LESSON 26: Swimming Specks
ESTIMATED TIME Setup: 5 minutes | Procedure: 5–10 minutes
• DESCRIPTION
• MATERIALS
Add black pepper to the surface of a bowl of water
to observe the properties of density and surface
tension.
o
Bowl
o Water
Ground black pepper
Liquid dish soap
o Cornstarch (optional)
o Baby powder (optional)
o
o
• OBJECTIVE
This lesson demonstrates the properties of density
and surface tension. Students observe as black
pepper is added to the surface of a bowl of water
and remains spread across the surface until soap is
added. The lesson can be simplified to focus on
measurements and calculating density.
Always remember to use the appropriate safety
equipment when conducting your experiment.
Refer to the Safety First section in the Resource Guide
on pages 391–393 for more detailed information about
safety in the classroom.
• CONTENT TOPICS
Scientific inquiry; measurement; states of matter;
properties of matter; attractive forces (surface tension)
Jump ahead to page 333 to view the
Experimental Procedure.
NATIONAL SCIENCE EDUCATION STANDARDS SUBJECT MATTER
This lesson applies both Dimension 1: Scientific and Engineering Practices and Dimension 2: Crosscutting Concepts
from “A Framework for K–12 Science Education,” established as a guide for the updated National Science Education
Standards. In addition, this lesson covers the following Disciplinary Core Ideas from that framework:
• PS1.A: Structure and Properties of Matter
OBSERVATION & RESEARCH
BACKGROUND
Matter is anything that has mass and takes up space.
Mass and volume are common units of measure. Mass is a
measure of the amount of matter in a substance. It’s the
amount of “stuff” in a substance. Volume is the amount of
space an object occupies. Mass and volume can be used to
determine a useful physical property of matter—density.
Density is an important concept in chemistry that is
defined as the mass of an object per unit volume.
Density is a physical property of matter that describes
how closely packed together the atoms or molecules of a
substance are. Scientists use density in different ways.
They may use it to identify unknown substances or to
separate different substances.
The approximate density of pure water is 1.0 gram per
milliliter (g/mL). This means that one milliliter of water
will have a mass of one gram and, therefore, weigh one
gram on the earth. In general, a substance that is less
dense than water will rest on top of the water, and a
substance that is denser than water will sink.
Matter exists primarily as a solid, liquid, or gas on the
earth. Solids have a definite volume and a definite shape.
Examples of solids are chairs, books, and bowls. Liquids
have a definite volume but no definite shape. Examples of
liquids are water and orange juice. Gases have no definite
shape and no definite volume. Examples of gases are the
oxygen we breathe and the helium that fills balloons.
Along with differences in shape and volume, the
different states of matter have other unique properties.
For example, surface tension is a property of liquids that
describes the attraction of liquid particles at the surface.
The strong attraction of particles at the surface of the liquid
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Putty
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LESSON 26: Swimming Specks
creates a surface “film” that makes moving an object
through the surface of a liquid more difficult than moving
the object when it is completely submerged in the liquid.
Surface tension is also the reason liquids tend to keep a low
surface area. For example, water droplets will tend to form
into a sphere rather than spreading out flat.
Water has a very high surface tension because of strong
attractions between the water molecules (hydrogen
bonding). In contrast, soap is a surfactant. A surfactant
(or surface active agent) is a substance that has the ability
to reduce the surface tension of a liquid. Therefore, when
a drop of liquid dish soap is added to the water, the surface
tension of the water is reduced. As this occurs, the water
molecules can move more freely and easily.
In this experiment, black pepper is sprinkled on top of a
bowl of water. The pepper rests on the surface. When a
drop of liquid dish soap is added to the center of the bowl
of water,the surface tension in the center is reduced, but
the surface tension toward the outside of the bowl remains
higher. As a result, even though the surface tension is
reduced in the center, the pepper will remain on top of
the water because it is less dense. However, the pepper is
pulled out to the sides where the surface tension is higher.
Eventually, the surface tension becomes balanced again,
and some pepper will move back toward the center of
the bowl.
CONNECT TO THE YOU BE THE
CHEMIST CHALLENGE
For additional background information, please
review CEF’s Challenge study materials online at
http://www.chemed.org/ybtc/challenge/study.aspx.
• Additional information on mass, volume, and
density can be found in the Measurement section
of CEF’s Passport to Science Exploration: The
Core of Chemistry.
• Additional information on states and properties of
matter, including surface tension, can be found in
the Classification of Matter section of CEF’s
Passport to Science Exploration: The Core of
Chemistry.
