Developers:


Sharon J. Kelly, M.Ed. Upper Dublin Schools Ft. Washington,
PA

Andrew L. Meyer, Ph. D., Senior Scientist Rohm and Haas
Company Spring House, PA 




Grade Levels: 

Primary: This unit is a series of four experiments that allows
primary students to use the study of density to learn the scientific
process. The children will have an opportunity to manipulate the materials
and measures themselves, record data, and draw conclusions while working
in whole group situations with teacher direction and/or in small
cooperative groups. They will record their data on a rank order line where
they will work on the math concepts of greater than, less than, and equal
to. They will write up their conclusions using correct scientific language
and format. They will draw a concluding picture of how the density of
materials affects their lives. Questions and ideas need to be encouraged
and seen as both an opportunity to teach and as a springboard for further
investigation. "Let's find out" is the optimal phrase.
Fourth and Fifth Grades: This unit could be used as an opportunity
to reinforce the math skill of division. By using a scale or a primary
balance with weights and by measuring volume, the students can obtain the
numerical values needed to calculate the bulk density of materials. In
another of the experiments, the students obtain the data needed to
determine the absolute density. For example, if 30 ml of corn syrup
weighed 36 g, the absolute density of corn syrup is calculated by 36 g /
30 ml = 1.20 g/ml.
Older students could also write up the experiments and their
conclusions whereas the younger students would be dependent on oral
discussion to express their ideas. 



Disciplines: 

General Science, Language Arts, Math 



Student Goals: 

On completion of this unit, the student will have:
 Understood that science is a tool we use to make sense of what we
observe and to gain new knowledge.
 Learned the elements of the scientific method, i.e., careful
measurements and observation, changing one variable at a time,
generating and testing hypotheses, and repeatability.
 Learned to record results.
 Learned to draw inferences and connect his knowledge to his
world.
 Understood that the scientific process is something he can do.




Student Objectives: 

On completion of this unit, the student will be able to:
 Explain orally the relationship between density, weight, and volume.
 Explain by way of example the difference between bulk and relative
density.
 Explain orally how the relative density of a material determines
whether it sinks or floats.
 Make hypotheses about the density of materials.
 Work independently or in a small group to experimentally obtain data
on the density of self selected materials.




Background: 

Density is defined as the ratio of the mass of a material to its
volume. It may be understood in three senses:
 Absolute Density: the density of a material in its closest packed
form.
 Relative Density: the density of a material relative to another
material, commonly water.
 Bulk density: the average density of a material which consists of
individual macroscopic particles, i.e. not atoms or molecules. Bulk
density may change with the degree of compaction, e.g. freshly fallen
snow vs. a packed snow ball.
For most purposes the absolute density of a liquid is equal to the
relative density of that liquid compared to water.
For a given volume, the weight of a material is directly proportional
to its density. Thus, a more dense material is heavier for a given volume.
In other words, a more dense material weighs more for a given volume than
does a less dense material.
For a constant weight, the volume of a material is inversely
proportional to its density. Thus a more dense material occupies less
volume than does the same weight of a less dense material. 



Introduction of These Concepts to the
Students: 

 Teach the concept of volume: the space that something
occupies.
Use a balloon to demonstrate that the volume of an empty
balloon is less than the volume when it is blown up.
 Show the students that they are already aware of bulk
density.
For example: Act out the snowflake example. Get 10
sheets of paper, crumple 5. Compare the volumes of the crumpled paper to
the flat paper. Weigh a given volume of Rice Crispies and then
measure the weight and volume after crushing them. Weigh and compare
the volume of popcorn before and after popping. (Then eat it.) These
examples should lead the children to conclude that the same material has
a greater density when its parts are close together than when they are
far apart.
 The concept of relative density will be introduced when the students
observe that materials like rice, which were observed to have a bulk
density less than water, actually sink in water. They should either
conclude on their own or will be told that the low bulk density occurs
because there is air between the particles. (Bulk density is actually
the average density; it is the sum of the weight of the particles and of
the air between them divided by the volume.)




Experimental Notes: 

It is critical that the volume of the materials used for experiments
1,2, and 4 be the same for each determination. Any measuring device may be
used so long as the volume can be controlled to be the same for all
experiments. We are recommending baby food jars because they are readily
available and in our experience are relatively uniform in volume; our jars
averaged 124+/ 1 ml. In each case care must be taken to fill the jars
completely to the top. (Of course, materials whose particles are large
like stones or marbles will not allow complete filling of the jars.
When working with primary students, their inability to accurately
measure materials and read a calibrated cylinder will result in data that
would be found incorrect if more sophisticated methods were used by more
sophisticated students. They will not be able to measure the small
difference in water and cooking oil. The concept of the density of one
material being similar to another material is acceptable for our purposes
in the primary grades.
If using a double pan balance, we recommend pennies as the weights
because they are uniform in weight. Moreover, the weight of each penny is
relatively small allowing small increments to be measured. 





Experiment 1: Ranking Substances by Density 

Goals: 

 The students will learn the experimental technique.
 The students will become active scientists.
 The students will understand that the study of science requires
precision and repeatability.


