SWENext
Engineering Challenge
SWENext Engineering Challenge #17
Dynamic Density Bottle
This month you will get to make your own toy, Dynamic Density Bottle, using simple household
material. The challenge will be to study the behavior of both solids and liquids and see how you can use
this information to build your own toy just the way you want it! This activity includes a great worksheet
which will guide you through making observations and learning about how the toy works! Think about
how this experiment relates to what you are learning in school and ask your parents or teachers to help
along … they will enjoy this challenge as well 
After you finish making your first bottle from the directions, start thinking of how you can change
what’s inside. How will your design change if you change what is high density and low density …
using different colored beads or Legos? What if you use different ratios of liquids? Make a prediction
and test it out!
How did your Dynamic Density Bottle turn out?
Send us pictures of your creation. Good luck and have fun.
#SocialMedia
Share your pictures and videos from this activity on Facebook, Instagram or other social media with us
and other SWENexters.
Use the hashtags:
#SWENext #BeThatEngineer
Activity
pubs.acs.org/jchemeduc
The Dynamic Density Bottle: A Make-and-Take, Guided Inquiry
Activity on Density
Thomas S. Kuntzleman*
Department of Chemistry, Spring Arbor University, Spring Arbor, Michigan 49283, United States
S Supporting Information
*
ABSTRACT: An activity is described wherein students observe dynamic floating and sinking behavior of plastic
pieces in various liquids. The liquids and solids are all contained within a plastic bottle; the entire assembly is
called a “density bottle”. After completing a series of experiments that guides students to think about the relative
densities of both the liquids and solids in the bottle, students are able to explain the curious floating and sinking
phenomena. As a part of the activity, students construct their own bottles and are encouraged to describe to
others how the density bottle works. These bottles can be constructed using inexpensive and easily obtained
materials. The level of inquiry involved in the activity can be tailored to meet the particular interests and needs of
students. Modifications to the density bottle, including an engaging one that uses LEGO pieces, are discussed.
KEYWORDS: General Public, Elementary/Middle School Science, High School/Introductory Chemistry,
First-Year Undergraduate/General, Public Understanding/Outreach, Hands-On Learning/Manipulatives,
Inquiry-Based/Discovery Learning, Physical Properties
■
INTRODUCTION
In this activity students construct a density bottle1 that contains
salt water (17% NaCl by mass, density = 1.12 g mL−1),2
isopropyl alcohol (density = 0.79 g mL−1), plastic pieces
composed of low density polyethylene (LDPE, density = 0.92
to 0.94 g mL−1),3−5 and plastic pieces composed of polystyrene
(PS, density = 1.05 to 1.07 g mL−1).3,5,6 When the contents of
the bottle are at equilibrium, the salt water and isopropyl
alcohol are separated7 into two layers and the plastics rest at the
interface between the two liquid layers (Figure 1, panel at 0 s).
When the bottle is thoroughly shaken, the salt water and
alcohol mix to form an emulsion. Because the density of the
pieces composed of LDPE (Figure 1, blue pieces) is less than
the density of the emulsion, these pieces float on top of the
emulsion (Figure 1, 10 s). On the other hand, the pieces of PS
(Figure 1, yellow pieces) are more dense than the resulting
emulsion, so these pieces sink to the bottom of the emulsion
(Figure 1, 10 s). Over time, the emulsion separates back into
distinct layers of salt water and isopropyl alcohol. As this
separation occurs, the LDPE plastics float at the isopropyl
alcohol−emulsion interface (below the alcohol but above the
emulsion; Figure 1, 20 s and 40 s), because LDPE is less dense
than the emulsion but more dense than the alcohol.
The PS pieces float at the salt water−emulsion interface
(below the emulsion but above the salt water; Figure 1, 20 and
40 s), because PS is less dense than salt water and more dense
than the emulsion. As the emulsion continues to separate back
into salt water and isopropyl alcohol, the plastic pieces move
closer to one another until they meet at equilibrium at the salt
water−isopropyl alcohol interface (Figure 1, 60 s).
To introduce this activity, students are shown a large (1 L
volume), completed density bottle. The bottle is shaken, and
students are instructed to observe the dynamic process of
plastics separating and recombining as previously described.
Almost all observers find the separation and recombination of
plastics that occurs to be a very curious effect. However, no
explanation for how this effect takes place is provided. Rather,
Figure 1. Time dependent floating and sinking behavior observed for
LDPE plastic (blue pieces) and PS plastic (yellow pieces) in a
thoroughly mixed isopropyl alcohol−salt water mixture. Left to right, 0
s, 1 s, 10 s, 20 s, 40 s, and 60 s after mixing. Bottle shown is 23 cm
high, 19 cm in diameter, and holds 500 mL of fluid.
© 2015 American Chemical Society and
Division of Chemical Education, Inc.
Published: February 25, 2015
1503
DOI: 10.1021/ed500830w
J. Chem. Educ. 2015, 92, 1503−1506
Journal of Chemical Education
■
INTEGRATING THE ACTIVITY INTO THE
CURRICULUM
This activity provides students with an opportunity to
participate in inquiry-based learning, an important component
of the Next Generation Science Standards (NGSS).9,10 Within
the NGSS, science teachers are encouraged to provide students
with opportunities to engage in inquiry-based learning through
(1) asking questions, (2) developing and using models, (3)
planning and carrying out investigations, (4) analyzing and
interpreting data, and (5) constructing explanations. As
students complete this activity they participate in the latter
two of these five practices. Because this lesson does not delve
into higher levels of inquiry learning, and because it has been
argued that students should be gradually introduced to inquirybased activities,11 this lesson could provide a nice starting point
for students and instructors just starting to incorporate inquirybased practices in the classroom.
students are encouraged to complete a laboratory experience
that guides them through an understanding of how the density
bottle works. This activity is best characterized as a guided
inquiry lesson as defined by Buck, Bretz, and Towns:8 the
instructor poses a question and students are provided with
procedures to perform, while students independently analyze
results and draw their own conclusions on how the density
bottle works.
