LESSON 26: Swimming Specks ESTIMATED TIME Setup: 5 minutes Procedure: 5 10 minutes DESCRIPTION Add black pepper to the surface of a bowl of water to observe the properties of density and surface tension. 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. CONTENT TOPICS Scientific inquiry; measurement; states of matter; properties of matter; attractive forces (surface tension) MATERIALS o Bowl o Water o Ground black pepper o Liquid dish soap o Cornstarch (optional) o Baby powder (optional) 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. 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 You Be The LESSON Chemist 1: Activity Goofy Guides Putty page 330 330
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 (cm 3 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. You Be The Chemist Activity Guide page 331
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. You Be The Chemist Activity Guide page 332
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. EXPERIMENTAL PROCEDURE 1. Fill a bowl about three-quarters full with water. 2. Sprinkle black pepper on the surface of the water to cover the surface with a thin layer of pepper. Fun Fact Black pepper comes from mature green peppercorn berries that have been harvested and sun-dried for 7 to 10 days. 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. You Be The Chemist Activity Guide page 333
LESSON 26: Swimming Specks ANALYSIS & CONCLUSION 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. 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. 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 5-8 5-8 on top of the water. When the surface tension is reduced, there is nothing to prevent the denser cornstarch from falling below the water. 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. See Lesson 15: Floating Paper Clips to explore the property of surface tension further. See Lesson 7: Milk Rainbow for a colorful lesson on surface tension. 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. You Be The Chemist Activity Guide page 334
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 You Be The Chemist Activity Guides page 335
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? You Be The Chemist Activity Guide page 336
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? You Be The Chemist Activity Guides page 337
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? You Be The Chemist Activity Guide page 338
ANSWER KEY Below are suggested answers. Other answers may also be acceptable. OBSERVE & RESEARCH 1. Write down the materials you see. Bowl, water, pepper, liquid dish soap, cornstarch or baby powder 2. How might these materials be used? A bowl may be used to hold a substance. Water may be used to drink, bathe, or clean. 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 Example (write or add image) 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 Surface tension 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. 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. You Be The Chemist Activity Guide page 339
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. uwrite your hypothesis. The pepper will rest on the surface of the water. When liquid dish soap is added to water, 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. You Be The Chemist Activity Guide page 340
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. 7. What do you think will happen if you sprinkle salt in the water? Why? Salt will fall to the bottom of the water because is it 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... You Be The Chemist Activity Guide page 341
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 Example (write or add image) 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. 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. You Be The Chemist Activity Guide page 342