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This experiment is courtesy of 
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Changing Viscosity
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Developers:
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Pamela G. Ponce
East Goshen Elementary
West Chester Schools
West Chester, PA
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Dr. Robert J. Smith
Polymer Process Research
Rohm and Haas Company
Spring House, PA
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Grade
Level:
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Fifth
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Discipline:
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Physical Science/Chemistry
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Goals:
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To have students investigate the
properties of flow and viscosity.
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Specific
Objectives:
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To strengthen observational skills. To
record observations in a science journal. To cooperate and
share findings.
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Background:
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One of the most obvious differences
between solids and liquids is that liquids, like gases,
flow. Because both gases and liquids tend to flow, whereas
solids do not, the gaseous state and the liquid state are
referred to as fluid states. Thus the solid state is a
condensed non fluid state, the liquid state is the condensed
fluid state, and the gaseous state is the noncondensed fluid
state. A measure of the tendency to flow is the viscosity.
There is a wide range of viscosities among liquids. Water
flows readily, glycerine is much slower, and a tar's pace is
even more leisurely. Viscosity is one of a group of
properties of fluids called transport properties, which are
related to the flow of matter or energy.
Viscosity is the property of fluids
responsible for their resistance to flow. A high viscosity
is characteristic of a liquid that flows slowly, and a
relatively low viscosity characterizes liquids that flow
freely. The measurement of viscosity is made by a number of
methods, including passing the liquid through a narrow tube
or dropping a ball through the liquid. The measurements are
usually made by measuring the time required for a flow
process to be completed and comparing this with the time
required for a standard liquid (often water). In the
falling-ball method the liquid is placed in a tube and a
ball is dropped into it. The viscosity is determined from
the length of time required for the ball to sink through a
calibrated distance on the tube.
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Materials:
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- glycerine
- water
- graduated cylinders (various sizes
but at least two 25 mL, two 50 mL, and two 100 mL)
- assorted beads 6mm in diameter
(glass, plastic, steel)
- stopwatch
- centimeter ruler
- funnels
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Procedure:
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With a magic marker mark a distance of
5 cm in the middle of the graduated cylinder. If you are
using clear beads, mark them with a "sharpy" (permanent
magic marker). This will help make them more visible. If you
are using different types of clear beads, color code
them.
Using a funnel, fill a graduated
cylinder. Then fill the same size graduated cylinder with
water. Drop a bead into each graduated cylinder and time the
descent through the calibrated distance with a stopwatch.
Calculate the cm/sec for each. Repeat this operation with
different types of beads. Then try different sizes of
graduated cylinders. Develop a chart to keep track of your
results.
Now let's see if we can change the
viscosity by mixing water and glycerine. Fill one graduated
cylinder with 25% water and 75% glycerine, one with a 50-50
mix, and a third with 75% water and 25% glycerine. Again
time the descent of the bead through the calibrated distance
for each. Now develop a graph to show your results. Be sure
to include your times for 100% glycerine and 100%
water.
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Discussion
Questions:
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- Did different types of beads fall
at different speeds? Why or why not?
- What kind of effect did different
sizes of graduated cylinders have on the speed of the
bead?
What might cause this?
- Explain your graphed results for
changing the viscosity of glycerine by adding
water.
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Extensions:
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- Design, conduct, and share another
experiment using these materials.
- Brighter math students can
calculate viscosity and see that it stays the
same.
- Make predictions for using 10%,
20%, 30%,...90% glycerine. What would the graph look
like?
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Thixotropy and Dilatancy
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Developers:
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Pamela G. Ponce
East Goshen Elementary
West Chester Schools
West Chester, PA
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Dr. Robert J. Smith
Polymer Process Research
Rohm and Haas Company
Spring House, PA
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Grade
Level:
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Fifth
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Discipline:
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Physical Science/Chemistry
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Goals:
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To investigate thixotropy and
dilatancy.
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Specific
Objectives:
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To strengthen observational skills. To
record observations in a science journal. To work together
in cooperative groups. To share findings and insight.
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Background:
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Thixotropy is the ability of certain
substances to liquefy when agitated and to return to a gel
form when at rest. The term thixotropy is derived from the
Greek words thixis, meaning "the act of handling," and
trope, meaning "change." Thixotropic substances are
colloidal gels when solid and sols when liquefied. Examples
of thixotropic substances include catsup, some hand creams,
certain paints and printer's inks, and suspensions of clay
in water. The reversibility and essentially isothermal
nature of the of the gel-sol-gel transformation distinguish
thixotropic materials from those that liquefy upon
heating--for example gelatin.
Thixotropic systems are quite diverse.
Therefore, it is unlikely that a single descriptive theory
can include them all. However, in general, the phenomenon is
found only in colloidal suspensions.