As students conduct this activity, they are introduced to
several topics and procedures often covered at the beginning of
many chemistry courses: density, heterogeneous mixtures,
solubility, miscibility, and filtration. As a result, this activity
makes for a good laboratory experience for students to conduct
at the beginning of the school year or semester. This activity
can also be used to discuss topics normally taught later in the
year, such as polarity and intermolecular forces. As an added
motivator, students are encouraged to keep their constructed
density bottles and to teach others how the bottle works.
■
Activity
■
MODIFICATIONS TO THE PRESENTATION OF THE
ACTIVITY
Modifications of this activity allow for higher levels of inquiry
learning (and more of the NGSS practices) to be integrated
into this lesson. For example, instructors might choose to only
show how the density bottle operates and then ask students to
design their own experiments to help explain the inner
workings of the bottle. Alternatively, students could be shown
the bottle and then asked to construct a working density bottle
on their own. Also, students could be asked to explore how
plastics of different composition respond in the density bottle
(see also Modifications to the Density Bottle). In all of these
learning scenarios students are immersed in all five of the
above-stated NGSS practices. The author indeed observed this
to be the case when some of his students successfully
constructed a working density bottle as part of a three-week
project conducted at the end of the general chemistry
laboratory sequence.12
This activity may also be modified to simply be a hands-on
science experiment without an inquiry component. For
example, K−8th grade students at a summer science camp
and other outreach events13 directed by the author have
successfully constructed density bottles by following written
instructions (see Supporting Information). In these venues,
explanations on how the bottle works are provided to the
students.
ABOUT THE ACTIVITY
After observing a demonstration of how a completed bottle
works, students are given a laboratory sheet (see Supporting
Information) that describes a series of experiments to carry out.
These experiments focus the student’s attention on the
scientific concepts necessary to describe the processes at
work in the density bottle. First, students measure the density
of isopropyl alcohol, water, and salt water. Keeping these results
in mind, students test to see if PS and LDPE pieces float in
alcohol, water, and salt water. From observations of floating and
sinking and the measured densities, students are asked to
predict the relative densities of the three liquids, LDPE, and PS.
After conducting observations related to density, students
test the miscibility of isopropyl alcohol, first with water and
then with salt water. Students are encouraged to carefully
observe what happens when salt water and isopropyl alcohol
are vigorously mixed together. When doing this, students note
that the two liquids initially mix and then begin to separate.
During the separation, three layers of liquid are discernible: A
bottom layer composed of salt water, a top layer composed of
isopropyl alcohol, and a middle layer composed of a
heterogeneous mixture of salt water and isopropyl alcohol: an
emulsion of salt water and isopropyl alcohol. From the position
of the emulsion relative to the isopropyl alcohol, students are
asked to predict if the density of the emulsion is greater than or
less than the density of the isopropyl alcohol. Similarly,
students predict if the density of the emulsion is greater than or
less than the density of the salt water.
After completing the density and miscibility experiments,
students move on to construct their own density bottle.
Students discard half of the water from a container of bottled
water, and salt is dissolved into the remaining water. The
resulting salt water is filtered to remove insoluble additives.
After the filtered water is returned to the water bottle, isopropyl
alcohol is added, pieces of PS and LDPE plastic are added, and
the bottle top is secured with duct tape. With the results of the
previously performed experiments in mind, students investigate
the movement of plastics within the density bottle after it is
vigorously mixed. Armed with their previous observations,
students describe why the PS plastics first sink, then rise and
why the LDPE plastics first rise, then sink within the bottle.
■
MODIFICATIONS TO THE DENSITY BOTTLE
Plastic pieces composed of high density polyethylene (HDPE,
density = 0.95 to 0.97 g mL−1)3,6,14 or polypropylene (PP,
density = 0.90 to 0.91 g mL−1)3,6,14 may be substituted for
plastic pieces made of LDPE in this activity. Similarly, plastic
pieces manufactured from acrylonitrile butadiene styrene (ABS,
density = 1.00 to 1.08 g mL−1)15 can substitute for plastics
made from PS. LEGO pieces, which are composed of ABS,16
are a substitution that students find particularly engaging.
Interestingly, a density bottle that uses only LEGO pieces can
be constructed. Because LEGO pieces are composed of ABS,
these pieces float on salt water and sink in the alcohol−salt
water emulsion (Figure 2, pieces on the bottom). However,
LEGO pieces can be made to float on the alcohol−salt water
emulsion in the following way. When two LEGO pieces are
connected, air is trapped between the interlocking pieces. The
trapped air causes the density of linked LEGO pieces to be less
1504
DOI: 10.1021/ed500830w
J. Chem. Educ. 2015, 92, 1503−1506
Journal of Chemical Education
■
Activity
ASSOCIATED CONTENT
S Supporting Information
*
Self-paced, guided inquiry laboratory instructions for students
to follow; recipe for constructing density bottles using 70%,
91%, or 99% isopropyl alcohol. This material is available via the
Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
I wish to thank Lynn Higgins and Bruce W. Baldwin for helpful
discussion. I also thank Lynn Higgins for graciously permitting
me to describe her outstanding original activity with the
extensions reported herein.