FORMULAS & EQUATIONS
Pure water is comprised of two hydrogen atoms and one
oxygen atom.
The chemical formula for pure water is H2O.
Tap water is a mixture of pure water, dissolved minerals,
and other substances.
Black pepper, known as piper nigrum, comes from a
plant. The piperine molecule gives the pepper its spicy
taste.
The chemical formula for piperine is C17H19NO3.
Once the mass and volume of an object have been
determined, the density of an object can be calculated
using the following equation:
d = m/v
Density is measured in grams per milliliters or grams
per cubic centimeters. One cubic centimeter (cm3 or cc) is
equal to one milliliter (mL). Water has a density of
approximately 1 g/mL. The density of pepper ranges from
approximately 0.4 g/cc to 0.5 g/cc, which is less than the
density of water. Therefore, pepper rests on top of water
because pepper is less dense than water.
Soaps and detergents are surfactants. One part of a soap
molecule is attracted to water, while the other parts are
repelled by water but attracted to other substances, such
as fats.
HYPOTHESIS
uWhen black pepper is added to a bowl
of water, the pepper will rest on the surface
of the water because of its lower density and the
surface tension of water. In addition, when liquid
dish soap is added to the center of the bowl of water,
the soap will reduce the surface tension at the center
and cause the pepper to move to the outside.
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LESSON 26: Swimming Specks
DIFFERENTIATION IN THE CLASSROOM
LOWER GRADE LEVELS/BEGINNERS
DESCRIPTION
Calculate the density of a sample of black pepper, and
add the pepper to the surface of a bowl of water.
OBJECTIVE
This lesson explores the concept of density and how to
calculate a substance’s density. Students take different
measurements and use those measurements to calculate
density.
OBSERVATION & RESEARCH
Measurement is perhaps one of the most fundamental
concepts in science. It is the process of determining the
ratio of a physical quantity, such as length or mass, to a unit
of measurement. Without the ability to measure, it would
be difficult for scientists to conduct experiments or form
theories. Not only is measurement important in science,
but it is also essential in industry, farming, engineering,
construction, manufacturing, commerce, and numerous
other occupations and activities.
There are several properties of objects that scientists need
to measure, but the most useful and common properties are
length and mass. Length is a measure of how long an object
is, and mass is a measure of how much matter is in a
substance. Mass and length are base units, meaning that
they are independent of all other units. Most units of
measure, such as volume and density, are derived or
calculated from base units. Volume is the amount of space
an object occupies.
Density is defined as the mass of an object per unit volume.
Density is a physical property of matter that describes how
closely packed together the atoms or molecules of a
substance are. Scientists use density in different ways.
They may use it to identify unknown substances or to
separate different substances.
The approximate density of pure water is 1.0 gram per
milliliter (g/mL). This means that one milliliter of water
will have a mass of one gram and, therefore, weigh one
gram on the earth. In general, a substance that is less dense
than water will rest on top of the water, and a substance that
is denser than water will sink .
For this experiment, students can first take a sample of
black pepper and measure its mass and volume. The mass
of a graduated cylinder or beaker could be measured using
a balance. Then, the sample of pepper could be poured into
the graduated cylinder or beaker to determine its volume.
The mass of the cylinder or beaker with the pepper inside
should be measured on the balance. Then, the mass of the
cylinder or beaker should be subtracted from the combined
mass to determine the mass of the pepper alone. Now, the
students know the mass and volume of the sample. Then,
students can predict whether the pepper will sink or float
when added to the water and test their hypotheses by
performing the experiment.
HIGHER GRADE LEVELS/ADVANCED STUDENTS
Perform the experiment as described on page 333, but
spend more time on states and properties of matter.
In addition to solids, liquids, and gases, discuss fluids.
A fluid is any substance made up of particles that flow or
move freely. They easily change shape when a force is
applied. Liquids and gases are fluids.
CONNECT TO THE YOU BE THE
CHEMIST CHALLENGE
For additional background information, please
review CEF’s Challenge study materials online at
http://www.chemed.org/ybtc/challenge/study.aspx.
• Additional information on mass, length, volume,
and density can be found in the Measurement
section of CEF’s Passport to Science Exploration:
The Core of Chemistry.
• Additional information on states and properties of
matter can be found in the Classification of Matter
section of CEF’s Passport to Science Exploration:
The Core of Chemistry.