Objectives: 

 The students will learn the experimental techniques necessary for
Experiment
 a. The students will use a primary balance or a scale
correctly.
b. The students will transfer the materials completely.
 The students will record data by using the rank order line for
primary grades or by tabulating the calculated densities when the
students are capable of division.
 The students will be actively involved in the experiment.
 The students will cooperate with others to complete the scientific
tasks.


Materials: 

baby food jars primary balance pennies used as weights jars
filled with water, sand, corn syrup, Rice Crispies, ground
coffee colored paper in the shape of baby food jars white paper in
the shape of baby food jars rank order line made by the
teacher


Procedure: 

 Estimate the order of density of the substances:
A. With all the
jars marked as to their contents, pass the jars around to the children
and ask them to make a judgement about Encourage the children to use the
terms greater than, less than, shapes and placing them on the rank order
line.
 Test the student's judgments by weighing the contents of the
jars:
With teacher supervision: A. Have the students weigh the
jars using either a primary or a scale. B. Have the students record
the weight or the number of pennies on a white jar shape and place it on
a rank order line. White jar shapes are used to distinguish data
determined by measurement from data determined by estimation or
hypothesis which was recorded using the colored jar shapes. C. Ask
the students if they have drawn any conclusions or want to make a
hypothesis about their observations. Keep in mind that some conclusions
are also hypotheses which may need to be tested. (A hypothesis is an
unproved statement which accounts for the observations or
conclusions.) D. Record conclusions and/or hypotheses for future
reference.
 Group ranking of additional materials:
A. Divide the class into
groups. B. Give each group a jar containing one of the initial test
materials. Also give each group jars containing 3 additional materials.
Each group should get the same 3 additional materials. C. Have the
students repeat Step 2 in their groups. D. Bring the class together
and report and record their results on the rank order line. All groups
should get approximately the same results if the differences in
densities between materials are not too small to measure. The teacher
could ask the students to make hypotheses as to why their results
differed. For older students it would be good to emphasize that
repeatability is the hallmark of good science and they should be able to
duplicate their results. F. Ask the class to suggest ways to solve
the problem of how to rank materials whose density is similar. Good ways
might be to measure them using weights with a smaller increments between
them such as plastic poker chips or matches, or to use larger amounts of
the materials so that the incremental difference in weights would be a
smaller fraction of the total weight. Discuss the merits of other ideas
and test them if time allows. Younger students may be satisfied to know
that the density of some material is equal to the density of
another.






Experiment 2: Further Ranking Substances by Density 

Objectives: 

 To further involve the students in recognizing the concept of
density in their everyday experience.


Materials: 

Have the students bring in materials of their choosing from home in
filled jars.Other materials to be included are rice, cooking oil, wood
(These materials and their density will be needed for Experiment 3 also.)


Procedure: 

 1. Have the students estimate the rank order of their
materials.
A. Circulate some of the jars containing materials from
Experiment 1 and the jars containing materials the students brought
in. B. Have the children write a sentence or two predicting where
their materials would be positioned on the rank order line and why.
Remind them to think of their conclusions or hypotheses from Experiment
1. This could be done individually or in small groups.
 Rank the materials by density.
A. Repeat steps 2 A, B, and C from
Experiment 1. B. Ask the children to discuss why their prediction was
close or not to the observed ranking.






Outcomes from Experiments 1 and 2 



In these two experiments the students have made subjective judgments
about the rank order of several materials by density and have tested
whether these judgments were born out by experimentation.
They have learned the usefulness of recording their results.
They have been asked to draw conclusions and make hypotheses based on
the observations from Experiment 1 which were or could be tested in
Experiment 2. 







Experiment 3: The Inverse Relationship of Volume and Density


Goals: 

 The student will observe that if the weight of several materials is
held constant, the volume will be found to be greater for less dense
substances.
 The students will discover the concept of relative
density.


Objectives: 

 The students will be able to provide reasons orally or in writing
for the rank order established by testing.


Background: 

In this experiment the students will be learning another way of
evaluating or ranking materials by density. By placing solid materials in
water only the volume taken up by the particles is measured. The effect of
the air between the particles is eliminated. This will allow the students
to recognize the differences that will lead to the concept of relative
density. Once this concept is understood, it will allow them to better
understand their world by understanding the principle which explains why
things float or sink in water. 

Materials: 

primary balance or scale 100 ml. plastic graduated
cylinders selected materials from Experiment 1 and 2 or other
appropriate materials. These should include solids that float such as
wood or butter and at least one liquid that floats such as cooking oil.
Rice should also be included. (See below.) pennies for
weights


Procedure: 