■
(1) The density bottles constructed are similar to the poly density kit
sold by Educational Innovations. See: http://www.teachersource.com/
product/poly-density-kit/density. The poly density kit sold by
Educational Innovations was originally developed by Lynn Higgins.
See: http://www.polymerfun.org/.
(2) CRC Handbook of Chemistry and Physics, 67th ed.; Weast, R. C.,
Ed.; CRC Press: Boca Raton, FL, 1986.
(3) Kolb, K. E.; Kolb, D. K. Method for Separating or Identifying
Plastics. J. Chem. Educ. 1991, 68, 348.
(4) Perler beads (see http://www.eksuccessbrands.com/perlerbeads/
) are a good, decorative source of LDPE for this activity.
(5) Craft “Pony” Beads (see, for example http://www.walmart.com/
ip/19515698) work well as a decorative source of PS for this activity.
Plastic cutlery, cups, plates, and bowls are also a good source of PS.
(6) Harris, M. E.; Walker, B. A Novel, Simplified Scheme for Plastics
Identification: JCE Classroom Activity 104. J. Chem. Educ. 2010, 87,
147−149.
(7) The alcohol and salt water separate due to the “salting out” effect.
See: Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for
Teachers of Chemistry; University of Wisconsin Press: 1989; Vol. 3, pp
266−268.
(8) Buck, L. B.; Bretz, S. L.; Towns, M. H. Characterizing the Level
of Inquiry in the Undergraduate Laboratory, The Many Levels of
Inquiry Science and Children. J. Coll. Sci. Teach. 2008, 38, 52−58.
(9) For more information on the Science and Engineering Practices
in the NGSS, see: http://www.nextgenscience.org/sites/ngss/files/
Appendix%20F%20%20Science%20and%20Engineering%20Practice
s%20in%20the%20NGSS%20-%20FINAL%20060513.pdf. Accessed
October 2014.
(10) Cooper, M. M. Chemistry and the Next Generation Science
Standards. J. Chem. Educ. 2013, 90, 679−680.
(11) Buck, L. B.; Towns, M. H. Preparing Students to Benefit from
Inquiry-Based Activities in the Laboratory: Guidelines and Suggestions. J. Chem. Educ. 2009, 86, 820−822.
(12) For more information on these three week projects, see: http://
www.jce.divched.org/blog/small-research-projects-chemistryclassroom and also http://www.jce.divched.org/blog/my-favoritesmall-research-project-year.
(13) Kuntzleman, T. S.; Baldwin, B. W. Adventures in Coaching
Young Chemists. J. Chem. Educ. 2011, 88, 863−867.
(14) Plastic, one-gallon milk jugs or opaque detergent bottles are
good sources of HDPE; bendable plastic drinking straws are a good
source of PP.
(15) Liu, X.; Namura, M.; Liu, Y.-H.; Ishitani, K.; Fujita, K.
Saturation Swelling of ABS Latex Particles by Styrene and
Figure 2. Floating and sinking of LEGO pieces in the density bottle.
than the density of solitary LEGO pieces. Connecting a LEGO
plate to another LEGO plate17 (Figure 3C) confers just the
Figure 3. LEGO pieces. A: 1 × 2 plate (sinks in emulsion). B: 1 × 2
brick (sinks in emulsion). C: connected 1 × 2 plates (floats on
emulsion). D: A and B connected brick-on-plate (not used; floats on
alcohol). E: A and B connected plate-on-brick (floats on emulsion). F:
two connected 1 × 2 bricks (not used, floats on alcohol).
right amount of trapped air to give the assembled pieces a
density sufficient to sink in isopropyl alcohol but float on the
emulsion. Likewise, the grouping that results from connecting a
LEGO plate piece on a LEGO brick piece (Figure 3E) will also
sink in alcohol and float on the emulsion (Figure 2, pieces at
the top). On the other hand, connecting a brick on a plate
(Figure 3D) or two bricks (Figure 3F) traps too much air, and
the resulting LEGO pairs float on alcohol. As a result, LEGO
pieces attached plate-on-plate (Figure 2C) or plate-on-brick
(Figure 3E) are a fun substitution for LDPE pieces in this
activity.18 Discovering and explaining how some LEGO pieces
float, then sink in the density bottle while others sink, then float
is quite a challenging exercise for students.
■
REFERENCES
SAFETY
Isopropyl alcohol is flammable and should be kept away from
open flames. Completed bottles should be secured with duct
tape to keep students from accessing the contents within the
density bottles.
1505
DOI: 10.1021/ed500830w
J. Chem. Educ. 2015, 92, 1503−1506
Journal of Chemical Education
Activity
Acrylnonitrile Monomer Mixtures. Ind. Eng. Chem. Res. 1997, 36,
1218−1223.
(16) See: http://parents.lego.com/en-us/lego-and-society/50thbirthday.
(17) LEGO 1 × 2 plates (design ID 3023) and 1 × 2 bricks (design
ID 3004) can be ordered at the LEGO website: http://shop.lego.com/
en-US/Pick-A-Brick-ByTheme.
(18) It should be noted that the connected LEGO pieces are not
completely airtight. Over the course of several weeks, fluid will leak
into the trapped air pockets, causing the connected LEGO pieces to
increasingly behave like unconnected LEGO pieces. This situation is
easily corrected by periodically replacing connected LEGO pieces.
Also, it has been observed that some LEGO pieces (specifically red,
green, and yellow) contain dyes that will slowly dissolve into the
isopropyl alcohol, turning it a slight yellow color. This yellow color is
usually observed after a few days. Finally, the neck of the LEGO
Erlenmeyer flask held by the scientist was observed to disintegrate
after a few hours in density bottle.