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LESSON 26: Swimming Specks
DIFFERENTIATION IN THE CLASSROOM
Likewise, address other properties of matter that relate to
whether a substance will sink or float in water, such as
buoyancy and displacement. An object suspended by
water’s surface tension is different from an object that is
floating because of buoyancy. Buoyancy is the upward
force that a fluid exerts on an object that enables the object
to float. The buoyant force on an object is equal to the
weight of the fluid displaced by the object. Displacement is
the act of moving something out of its original position or
of one substance taking the place of another. Weight is a
measure of the pull of gravity between an object and the
earth. Therefore, an object will float when the upward force
on the object (buoyant force) is greater than the downward
force on the object (gravity or the object’s weight).
EXPERIMENTATION
As the students perform the experiment, challenge them
to identify the independent, dependent, and controlled
variables, as well as whether there is a control setup for
the experiment. (Hint: If a different substance is added to
the water’s surface, will the results change?) Review the
information in the Scientific Inquiry section on pages
14–16 to discuss variables.
Fun Fact
Black pepper comes from
mature green peppercorn berries
that have been harvested and
EXPERIMENTAL PROCEDURE
sun-dried for 7 to
1. Fill a bowl about three-quarters full with water.
10 days.
2. Sprinkle black pepper on the surface of the water
to cover the surface with a thin layer of pepper.
3. Place a drop of liquid dish soap in the middle
of the bowl of water. Observe what happens.
Try adding more drops and observe.
DATA COLLECTION
Have students record data in their science notebooks or
on the following activity sheet. What are the physical
properties of the water? What are the physical properties
of the pepper? What happened when the liquid dish soap
was added to the water? Have students answer the
questions on the activity sheet (or similar ones of your
own) to guide the process.
Fun Fact
Rain droplets form in the
shape of a sphere because of
surface tension. The attractive
forces of the surface layer pull
the droplets into the shape
of a sphere.
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LESSON 26: Swimming Specks
ANALYSIS & CONCLUSION
on top of the water. When the surface tension is
reduced, there is nothing to prevent the denser
cornstarch from falling below the water.
Use the questions from the activity sheet or your own
questions to discuss the experimental data. Ask students
to determine whether they should accept or reject their
hypotheses. Review the information in the Scientific
Inquiry section on pages 14–16 to discuss valid and
invalid hypotheses.
• Likewise, follow the instructions from the previous
bullet, but this time use baby powder. Baby powder is
made primarily of either talcum powder or cornstarch.
Talcum powder is a mineral composed of mainly
hydrated magnesium silicate. The chemical formula
for talc, the main ingredient in talcum powder, is
Mg3Si4O10(OH)2. Therefore, different types of baby
powders may behave differently during the
experiment. Cornstarch-based baby powder is more
prevalent and is generally the preferred type of baby
powder used today.
ASSESSMENT/GOALS
Upon completion of this lesson, students should be able
to …
• Apply a scientific inquiry process and perform an
experiment.
• Define and identify different types of measurements,
such as mass and volume.
• Calculate the density of an object if given the mass
and volume.
• Compare and contrast solids, liquids, and gases and
give examples of each.
5-8
See Lesson 15: Floating Paper Clips to explore the
property of surface tension further.
5-8
See Lesson 7: Milk Rainbow for a colorful lesson on
surface tension.
• Understand the property of surface tension.
• Describe the effect of a surfactant on surface tension.
• Define and give examples of fluids
(see Differentiation in the Classroom).
• Understand density, displacement, and buoyancy as
they relate to whether an object will sink or float in a
liquid (see Differentiation in the Classroom).
MODIFICATIONS/EXTENSIONS
Modifications and extensions provide alternate methods
for performing the lesson or similar lessons. They also
introduce ways to expand on the content topics presented
and think beyond those topics. Use the following
examples or have a discussion to generate other ideas as
a class.
• Perform steps 1–3 of the experiment and then go a
step further. Wash out the bowl well. (Make sure no
soap residue is left behind.) Then refill the bowl with
water, but this time, add cornstarch to the surface.
Finally, add a drop of soap and observe. The density
of cornstarch is approximately 1.2 g/cc, which is
greater than the density of water. The cornstarch is
initially suspended on top of the water because of
surface tension. When soap is added to the water,
the cornstarch will fall to the bottom of the bowl.
This occurs because the cornstarch is denser than
water, so it relies on the surface tension to remain
REAL-WORLD APPLICATIONS
• In general, dirt, sand, and soil are denser than water.
Therefore, these materials will sink. However, you
may have seen some dirt, sand, or soil particles resting
on the surface of water at some point. This occurrence
was most likely the result of surface tension allowing
some of the particles to rest on top of the water.