 Select an arbitrary weight to be used for the experiment. This
should be large enough so that materials with large, heavy particles can
be used and small enough so that amount of the smaller, less dense
particles is not too great to put into a cylinder.
 Have the students predict whether the materials used will float or
sink using the chart in the appendix to record their prediction. The
chart will be completed as the experiment continues.
 Put about 40 ml of water in the plastic graduated cylinder. You may
want to mark that level with tape so the students can clearly see the
beginning water level. Younger students may have difficulty reading
small increment marks.
 If a primary balance is used, balance an empty jar with pennies or
other suitable weights. In the case of a scale weigh the empty jar; the
teacher will then have to make the children understand that the weight
of the material is the difference between the weight of the jar with its
contents and the weight of the empty jar.
 Add the amount of each material to the jar until it balances or
until it has the required weight.
 Place the contents of the jar into the cylinder and measure the
increase in the volume of the water. If the material floats, it will be
necessary to use a rod or some other device to push it just under the
surface of the water, taking care not to displace additional water with
the rod.
For liquids, be they less dense or more dense than water,
the water displacement is superfluous because their volume could be
measured directly in the cylinder. However it is recommended to use the
same experimental method for all the determinations so as not to confuse
the students. Older students might not be confused by using different
procedures to get the volume. For them it could be instructive to
measure the volume of the liquids by both methods to show that the
results are the same. Likewise, they should get the same numerical value
for the density of the liquids that they determined in Experiments 1 and
2.
 Record the number of ml of water displaced on a white cut out jar
and place it on the ran order line.
 Repeat the procedure with the remaining materials completing the
chart after each material.
 With all the students able to see the rank order line from
Experiment 1 and 2 and the rank order line from Experiment 3, discuss
the similarities and differences and why they may have occurred.
Notes: This experiment can be thought of as testing the
hypothesis: If popped popcorn is less dense than the same weight of
unpopped corn, then its volume should be greater.
Plastic graduated cylinders are recommended so the students can handle
them safely. They are inexpensive and available from scientific supply
houses. Other measuring devices could be substituted so long as they have
narrowly spaced gradations.
In this experiment students should conclude that materials less dense
than water will float on it. But they also will observe that rice which
had a bulk density less than water sinks. This observation is an
indication that the students' current understanding of density is
incomplete and that their "theory" must be modified to incorporate the
fact that some materials which seem to be less dense than water
sink. In this way they can come to an understanding of the concept of
relative density. Perhaps they will independently come up with the idea
that the air between the particles makes the bulk material appear less
dense than the particles that make it up.






Experiment 4: Using Flotation as a Way of Evaluating the Density of
Materials 

Background: 

The greater the weight of the cargo, the lower a boat floats in water.
Thus the depth to which a floating container sinks in water is a measure
of the weight in the container. This idea can be used in place of, or in
addition to, a balance and weights to rank the weight of a given volume of
a material.


Objective: 

 The children will discover another way of determining the rank order
of materials by density.


Materials: 

a 1 or 2 liter transparent soda bottle with the neck cut off many
smaller diameter plastic bottles whose length are much greater than their
width or diameter and which fits inside the soda bottle with out too much
friction. A scale in centimeters or quarter inches should be marked on the
side of these bottles. (See drawing below.) test materials from
Experiment 1 and 2 baby food jars from Experiment 1 and 2 pennies or
other suitable ballast to keep the inner bottle floating upright if
necessary rank order line jar shapes for recording data


Procedure: 

Review with the students what they learned about density when they used
the balance. Discuss that now they are going to use another method of
determining the rank order of materials. As an introduction the children
could be asked to discuss how the weight of the material in a boat effects
how it floats.
With the students doing the work, but the teacher directing the
experiment:
 Fill the soda bottle half full of water.
 Have the children predict the order of the materials based on their
previous experiments and record their predictions using colored cut outs
as before.
 Place the contents of the baby food jar into the inner container.
 Float the inner container in the soda bottle. Observe how low it
floats based on the markings on the side of the inner container.
 Have the children record the observation on white cut outs and place
them on the rank order line.
 Alternatively, after a few demonstrations, the children can be
divided into groups. They would be instructed to make the measurements
on several materials themselves and then report back to the whole group
later.


Conclusion: 

Bring the children together with the three rank order lines and ask
them what conclusions they can draw from the experiments about the weight
of the materials and their densities. They should be asked to compare the
rank order of the materials as determined in Experiment 1 and 2 and in
Experiment 4. They should see that the rank order is the same and conclude
that this method also confirms the results of Experiment 2.
What did they learn from Experiment 3? Help them see the inverse order
of the materials on the rank order line. The densest material, the one
that required the least volume to balance the assigned weight, took up the
least amount of space when put into the water.
The children should be asked to think about how this knowledge of
density could be useful in their lives. Examples might be deciding how
full they can fill their cereal bowls with cereal that is more or less
dense than milk. Or how to pack their suitcase to get the most clothes in
it. Younger children might enjoy drawing a picture depicting how the
density of a material affects their lives. 

Extensions: 

 Test different kinds of cereals to see which are more dense or less
dense by adding them to milk. Have the children bring in many different
kinds of cereals.
 Giving each student or each group of students a ball of clay or a
piece of aluminum foil, have them form a boat that will float when an
equal amount of a designated material is put in it. Have a big tub of
water and allow the students to learn by trial and error. When they have
built a boat that floats with the material in it, have them draw a
picture of it. Tell them to take care to get the length and width
measured exactly. When all the pictures have been drawn, discuss as a
group what kind of boat was needed to float the material. Have the
children predict what kind of boat would be needed to carry a heavier
material or a lighter material. They could make those boats as time
allowed.