1506
DOI: 10.1021/ed500830w
J. Chem. Educ. 2015, 92, 1503−1506
Supporting Information for:
The Dynamic Density Bottle: A Make-and-Take, Guided Inquiry Activity on Density
Thomas S. Kuntzleman, Department of Chemistry, Spring Arbor University, Spring Arbor, MI
Information for Teachers for the Dynamic Density Bottle Activity
Introduction – p. 2
Presentation – p. 3
Modifications to Increase Opportunities for Inquiry Learning – p. 4
Sources of PS and LDPE – p. 4
Alternative Sources of Plastics – p. 5
Extensions using LEGO Pieces – p. 6
Considerations when using LEGO Pieces – p. 6
Different Recipes – p. 7
Student Worksheet: Construction of a Density Bottle – pp. 9 – 13
Typical Answers to Questions in Student Worksheet- pp. 14 – 15
Dynamic Density Bottle – 70% isopropyl alcohol recipe – p. 16
Dynamic Density Bottle – 70% isopropyl alcohol recipe – p. 17
Dynamic Density Bottle – 70% isopropyl alcohol recipe – p. 18
Information for Teachers for the Dynamic Density Bottle Activity
Introduction In this experiment students use simple materials to make an interesting toy called a
Dynamic Density Bottle (Figure 1). This toy was invented by Lynn Higgins, a retired public school
teacher from Chicago. You can learn more about Lynn (and also more about polymers) at
www.polymerfun.org. The Dynamic Density Bottle illustrates the concept of density in a thoughtprovoking way. Inside the bottle are two different liquids and two different plastics. These liquids
and plastics all have different densities and as a result layer according to density (Figure 1). The two
liquids in the bottle are isopropyl alcohol (D = 0.79 g mL-1) and salt water (17% NaCl by weight, D
= 1.12 g mL-1). The two plastics in the bottle are low density polyethylene (LDPE, D = 0.93 g mL-1)
and polystyrene (PS, D = 1.05 g mL-1). Because salt water is more dense than alcohol, it sinks
beneath alcohol. Both plastics are more dense than alcohol but less dense than salt water. Thus, the
plastic pieces float at the interface between the two liquids (but notice the more dense, yellow PS
pieces are situated beneath the less dense, blue LDPE pieces in Figure 1). Alcohol and 17% NaCl
solution are immiscible; that is to say these liquids will separate from one another if they are mixed
(two miscible liquids will dissolve one another when mixed). However, upon first mixing salt water
and isopropyl alcohol, the two liquids initially appear to form a single, mixed liquid. This metastable
mixture is called an emulsion (Figure 2, page following, panel at ~1 s).
Figure 1: The Dynamic Density Bottle. The liquids and solids layer according to density. From
top to bottom: alcohol, blue pieces comprised of LDPE, yellow pieces comprised of PS, 17 % NaCl
solution (see also Figure 2, panel at 0 s). Bottle shown is 23 cm high, 20 cm in diameter and holds
500 mL of fluid. See http://www.sparklingice.com/product/blackraspberry for an example of a
product with a bottle of these dimensions.
Figure 2: Time dependent floating and sinking behavior observed in LDPE plastic (blue pieces) and
PS plastic (yellow pieces) in a thoroughly mixed isopropyl alcohol-salt water mixture. Left to right, 0
s, 1 s, 10 s, 20 s, 40 s and 60 s after mixing. Bottle shown is 23 cm high, 20 cm in diameter and
holds 500 mL of fluid.
The emulsion has a density that is approximately 1.0 g mL-1. Thus, the LDPE plastic (D = 0.93 g
mL-1) pieces float in the emulsion, whereas the PS pieces (D = 1.05 g mL-1) sink in the emulsion
(Figure 2, panel at 10 s). Over time, the emulsion separates back into alcohol and salt water and the
plastic pieces return to their original position (Figure 2, panels at 20 s – 60 s).
Presentation:
The Dynamic Density Bottle can be used as an instructional tool in a variety of ways. I happen to
currently use it during laboratory instruction. I show my students a large, completed Dynamic
Density Bottle. After shaking the bottle and observing the behavior of the contents within the
bottle, students are challenged to describe their observations using scientific principles. Students
complete a self-paced worksheet (see pages 9 – 13 of this document) to help them complete this
task during a single laboratory period. This worksheet guides students through a series of
experiments that facilitates an understanding of how the Dynamic Density Bottle works. The
worksheet follows a guided inquiry model in which students are provided with a question to answer,
the necessary background to answer the question, and procedures to carry out that will help to
explain the question posed. Equipped with the results of experimentation, students are challenged
to explain the physics and chemistry behind how the Dynamic Density Bottle works. As written, I
find that most students require between one and two hours to complete the worksheet, construct a
working Dynamic Density Bottle, and to sufficiently explain what is going on when the bottle is
shaken. In the student worksheet, students make an 8 ounce Dynamic Density Bottle using 91%
isopropyl alcohol.
Modifications to Increase Opportunities for Inquiry Learning:
Teachers are free to modify the student worksheet as they see fit to either increase or decrease
opportunity for student inquiry. For example, instead of explicitly stating which particular plastic is
high density and which is low density, the plastics could be identified by color (or type). In this case,
students would need to discover the relative densities of the plastics on their own. A particularly
tricky twist on this lesson is to eliminate the discussion of emulsions. This allows students to figure
out what is happening on their own as the alcohol and salt water is mixed. If you make these
modifications to the lesson, be sure to modify the student worksheet accordingly. Finally, students
can simply be shown a working Dynamic Density Bottle and challenged to construct one on their
own – no student worksheet provided. This latter approach is most challenging if students are not
allowed to open the Dynamic Density Bottle to explore the properties of its contents. It should be
recognized that each of these modifications require increasing amount of time for students to
complete the task. I have found that students require three to four, 3-hour laboratory periods to
construct a working Dynamic Density Bottle using their own.
