MICRODENSITY OF PLASTICS
Standards:
3.4.12.A –
3.7.7.B –
3.1.12.D –
3.2.10.B –
3.4.7.A –
Apply concepts about the structure and properties of matter.
Use appropriate instruments and apparatus to study materials.
Analyze scale as a way of relating concepts and ideas to one another by some measure.
Apply process knowledge and organize scientific and technological phenomena in vaired ways.
Describe concepts about the structure and properties of matter.
Introduction:
Density is defined as the mass per unit volume of a substance. It is one of the most
important properties used in the identification of substances. However, if the sample of
material is very small and of irregular shape, determination of mass and volume may be
difficult to accomplish with precision.
Usually, density is found by massing the object, measuring its volume, and then dividing
mass by volume. However, this lab demonstrates a different technique of determining
density. It uses the relative densities of substances and the properties they demonstrate.
An object with a low relative density will float on top of a liquid with a high relative
density. But if the liquid's density is somehow lowered, a point could be reached where
the densities are equal. At this point, the object will be suspended in the liquid, neither
sinking nor floating. This same idea is used for a relatively low density liquid and a high
density object; only the liquid's density must be increased to suspend the material. Once
the object is suspended, the object and the liquid possess exactly the same density. The
density of the object can be determined indirectly by measuring the density of the liquid
in which the object is suspended.
Guiding Question:
1. Explain how it is possible to determine the density of irregular shaped objects.
2. Are there possibilities of measuring the density of an object without really
taking measurements of the object itself? Explain your answer.
Materials:
small piece of plastic
forceps
4” test tube
scissors
50 mL beaker
methanol
saturated NaI (sodium iodide) solution
stirring rod
2 medicine droppers or Beral pipets
1 mL micropipet
weigh boats
analytical balance
Microdensity of Plastics
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Science In Motion
Juniata College
Safety:
 Always wear safety glasses in the chemistry lab.
 Never eat or drink in the chemistry lab.
Procedure:
1. Obtain a small bottle of methanol and a beaker.
2. Fill a test tube half full with water.
3. Obtain a plastic given to you by the instructor. Record the type of plastic.
4. Bubbles can form on the plastic and change its apparent density. In order to reduce
the number of bubbles, immerse the sample in methanol. Then transfer the plastic to
the test tube of water by using the forceps. Be careful not to lose the plastic in the
bottle.
If the plastic floats:

Add methanol - a few drops at a
time - to the test tube, and carefully
swirl or stir the solution until the
liquid is homogeneous.

Repeat this step until the plastic is
suspended in the solution.
If the plastic sinks:
 Add the NaI - a few drops at a time to the test tube, and carefully swirl or
stir the solution until the solution is
homogeneous.
 Repeat this step until the plastic is
suspended in the solution
Note: During the stirring process, be careful not to create tiny air bubbles which can
attach to the piece of plastic. If this happens, the plastic will float regardless of the
solution’s density. Using the stirring rod, carefully remove the bubbles from the
plastic piece.
6. When the plastic is suspended in the solution, the density of the plastic should equal
that of the solution. Place a weigh boat on the analytical balance and tare. Remove
1.000 mL of the solution with a micropipet, and place it in the weigh boat. Record the
mass of the solution.
7. From the information you have gathered, calculate the density of the plastic and
compare it to the chart to see how accurate your measurements were. Repeat this
process with an unknown plastic given to you by the instructor.
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Data Table:
Plastic
1 = PETE
2 = HDPE
3 = PVC
4 = LDPE
5 = PP
6 = PS
7 = Other
Density (g/mL)
polyethylene terephtalate
high density polyethylene
polyvinyl chloride
low density polyethylene
polypropylene
polystyrene
(often a mixture)
1.39
0.95 - 0.97
varies
0.92 - 0.94
0.90 - 0.91
1.05 - 1.07
varies
Fill in the data for the Known and Unknown plastics given to you by the instructor.
Please be sure to write the identity of the plastic down right away so you do not forget
which plastic was given to you.
KNOWN
UNKNOWN
Identity of plastic
Mass of solution
Volume of solution
Density of solution
Density of plastic
***NO CALCULATORS TO SOLVE FOR DENSITY***
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Questions:
1. Why is this method of determining density referred to as an indirect method?
2. a. What is the relative density of a piece of plastic if, when it is placed in water, the
piece of plastic is suspended?
b. What is the relative density of a piece of plastic if, when it is placed in water, the
piece of plastic floats?
