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Developers:
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Joseph J. Leo
Log College Middle School
Centennial School District
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Allen P. Marks
Research Scientist
Rohm and Haas Company
Spring House, PA
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Grade
Level:
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6 to 8
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Discipline:
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Physical Science, General
Science
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Objectives:
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To determine the density of Plaster of
Paris that has been prepared with a foam generated from
dishwashing liquids and shampoo.
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Background:
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Calcium Sulfate products are generally
thought to be heavy, with a high density and sink in water.
This experiment will provide a way to decrease the density
of calcium sulfate by adding a solution of foam to a
prepared slurry of Plaster of Paris. Gypsum
(CaSO4 2H2O) is not a
satisfactory product because the 2 moles of water of
hydration reduce the setting time or working time
considerably. The only product to use for this experiment is
Plaster of Paris.
This product and experiment must be
carried out under adult supervision. It is not to be used
for casting of fingers, hands or other body parts because
heat is generated while setting and it may cause burns.
Also, avoid eye contact and wear safety goggles at all
times.
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Materials:
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Student laboratory groups of 5
students.
Safety goggles or glasses - 1 pair for
each student
25-50 lbs. of Plaster of Paris - Can be found in a hardware
store, building supply store or craft shop.
Dishwashing liquids - Dawn � or Ivory � are
preferred
Shampoo - Head and Shoulders � preferred
150-200 waxed paper cups - 4 oz. sample cup
25-50 waxed paper cups - 32 oz. mixing cup
100 tongue depressors 6" x 3/4 " - Fisher � brand
1/2 -gallon or 1-gallon plastic jugs
10-, 25-, or 50-ml. graduated cylinder
100-ml beaker
Small fish tank
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Procedure:
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- Prepare a 5% solution by volume of
foaming agent using either a dishwashing liquid or a
shampoo. Add 5 mL of soap to 95 mL of water in a 1 �2
-gallon jug. Swirl gently to dispense the soap in the
water. Then shake vigorously for about 3 minutes just
before adding the soap to the Plaster of Paris in Step 5.
I noted my preferences above and found the best results
with Dawn � and Head and Shoulders � .
- Weigh the 4-oz. waxed cup to
obtain the tare of the cup.
- Weigh the cup filled with water to
get the volume of the cup. The weight of water in grams
is equal to the volume in mLs.
- Prepare a control sample of
Plaster of Paris (8-oz. by volume) and 1 4-oz. cup of
water. Mix thoroughly and fill several sample cups and
label control. Allow the sample to set at least 24
hours.
- Prepare a second mixture of
Plaster of Paris (2 4-oz. cups by volume, 1 cup of
water). Mix thoroughly and stir in 3 4-oz. cups of foam
from step 1. Make sure the mixture from one is shaken
well and measure out the foam exactly.
- Make at least 3 samples of the
mixture and set aside for 24-48 hours to allow the
samples to dry.
- Samples in step 6 should be
weighed on a triple beam balance to see if they lose
weight over a period of time.
- After waiting at least 24 hours,
weigh and record each sample less the tare weight of the
waxed cup.
- We are now ready to compare the
density of each sample and the control. Density is a term
used to compare the heaviness of the same volume of
different substances.
A solid that has a higher density than water will sink to
the bottom of the beaker.
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D =
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M
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=
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130 g of
sample
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= 1.11 g/mL
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V
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117 mL of H2O
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A solid that has a lower density than
water will float to the top of the beaker.
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D =
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M
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=
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64 g of
sample
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= 0.55 g/mL
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V
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117 mL of H2O
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- Calculate the density of the
control. Is it more than one? Will it float?
- Calculate the density of Samples
1, 2 and 3. Will they float? Why does it act the way it
does?
- Place all four samples in the
container of water and observe the reaction of each
sample. From your calculations, did you get the right
answers?
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