Sources of PS and LDPE:
Craft stores or craft sections in retail stories almost all carry Pony beads, which are a nice decorative
source of PS (Figure 3), the higher density plastic in this activity. I have found just about any
translucent, decorative plastic found in craft stores works as a source of (or at least behaves like) PS
in this activity.
Figure 3: Decorative Pony Beads as a source of PS.
Perler or “melty” beads are an outstanding source of LDPE (Figure 4), the lower density plastic in
this activity. These can also be found in craft stores or craft sections in retail stores.
Figure 4: Perler “Biggie” beads (left) and melty beads (right). Perler beads are also sold in sizes
essentially identical to melty beads.
Alternative Sources of Plastics
UV beads (sold by Educational Innovations http://www.teachersource.com/product/ultravioletdetecting-beads/light-ultraviolet or Steve Spangler Science
http://www.stevespanglerscience.com/uv-color-changing-beads.html) may be used as a source of
lower density plastic for this activity (Figure 5). My guess is that these beads are made of HDPE,
but I’m not sure about this. UV beads are a nice option if you want a source of beads that is
essentially identical in size and shape to Pony beads (Figure 3)
Figure 5: UV beads
Plastics pieces comprised of high density polyethylene (HDPE, D = 0.96 g mL-1) or polypropylene
(PP, 0.91 g mL-1) may substitute for LDPE plastic pieces in this activity. Bendable plastic drinking
straws cut into pieces tend to be a good source of PP. Pieces cut from plastic gallon milk jugs and
laundry detergent bottles (the opaque kind) tend to be a good source of HDPE. (Be careful,
because it sometimes gets confusing that HDPE is used as a LOW density plastic in the density
bottle). LDPE, HDPE and PP are all good sources of low density plastic in this activity.
Extension Using LEGO Pieces:
LEGO blocks are composed of acrylonitrile butadiene styrene (ABS, D = 1.00 – 1.08 g mL-1).
Therefore, LEGO blocks can substitute for higher density plastic pieces in this activity.
It is also fun to experiment with LEGO pieces that are connected to one another. When two
LEGO pieces are connected, air is often trapped in between the connected pieces. The trapped air
lowers the density of the assembled pieces. As a result, students can experiment with different
combinations of LEGO pieces and observe resulting floating and sinking behavior (Figure 6).
A
B
C
D
E
F
Figure 6: LEGO pieces. A: 1 x 2 plate (sinks in emulsion). B: 1 x 2 brick (sinks in emulsion). C:
connected 1 x 2 plates (floats on emulsion). D: A and B connected brick-on-plate (not used; floats
on alcohol). E: A and B connected plate-on-brick (floats on emulsion). F: two connected 1 x 2
bricks (not used, floats on alcohol).
Considerations when using LEGO pieces:
It should be noted that connected LEGO pieces are not completely air tight. If LEGO pieces are
left in a density bottle over the course of several weeks, fluid will leak into the trapped air pockets,
causing the connected LEGO pieces to increasingly behave like unconnected LEGO pieces. This
situation is easily corrected by periodically replacing connected LEGO pieces. (However, it is also
fun to have students try to figure out why connected LEGO pieces increasingly behave like
unconnected pieces!). Also, some colors of LEGO pieces (I have specifically found that red, green
and yellow) contain dyes that will slowly dissolve into the isopropyl alcohol, turning it a slight yellow
color. This process occurs over one or two days. Finally, add LEGO minifigure accessories at your
own risk! I once added a minifigure scientist holding an Erlenmeyer flask to a Dynamic Density
Bottle. The neck of the Erlenmeyer flask disintegrated when left in the bottle much longer than a
few hours.
Different recipes:
Three different recipes are provided (see the last three pages of this packet) for constructing
Dynamic Density Bottles. This is because isopropyl alcohol is commonly found at three
concentrations (70%, 91% and 99%). The student worksheet written to accompany this lesson uses
91% isopropyl alcohol, so be sure to change the student worksheet accordingly if you use a different
alcohol concentration. To conserve funds and materials, I have my students make an 8 ounce
Dynamic Density Bottle (Figure 7, bottle on the right).
Figure 7: Comparison of Dynamic Density Bottle sizes. L to R: 1 L, 500 mL, 8 ounces. The
bottles on the left and right have UV beads as a source of LDPE and Pony beads as a source of PS.
However, I use a large Dynamic Density Bottle for classroom demonstrations. To make larger
bottles, the measurements in each recipe need to be converted. Each recipe describes how to make
about 8 ounces of fluid. To increase the amount of fluid yielded, all measures in the recipe are
multiplied by the following conversion factor:
Conversion factor =
ounces desired
8
Example: Suppose you are using 70% isopropyl alcohol and wish to make a 500 mL Dynamic
Density Bottle. Because 500 mL = 17.6 ounces (500 mL / 28.4 mL oz-1 = 17.6 ounces), the
conversion factor is 2.2:
Conversion factor =
17.6
 2.2
8
The recipe using 70% isopropyl alcohol calls for 68 mL of water, 24 g of salt, and 172 mL of 70%
isopropyl alcohol. Multiplying each of these amounts by the conversion factor, we get 150 mL of
water, 53 g of salt and 378 mL of isopropyl alcohol. Thus, 53 grams of salt are dissolved in 150 mL
of water, and the salt water is filtered. Next, 378 mL of 70% isopropyl alcohol are added to the
filtered salt water.