Various mechanisms can cause
thixotropic behavior. For a gel system, agitation disrupts
the three-dimensional structure that binds the system into a
gel. Agitation might also introduce order into the system.
In a system containing long polymeric molecules, these
molecules can be disordered in the gel. When the gel is
agitated, the molecules can align in the direction of flow,
reducing the resistance to flow.
Some substances possess a property
which is nearly the opposite of thixotropy. This property is
called dilatancy. A dilatant substance is one that develops
increasing resistance to flow as the rate of shear
increases. A household example of a dilatant material is a
thick dispersion of cornstarch in water. This appears to be
a free-flowing liquid when poured, but when it is stirred,
it becomes very firm. Another familiar example of dilatancy
is the phenomenon of wet sand appearing to dry and become
firm when it is walked on.
A very versatile commercial product
that can perform multiple functions simultaneously in
colloidal formulations is CAB-O-SIL. This is a
submicroscopic, fire dried fumed silica.
Because fumed silica is inert, up to
2% by weight is allowed in foods. CAB-O-SIL is used as an
emulsifier in salad dressings..The great efficiency in the
application allows incorporation of more water in the
product, resulting in "light" dressings. CAB-O-SIL can be
added to catsup to make it thixotropic. It serves as an
anticaking agent in cocoa, non dairy creamers, malted milk
powder, baking soda, and so on. In fact, vinegar can be
changed to a powder by adding 33% CAB-O-SIL . The powder can
be added, for example, to dry sweet-sour mix; the acid is
released, and the CAB-O-SIL amount drops to the 2% level
when the mix is used. CAB-O-SIL acts as a thickener in
heat-resistant margarine.
Lubricating oil is a nonpolar
hydrocarbon liquid. The viscosity is increased by orders of
magnitude with the addition of CAB-O-SIL. This allows the
grease formed to be used at elevated temperatures without
chemical degradation and loss of viscosity. The change of
viscosity with temperature in bearing lubricants can be
stabilized by adding a few percent of CAB-O-SIL, allowing
higher speeds, temperatures, and pressures without oozing
out. Paints can be stabilized in the same manner, permitting
applications at room temperature and drying out at higher
temperatures without runs. Also thicker films can be
applied. Combined control of thixotropy and viscosity can be
achieved.
Gelling of liquid cleaners by adding
CAB-O-SIL allows the product to remain where it is applied
so chemical action can take place where intended.
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Materials:
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For each group of 4-6 students or have
students rotate in small groups to each procedure.
Procedure A
- 500 mL catsup
- 600 mL beaker
- stand with ring small enough to
support beaker
- mirror, ca. 20 cm in diameter 10
(or more) small steel balls, ca. 6mm in diameter (eg.
bicycle ball bearings)
- stopwatch
- stirring rod
Procedure B
- 125 mL cornstarch
- 50 mL water
- 2 600 mL beakers
- stirring rod
- spoon
Procedure C
- 150 mL distilled water
- 23 g fumed silica (eg.,
CAB-O-SIL)
- Erlenmeyer flask
- rubber stopper for flask
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Procedure:
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Procedure A
Pour 500 mL of catsup into
the 600 mL beaker. Set the beaker on the ring stand and
place a mirror under the beaker so the bottom of the beaker
can be seen. Allow the catsup to rest for at least 5
minutes.
From about 3 cm above the surface of
the catsup, drop one of the steel balls into the beaker of
catsup and time how long it takes for the ball to reach the
bottom of the beaker. Repeat this with four more balls, and
compute the average of the times. Do not drop more than one
ball at any one point at the surface of the catsup, because
each ball leaves a track through the catsup that another
ball may follow. A significant variation in the times is to
be expected.
Stir the catsup for one minute. Drop
another ball and time its travel. It will sink much more
quickly than the previous balls. Time the falls of several
more balls dropped at one-minute intervals. The time
required for the ball to sink to the bottom increases with
each ball.
Procedure B
Place 125 mL (about half a
cup) of cornstarch in a 600 mL beaker. While stirring the
corn starch, slowly add about 50 mL of water. Stir the
mixture with the stirring rod until it is homogeneous. The
mixture will be very stiff and difficult to stir. Pour the
mixture into the other beaker. It pours freely, although
slowly. Stir the mixture again, and again it will be
difficult to stir. Pour some of the mixture into the palm of
your hand. Strike the mixture in your hand with the spoon.
It will not splatter.