3. What is the purpose of dipping the piece of plastic in methanol prior to placing it in
the test tube of water?
4. If the plastic piece sinks in water, why is a saturated solution of sodium iodide added?
5. If you had to make a floatation device from plastics, which types of plastics used in
the lab would be most effective in your design?
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MICRODENSITY OF PLASTICS
Teacher Notes
LAB TIME:
This lab can be completed in one class period, possibly part of another class
period to discuss the questions.
TARGET GRADE: 6-12 (can be adapted for both middle and high school students).
Objectives:
The purpose of this experiment is to find the density of a small piece of plastic
through an indirect method. The density is then used to identify the type of plastic.
Students will also gain lab skills with equipment (pipetting) as well as measurement
(using a balance).
Major Concepts:
This lab is easily fit into a classroom to explain the concept of density. One of the
items included to explain how solids look when they are suspended in a liquid is a drink
called Orbitz. This drink has tiny gelatinous spheres suspended in the drink. A good
opening question for the lab is “What is the density of the spheres in the Orbitz drink”?
A general response is that they don’t know but guiding them to consider the liquid (how
do you get the density of a liquid vs. a solid), they may come up with the response that
the density of the spheres is equal to the density of the liquid.
A lot of plastics are less dense than water, so the amount of NaI needed may not be
too great. It may be useful to demonstrate to the students that if they pass up the point
where the densities are equal, the addition of the other solution (of opposite relative
density) to the test tube will exhibit the same result.
Calculations are simple for this lab. Students should not use calculators due to
simple division (all calculations will be to divide the mass by the volume; the volume
will always be 1.000 mL if measured correctly).
Archimedes’ principle (water displacement) supports this lab, but cannot be seen due
to the extremely small size of the plastic the students will measure.
Preparation:
Prepare the NaI solution by dissolving 185 g per 100 mL of water. This will be
enough for at least 10 lab groups. This will also correspond to 150 mL of methanol.
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Typical Results:
Answers will vary based on student’s accuracy. However, the volume of the
solution should always be 1.000 mL if the students properly uses the micropipets.
Data Table:
Known plastic
KNOWN
UNKNOWN
Identity of plastic
Mass of solution
Volume of solution
1.000mL
1.000mL
Density of solution
Density of known plastic
Answers to Questions:
1. Why is this method of determining density referred to as an indirect method?
The density of the plastic is being found but not by actually measuring the mass
and volume of the piece of plastic.
2. a. What is the relative density of a piece of plastic if, when it is placed in water, the
piece of plastic is suspended?
It has the same density as the water.
b. What is the relative density of a piece of plastic if, when it is placed in water, the
piece of plastic floats?
It has a density less than that of water.
3. What is the purpose of dipping the piece of plastic in methanol prior to placing it in
the test tube of water?
To prevent air bubbles from forming on the surface of the piece of plastic.
4. If the plastic piece sinks in water, why is a saturated solution of sodium iodide added?
Sodium iodide is used to increase the density of the water, thus causing the
piece of plastic to become suspended.
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5. If you had to make a floatation device from plastics, which types of plastics used in
the lab would be most effective in your design?
Students will have various answers, but they should use the plastics with density that
is less than 1 (water)
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Pre/Post Quiz
Microdensity of Plastics
Science In Motion
Name_________________________
Date__________________________
1. Density of an object is typically calculated using which of the following
formulas?
a.
b.
c.
d.
Density = Length × Width × Height
Density = mass ÷ volume
Density = total of all sides
Density = Length × Width
2. Finding an object’s density indirectly can be done using which of the following
methods?
a.
b.
c.
d.
Small metric rulers and a balance.
Your eye while holding the object to get both the mass and visual volume.
Measuring the density of a solution that has the same density as the object.
Asking for a second opinion on someone else’s density estimate and
averaging your opinion with theirs.
3. Density is useful to do which of the following?
a.
b.
c.
d.
Identify the substance
Know if it will float in water
Know if it will sink in water
All of the above
4. What does it mean to have a solid suspended in a liquid?
a.
b.
c.
d.
The solid is melting and will soon be a liquid.
The solid sinks quickly to the bottom.
The solid is no longer permitted in the liquid due to chemical behavior.
The solid is neither sinking nor floating in the liquid.
5. What would cause an object to float on water?
a.
b.
c.
d.
The object’s density is greater than the water.
The object’s density is equal to the water.
The object’s density is less than the water.
The object needs to breath.
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