Try this: How much water, salt, and 91% isopropyl alcohol are necessary to make 500 mL of fluid
for a Dynamic Density Bottle?
Answer: 238 mL of water, 53 g of salt, and 290 mL of 91% isopropyl alcohol.
Although I have not tried this particular modification, Lynn Higgins reports that the filtering step
can be eliminated by using Kosher salt or canning salt in place of table salt. When using table salt
the filtering step is carried out to remove some solids that remain undissolved. These undissolved
solids give the fluid layers an undesirably cloudy appearance. I prefer to have students do the
filtering step simply to reinforce (or introduce) this technique.
Student Worksheet: Construction of Density Bottle
Introduction In today’s experiment, you will be using simple materials to make a Density Bottle.
First, be sure to observe how the Density Bottle behaves. Next, use the following worksheet as a
guide to study the properties of water, salt water, isopropyl alcohol, and two different types of
plastic in order to explain how the Density Bottle works.
Materials 91% isopropyl alcohol, table salt, duct tape, digital balance, 8 oz., clear and colorless
water bottle, water, test tubes and stoppers, 10 mL graduated cylinder, 100 mL graduated cylinder,
filter paper, funnel, beakers, several colored plastic pieces of different composition.
Procedure Part A: Densities of the liquids in the density bottle
1.
2.
3.
4.
5.
Place a 10 mL graduated cylinder on the digital balance and tare it.
Measure 4 mL of 91% isopropyl alcohol into the graduated cylinder.
Record the mass of the 91% isopropyl alcohol.
Record the volume of the 91% isopropyl alcohol.
Calculate the density of the 91% isopropyl alcohol using the equation:
D = m/V
(Equation 1)
Where D is density, m is mass and V is volume.
6.
7.
8.
9.
10.
11.
12.
Save the isopropyl alcohol for parts B and D.
Clean the graduated cylinder and allow it to dry.
Measure 5 mL of water into a test tube.
Add 1.1 grams of salt into the test tube.
Mix the contents of the test tube until the salt is dissolved.
Find the density of the salt water using the same procedure used for the isopropyl alcohol.
Save the salt water for later experiments.
Data
Liquid
Density / g mL-1
Isopropyl alcohol
Salt water
Part B: Miscibility of the liquids in the density bottle
1.
2.
3.
4.
5.
Measure 4 mL of water into a test tube.
Add 4 mL of isopropyl alcohol into the same test tube. Mix well.
Observe the contents of the test tube for 1 – 2 minutes.
Are water and alcohol miscible (do they completely mix)? Record your observations.
Now add 1.0 g of salt to the test tube containing water and alcohol. Mix well. If the liquids
do not separate, dissolve more salt until they do separate. Are alcohol and salt water
miscible? Record your observations.
6. Why do you think there is a difference in the behavior of the alcohol/water vs. the
alcohol/salt water mixtures?
______________________________________________________________________________
______________________________________________________________________________
7. Save this test tube containing the water/alcohol/salt mixture for Part E.
Observations:
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
Are water and isopropyl alcohol miscible? ______
Are salt water and isopropyl alcohol miscible? ______
Part C: Densities of the plastics in the density bottle relative to water.
1. Obtain 2 – 5 pieces of higher density plastic.
2. Fill a test tube to about 5 mL with water.
3. Place the plastic pieces in the water. Do the beads float or sink? Why do you think this is so?
Record your results (see next page).
4. Obtain 2 – 5 pieces of higher density plastic.
5. Fill a test tube to about 5 mL with water.
6. Place these plastic pieces in the water. Do the pieces float or sink? Why do you think this is so?
Record your results (see next page).
Data
Plastic
Float or Sink in Water?
Density (greater than or less than 1.0
g mL-1)
High density
plastic
Low density
plastic
Part D: Densities of the plastics in the density bottle relative to alcohol, water and salt water.
1. Obtain a single piece of high density plastic.
2. Place the plastic piece in 91% isopropyl alcohol. Does the plastic piece float or sink? Record
your observations.
3. Place the plastic piece in water. Does the plastic piece float or sink? Record your observations.
4. Place the plastic piece in salt water. Does the plastic piece float or sink? Record your
observations.
5. Repeat steps 1 – 4 with a lower density plastic piece.
Float or sink in 91%
isopropyl alcohol?
Float or sink in
water ?
Float or sink in salt
water?
High density
plastic
Low density
plastic
6. Review the densities you measured for the 91% isopropyl alcohol and salt water. From these
densities, the density of water (D = 1.0 g mL-1) and your observations in the table above, what can
you conclude about the relative densities of salt water, water, alcohol and the two plastics?
____________________________________________________________________________
____________________________________________________________________________
Part E: Observation and Characteristics of the Alcohol / Salt Water Emulsion
1.
2.
3.
4.
5.
6.
7.
8.
Obtain the test tube from part B containing the water/alcohol/salt mixture.
Stopper and vigorously shake the test tube; quickly observe its contents.
Notice that the alcohol and salt water take some time to separate.
Do you observe a third, cloudy layer that persists for some time before the alcohol and salt
water completely separate? Sometimes coloring the mixture with a drop of blue food
coloring (added at any time) helps to see this third layer.
How long does this third, cloudy layer persist?
When two immiscible liquids are vigorously mixed, they can form an emulsion. An
emulsion is a transient, heterogeneous mixture of two immiscible liquids (think well-shaken
salad dressing). Some emulsions take very long to separate, while others separate quickly.
Is the density of the emulsion greater than or less than the density of the alcohol?
Is the density of the emulsion greater than or less than the density of the salt water?
