaI and KI are mild skin irritants.
Materials:
Dropper bottles containing methanol (density 0.79g/mL), water (density 1.00g/mL), and concentrated KI or NaI
(density greater than 1.30g/mL), one small test tube, one adjustable micropipette, one glass stirring rod, one pair
of forceps, one weigh boat, one or more plastic samples, and access to an analytical balance.
Procedure:
1. Work in small groups to devise a plan (method) for measuring the density of a small plastic sample.
2. You are limited to using the materials listed above.
3. Write down your plan and review the plan with your teacher before proceeding.
4. Execute your plan to measure the density of a small plastic sample, record all your data and use your data
and the table below to identify the plastic.
5. Check your result with your teacher.
Recycling Code
1
2
3
4
5
6
Abbreviation
PETE
HDPE
PVC
LDPE
PP
PS
Type of Plastic
Density (g/mL)
polyethylene teraphthalate
high density polyethylene
polyvinyl chloride
low density polyethylene
polypropylene
polystyrene
ASIM Microdensity of Plastics 5E Student Handout revised 6/2010
2
1.39
0.95 – 0.97
1.30--1.34
0.92 – 0.94
0.90 – 0.91
1.05 – 1.07