• In the past, bulb thermometers were made using
mercury inside a glass tube. Mercury was found to be
toxic to humans, and shards of glass can easily pierce a
person’s skin. As a result, if the thermometer was
accidentally dropped, it was likely to break. When that
happened, the mercury would spill on the floor.
However, the high surface tension of mercury would
cause it to form small beads on the ground. Cleaning
up a broken mercury thermometer was difficult and
dangerous. Thus, most thermometers today are digital
and much safer to use.
COMMUNICATION
Discuss the results as a class and review the activity sheet.
Review the information in the Scientific Inquiry section on
pages 14–16 to discuss the importance of communication
to scientific progress.
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LESSON 26 ACTIVITY SHEET: Swimming Specks
OBSERVE & RESEARCH
1. Write down the materials you see. ____________________________________________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
2. How might these materials be used? __________________________________________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
3. Define the following key terms. Then, provide an example of each by writing the example or drawing/pasting an
image of the example.
Term
Definition
Example (write or add image)
Matter
Mass
Volume
Density
Solid
Liquid
Gas
Surface tension
Surfactant
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LESSON 26 ACTIVITY SHEET: Swimming Specks
4. Consider what will happen if liquid dish soap is added to a container of water with pepper sprinkled on the
surface and why.
uWrite your hypothesis. ______________________________________________________________
______________________________________________________________________________________
______________________________________________________________________________________
PERFORM YOUR EXPERIMENT
1. Fill a bowl about three-quarters full with water.
2. Sprinkle black pepper on the surface of the water. Use enough to cover the surface with a thin layer of pepper.
3. Place a drop of liquid dish soap in the middle of the bowl of water. Watch what happens.
4. Try adding more drops. Watch what happens.
ANALYZE & CONCLUDE
1. Describe the water.
______________________________________________________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
2. Describe the pepper.
______________________________________________________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
3. What happens when you sprinkle pepper on the surface of the water? Why?
________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
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LESSON 26 ACTIVITY SHEET: Swimming Specks
4. What happens to the pepper when you add one drop of liquid dish soap to the water? Why? ____________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
5. After one drop of liquid dish soap is added, does adding more soap have any effect? If so, what is the effect?
______________________________________________________________________________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
6. What do you think will happen if you sprinkle cornstarch or baby powder on the water? What will happen if you add
soap? Why?
____________________________________________________________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
7. What do you think will happen if you sprinkle salt in the water? Why? ______________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
8. Is your hypothesis valid? Why or why not? If not, what would be your next steps? ____________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
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LESSON 26 ACTIVITY SHEET: Swimming Specks
EXPAND YOUR KNOWLEDGE—ADVANCED
1. Define the following key terms. Then, provide an example of each by writing the example or drawing/pasting an
image of the example.
Term
Definition
Example (write or add image)
Fluid
Buoyancy
Displacement
Weight
2. Give an example of buoyancy.
______________________________________________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
3. What causes soap bubbles to form?______________________________________________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
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LESSON 26 ACTIVITY SHEET: Swimming Specks
ANSWER KEY Below are suggested answers. Other answers may also be acceptable.
OBSERVE & RESEARCH
Bowl, water, pepper, liquid dish soap, cornstarch or baby powder …
1. Write down the materials you see. ____________________________________________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
A bowl may be used to hold a substance. Water may be used to drink, bathe, or clean.
2. How might these materials be used? __________________________________________________________________
Pepper may be used to season food. Soap may be used to wash other things. Cornstarch may be used for cooking. Baby powder may be used
______________________________________________________________________________________________________
to absorb wetness. These materials may be used to observe the physical properties of water.
______________________________________________________________________________________________________
3. Define the following key terms. Then, provide an example of each by writing the example or drawing/pasting an
image of the example.
Term
Definition
Matter
Any substance that has mass and takes up space;
matter is generally found as a solid, liquid, or gas
on the earth.
Mass
A measure of the amount of matter in a substance.
Volume
A physical property that measures the amount of
space a substance occupies.
Density
A physical property of matter that describes how
closely packed together the atoms of an element or
the molecules of a compound are; the amount of
matter per unit of volume (d = m/v).
Solid
A state of matter characterized by a definite volume
and definite shape.
Liquid
A state of matter that has a definite volume but no
definite shape; a liquid will take the shape of the
container that holds it, filling the bottom first.
Gas
A state of matter that has no definite volume or
shape; a gas will take the shape of the container
that holds it, filling the entire container.
Surface tension
A property of liquids that describes the attraction of
liquid particles at the surface; the strong attraction
of particles at the surface of a liquid creates a
surface “film.”
Surfactant
A substance that has the ability to reduce the
surface tension of a liquid.