Observations:
___________________________________________________________________________
___________________________________________________________________________
Part F: Construction of the density bottle
1. Remove the label from the 8 oz. water bottle.
2. Take the cap off of the bottle. Be sure to keep the cap!
3. Measure out 108 mL of water from the bottle. Pour this water into a beaker. Label this beaker
A. Dump out the remaining water in the bottle.
4. Add 24 g of salt to the 108 mL of water you poured into beaker A. Mix the salt and water until
the salt is all dissolved. The salt water mixture will look cloudy because there is some undissolved
calcium silicate (and perhaps some other solids) in the water.
5. Filter the salt water mixture through a funnel and filter paper. The filtering process takes quite a
bit of time, about 10 minutes for every 50 mL of water. To speed up the filtering process, filter the
salt water through 2 or 3 separate filtering funnels.
6. Once the salt water mixture has filtered, pour all 108 mL of the filtered salt water back into the
original water bottle. The water bottle should be a little less than half full at this point.
7. Now add 132 mL of 91% isopropyl alcohol to the water bottle so that it is almost full.
8. Add about 20 of each type of plastic to your density bottle. Be sure to add pieces of both types
of plastic.
9. Tightly place the cap back on the water bottle. Secure the cap with duct tape.
Part G: Density bottle observations and explanations
1. Carefully observe your finished product. Next, shake the bottle well and observe. Describe what
happens to the contents of the bottle over the course of 1 – 2 minutes.
Observations:
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
2. In the space below, explain your observations in step 1 above. In your explanation, you will need
to discuss the relative densities of isopropyl alcohol, salt water, the alcohol/salt water emulsion, and
of both of the plastics. Include drawings.
Typical Answers to Questions in Student Worksheet: Answers are given in red.
Part A:
Liquid
Isopropyl alcohol
Salt water
Density / g mL-1
0.7 – 0.9 g mL-1
1.0 – 1.2 g mL-1
Part B:
6. Why do you think there is a difference in the behavior of the alcohol/water vs. the alcohol/salt
water mixtures?
Without knowledge of polarity and intermolecular forces, students may have difficulty answering
this question. Like dissolves like; the intermolecular forces between water and isopropyl alcohol
molecules (dipole-dipole interactions, hydrogen bonding) is not strong enough to displace the iondipole forces experienced between water and sodium/chloride ions. Thus, salt water does not mix
with isopropyl alcohol. However, the intermolecular forces between water and isopropyl alcohol are
sufficiently strong to displace water-water interactions.
Are water and isopropyl alcohol miscible? __Yes____
Are salt water and isopropyl alcohol miscible? __No____
Part C:
The high density beads sink in water, so the high density beads have a density greater than water.
The low density beads float in water, so the low density beads have a density less than water.
Plastic
Float or Sink in Water?
High density
plastic
Low density
plastic
Sink
Density (greater than or less than 1.0
g mL-1)
>
Float
<
Part D:
Float or sink in 91%
isopropyl alcohol?
High density
plastic
Low density
plastic
Sink
Sink
Float or sink in
water ?
Float or sink in salt
water?
sink
Float
Float
float
Review the densities you measured for the 91% isopropyl alcohol and salt water. From these
densities, the density of water (D = 1.0 g mL-1) and your observations in the table above, what can
you conclude about the relative densities of salt water, water, alcohol and the two plastics?
Most Dense
salt water
Least Dense
high density plastic
water
low density plastic
alcohol
Part G:
The position of the liquids and plastics observed are based on their densities. Alcohol (the least
dense) is always observed on top, while salt water (the most dense) is always observed on the
bottom. At the start, the liquids and solids layer as:
Alcohol
Low density beads
High density beads
Salt water
When the bottle is shaken, an emulsion forms. The emulsion has a density greater than the low
density beads but lower than the high density beads. When the emulsion is the only liquid present,
the liquids and solids layer as:
Low density beads
Emulsion
High density beads
As the emulsion separates back into alcohol and salt water, three liquids are present. In this case,
the liquids and solids layer according to density as:
Alcohol
Low density beads
Emulsion
High density beads
Salt water
Dynamic Density Bottle – 70% isopropyl alcohol recipe
1. Remove the label from the water bottle (for this recipe, use an 8 oz. water bottle).
2. Take the cap off of the bottle. Be sure to keep the cap!
3. Measure out 68 mL of water from the bottle. Pour this water into a plastic cup. Label this cup
A. Dump out the remaining water in the bottle.
4. Add 1 ¼ tablespoons (24 g) of salt to the 68 mL of water you poured into cup A. You may have
to gently heat the salt water mixture to get the salt to dissolve. Mix the salt and water until the salt is
all dissolved. The salt water mixture will look cloudy because there is some undissolved calcium
silicate (and perhaps some other solids) in the water.
5. Filter the salt water mixture through a funnel and filter paper. The filtering process takes quite a
bit of time, about 10 minutes for every 50 mL of water.
6. Once the salt water mixture has filtered, pour all 68 mL of salt water back into the original water
bottle. The water bottle should be about one-third full at this point.
7. Now add 70% isopropyl alcohol to the water bottle so that it is almost full. This will require
about 172 mL of isopropyl alcohol.
8. Add a few pieces of plastic to your density bottle. Be sure to add some plastic with a higher
density and some plastic with a lower density.
9. Tightly place the cap back on the water bottle. Secure the cap with duct tape.
10. Remember, before the liquids are mixed, the alcohol layer floats on the salt water layer. Both
plastics float on the salt water and both sink in the alcohol. When the salt water and alcohol are
mixed, a single liquid is formed. Some plastics are more dense than this mixed liquid and sink in it.