Procedure C
Pour 150 mL of distilled
water into a 500 mL Erlenmeyer flask. Sprinkle about 5 grams
of fumed silica onto the water, stopper the flask, and shake
it until the silica is dispersed in the water. Repeat this
process until all 23 g of the fumed silica have been added
to the water. After the last addition, allow the flask to
rest for several minutes, and the liquid in the flask will
form a gel. After the liquid has gelled, tip the flask to
show that its contents do not flow. Shake the flask
vigorously, and the contents will become a viscous liquid.
Allow the flask to rest for several minutes, and its
contents will again solidify.
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Discussion
Questions:
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A.
How can thickness of catsup be misleading in
advertising?
B.
How might cornstarch be used in products? Can you design a
product that would need these characteristics?
C.
CAB-O-SIL is used in many products. Contrast the positive
and the negative sides of its use.
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Extensions:
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Procedure A:
Compare different brands of catsup. Make sure to include one
with no additives or preservatives.
Procedure B:
Design another experiment using cornstarch. You might want
to observe it dry.
Procedure C:
1. Check labels on products for emulsifiers like CAB-O-SIL.
Report on your findings.
2. What are the advantages of being
able to turn vinegar into a powder with CAB-O-SIL? Design
some products that would use this powdered vinegar.
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References:
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Shakhashiri, Bazzam. Chemical Demonstrations: A Handbook
for Teachers of Chemistry, Vol. 3, The University of
Wisconsin Press, Madison, 1989.
CAB-O-SIL is manufactured by and available from the Cabot
Corporation, 125 High Street, Boston, Massachusetts 02110
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Viscosity of Some Petroleum-based
Materials
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Developers:
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Pamela G. Ponce
East Goshen Elementary
West Chester Schools
West Chester, PA
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Dr. Robert J. Smith
Polymer Process Research
Rohm and Haas Company
Spring House, PA
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Grade
Level:
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Fifth
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Discipline:
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Physical Science
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|
Goals:
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To have students investigate the
properties of flow and viscosity.
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Specific
Objectives:
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To strengthen observational skills. To
record observations in a science journal. To strengthen the
connection between math and science. To work cooperatively
and communicate findings.
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Background:
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Viscosity is one of a group of
properties of fluids called transport properties, which are
related to the flow of matter or energy. A material with a
high viscosity flows slowly and with difficulty, like honey.
A material with a low viscosity flows readily, like water.
You will determine relative viscosities, which means ranking
materials on a range from the most to the least
viscous.
The viscosity of a liquid is an
important design parameter in numerous practical
applications. The pipe dimensions and size and number of
pumping stations on the Alaskan pipeline are determined to a
large extent by the viscosity of crude oil. The oil is
heated to reduce the viscosity. The viscosity of blood
affects the throughput in artificial heart-lung machines.
When honey is packaged in a plastic dispenser bottle, its
high viscosity requires an increase in pressure, by
squeezing, to force the honey through the small
nozzle.
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Materials:
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(for each group)
- water
- mineral oil
- kerosene motor oil
- household lubricating oil
- 5 test tubes
- 5 tube caps
- plastic beads (6mm)
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Procedure:
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Copy this data table in your science
journal:
Data Table -- Viscosity
Measurements
Material
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carbon atoms per
molecule
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average time for bead to
fall (s)
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relative
viscosity
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water
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--
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mineral oil
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12-20
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kerosene
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12-16
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motor oil
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15-18
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household lubricating
oil
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14-18
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- Determine the average time for a
bead to fall from top to bottom within the capped tube
containing water. Follow this procedure:
a. Hold the capped tube upright until the ball is at the
bottom.
b. Gently turn the tube horizontally. (The bead will stay
at one end.)
c. Quickly turn the tube upright so the bead is at the
top.
d. Determine the number of seconds required for the bead
to fall to the bottom of the tube.
e. Repeat this procedure three more times. Calculate the
average time required for the bead to fall.
- Repeat this procedure for each
petroleum-based sample.
- Rank your samples in order of
relative viscosity, assigning the number 1 to the least
viscous material (the one in which the ball fell
fastest).
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Discussion
Questions:
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- Propose a rule based on
observations made in this activity regarding the relative
number of carbon atoms per molecule and the resulting
viscosity of the material.
- Petroleum refiners and
distributors must consider the viscosity of their
products when shipping diesel/fuels and motor oils to
different parts of the country. Explain why the
diesel/fuel shipped to a northern state (say Alaska) in
winter must be different from that shipped to southern
state (say Florida) in summer. How would you adjust the
products
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Extensions:
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- Use different sizes of test
tubes.
- Graph results
- Find out: Is viscosity a major
factor both in oil spills and oil fires? How does it
effect treatment and/or outcome?
- Have each group design their own
experiment to share with other groups.
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References:
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Chem Com: Chemistry in the Community, ACS Kendall/Hunt
Publishing Co. Iowa 1988.
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This experiment is courtesy of 
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