Example (write or add image)
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LESSON 26 ACTIVITY SHEET: Swimming Specks
ANSWER KEY Below are suggested answers. Other answers may also be acceptable.
4. Consider what will happen if liquid dish soap is added to a container of water with pepper sprinkled on the
surface and why.
The pepper will rest on the surface of the water. When liquid dish soap is added to water,
uWrite your hypothesis. ______________________________________________________________
the soap reduces the water’s surface tension. The pepper will remain on the surface, because it is less dense than water,
______________________________________________________________________________________
but is pulled to the outside of the container where the surface tension remains higher.
______________________________________________________________________________________
PERFORM YOUR EXPERIMENT
1. Fill a bowl about three-quarters full with water.
2. Sprinkle black pepper on the surface of the water. Use enough to cover the surface with a thin layer of pepper.
3. Place a drop of liquid dish soap in the middle of the bowl of water. Watch what happens.
4. Try adding more drops. Watch what happens.
ANALYZE & CONCLUDE
1. Describe the water.
Water is a clear, odorless liquid. It has no definite shape, so it will take the shape of the container it is in, filling
______________________________________________________________________________
the bottom first.
______________________________________________________________________________________________________
______________________________________________________________________________________________________
2. Describe the pepper.
Pepper is a brownish-black solid. It is lightweight (has a low density).
______________________________________________________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
3. What happens when you sprinkle pepper on the surface of the water? Why?
Pepper floats on the surface of the water.
________________________________
Pepper floats because it is less dense than water.
______________________________________________________________________________________________________
______________________________________________________________________________________________________
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LESSON 26 ACTIVITY SHEET: Swimming Specks
ANSWER KEY Below are suggested answers. Other answers may also be acceptable.
4. What happens to the pepper when you add one drop of liquid dish soap to the water? Why? ____________________
When you add soap to the
water, the surface tension of the water is reduced. The pepper remains on the top of the water because it is less dense than water. However,
______________________________________________________________________________________________________
the pepper moves away from the spot where the soap was added toward the edges where the surface tension remains higher.
______________________________________________________________________________________________________
5. After one drop of liquid dish soap is added, does adding more soap have any effect? If so, what is the effect?
Adding more soap to the water causes the pepper to move around the surface of the water.
______________________________________________________________________________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
6. What do you think will happen if you sprinkle cornstarch or baby powder on the water? What will happen if you add
soap? Why?
Cornstarch or baby powder is suspended on the surface of the water by the water’s surface tension. If soap is added, the
____________________________________________________________________________________
surface tension is reduced. As a result, the cornstarch or baby powder will most likely fall to the bottom because they are generally denser
______________________________________________________________________________________________________
than water.
______________________________________________________________________________________________________
Salt will fall to the bottom of the water because is it
7. What do you think will happen if you sprinkle salt in the water? Why? ______________________________________
denser than water. The surface tension is not stong enough to hold the salt on the surface.
______________________________________________________________________________________________________
______________________________________________________________________________________________________
8. Is your hypothesis valid? Why or why not? If not, what would be your next steps? ____________________________
Answer 1: Valid because the data supports my hypothesis.
______________________________________________________________________________________________________
Answer 2: Invalid because the data do not support my hypothesis. I would reject my hypothesis and could form a new one, such as ...
______________________________________________________________________________________________________
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LESSON 26 ACTIVITY SHEET: Swimming Specks
ANSWER KEY Below are suggested answers. Other answers may also be acceptable.
EXPAND YOUR KNOWLEDGE—ADVANCED
Have students complete this section if you used the advanced differentiation information, or challenge them to find the answers to these
questions at home and discuss how these terms relate to the experiment in class the next day.
1. Define the following key terms. Then, provide an example of each by writing the example or drawing/pasting an
image of the example.
Term
Definition
Fluid
Any substance made up of particles that flow or move freely,
such as a liquid or gas.
Buoyancy
An upward force that a fluid exerts on an object, enabling the
object to float.
Displacement
The act of moving something out of its original position or of
one substance taking the place of another.
Weight
A measure of the pull of gravity between an object and the
earth (or the planets, sun, etc.).
2. Give an example of buoyancy.
Example (write or add image)
Examples of buoyancy include boats, empty bottles, and rubber ducks placed in water.
______________________________________________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
3. What causes soap bubbles to form?
Soap bubbles form because of surface tension. The liquid soap and water molecules pull
____________________________________________________________________
together around the gas into the shape of a sphere.
______________________________________________________________________________________________________
______________________________________________________________________________________________________
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