Other plastics are less dense than this mixed liquid and float in it. Over time, the two liquids will
separate and return (along with the plastics) to the original configuration.
ONCE THE CAP IS SECURED WITH DUCT TAPE, DO NOT REMOVE THE CAP!!
To increase the amount of fluid yielded, all measures in the recipe should multiplied by the following
conversion factor:
Conversion factor =
ounces desired
8
Suppose you wish to make a 500 mL Dynamic Density Bottle. Because 500 mL = 17.6 ounces (500
mL / 28.4 mL oz-1 = 17.6 ounces), the conversion factor is 2.2. Thus, to make about 500 mL, 53
grams of salt (24 g x 2.2) are dissolved in 150 mL water (68 mL x 2.2), and the salt water is filtered.
Next, about 378 mL (172 mL x 2.2) of 70% isopropyl alcohol are added to the filtered salt water.
Dynamic Density Bottle – 91% isopropyl alcohol recipe
1. Remove the label from the water bottle (for this recipe, use an 8 oz. water bottle).
2. Take the cap off of the bottle. Be sure to keep the cap!
3. Measure out 108 mL of water from the bottle. Pour this water into a plastic cup. Label this cup
A. Dump out the remaining water in the bottle.
4. Add 1 ¼ tablespoons (24 g) of salt to the 108 mL of water you poured into cup A. Mix the salt
and water until the salt is all dissolved. The salt water mixture will look cloudy because there is
some undissolved calcium silicate (and perhaps some other solids) in the water.
5. Filter the salt water mixture through a funnel and filter paper. The filtering process takes quite a
bit of time, about 10 minutes for every 50 mL of water.
6. Once the salt water mixture has filtered, pour all 108 mL of salt water back into the original water
bottle. The water bottle should be a little less than half full at this point.
7. Now add 91% isopropyl alcohol to the water bottle so that it is almost full. This will require
about 132 mL of isopropyl alcohol.
8. Add a few pieces of plastic to your density bottle. Be sure to add some plastic with a higher
density and some plastic with a lower density.
9. Tightly place the cap back on the water bottle. Secure the cap with duct tape.
10. Remember, before the liquids are mixed, the alcohol layer floats on the salt water layer. Both
plastics float on the salt water and both sink in the alcohol. When the salt water and alcohol are
mixed, a single liquid is formed. Some plastics are more dense than this mixed liquid and sink in it.
Other plastics are less dense than this mixed liquid and float in it. Over time, the two liquids will
separate and return (along with the plastics) to the original configuration.
ONCE THE CAP IS SECURED WITH DUCT TAPE, DO NOT REMOVE THE CAP!!
To increase the amount of fluid yielded, all measures in the recipe should multiplied by the following
conversion factor:
Conversion factor =
ounces desired
8
Suppose you wish to make a 500 mL Dynamic Density Bottle. Because 500 mL = 17.6 ounces (500
mL / 28.4 mL oz-1 = 17.6 ounces), the conversion factor is 2.2. Thus, to make about 500 mL, 53
grams of salt (24 g x 2.2) are dissolved in 238 mL (108 mL x 2.2) of water, and the salt water is
filtered. Next, about 290 mL (132 mL x 2.2) of 91% isopropyl alcohol are added to the filtered salt
water.
Density Bottle – 99% isopropyl alcohol recipe
1. Remove the label from the water bottle (for this recipe, use an 8 oz. water bottle).
2. Take the cap off of the bottle. Be sure to keep the cap!
3. Measure out 120 mL of water from the bottle. Pour this water into a plastic cup. Label this cup
A. Dump out the remaining water in the bottle.
4. Add 1 ¼ tablespoons (24 g) of salt to the 120 mL of water you poured into cup A. Mix the salt
and water until the salt is all dissolved. The salt water mixture will look cloudy because there is
some undissolved calcium silicate (and perhaps some other solids) in the water.
5. Filter the salt water mixture through a funnel and filter paper. The filtering process takes quite a
bit of time, about 10 minutes for every 50 mL of water.
6. Once the salt water mixture has filtered, pour all 120 mL of salt water back into the original water
bottle. The water bottle should be about half full at this point.
7. Now add 99% isopropyl alcohol to the water bottle so that it is almost full. This will require
about 120 mL of isopropyl alcohol.
8. Add a few pieces of plastic to your density bottle. Be sure to add some plastic with a higher
density and some plastic with a lower density.
9. Tightly place the cap back on the water bottle. Secure the cap with duct tape.
10. Remember, before the liquids are mixed, the alcohol layer floats on the salt water layer. Both
plastics float on the salt water and both sink in the alcohol. When the salt water and alcohol are
mixed, a single liquid is formed. Some plastics are more dense than this mixed liquid and sink in it.
Other plastics are less dense than this mixed liquid and float in it. Over time, the two liquids will
separate and return (along with the plastics) to the original configuration.
ONCE THE CAP IS SECURED WITH DUCT TAPE, DO NOT REMOVE THE CAP!!
To increase the amount of fluid yielded, all measures in the recipe should multiplied by the following
conversion factor:
Conversion factor =
ounces desired
8
Suppose you wish to make a 500 mL Dynamic Density Bottle. Because 500 mL = 17.6 ounces (500
mL / 28.4 mL oz-1 = 17.6 ounces), the conversion factor is 2.2. Thus, to make about 500 mL, 53
grams of salt (24 g x 2.2) are dissolved in 264 mL (120 mL x 2.2) water, and the salt water is filtered.
Next, about 264 mL (120 mL x 2.2) of 91% isopropyl alcohol are added to the filtered salt water.