6th Grade Earth Science Curriculum Unit 1: Introduction to Scientific Methods Lesson 14: Layering Liquids Objectives: After completing the lesson, students will be able to: explain the meaning of density compute the density of a liquid use density to explain why one liquid floats on top of another Key Question: "Why does one liquid float on top of another liquid?" Overview: This lesson begins with concrete examples of mathematically derived measurements. These examples lead to an introduction to the property of matter called "density." They then are given an opportunity to measure the mass and volume of water, and use this information to determine the density of water (1g/ml, or 1g/cm3). Once students are comfortable with this concept, they investigate how four different liquids (of differing densities) interact when layered on top of each other. The lesson concludes with an opportunity for students to determine the density of each of the four liquids. They use this information to create a general rule for determining when one liquid will float on top of another liquid. Time Required: 3 class sessions of 45 minutes each (the third session is optional). Materials: for the class four stock solutions of liquids, prepared as follows: 16 ounces (about 1/2 liter) of glycerin combined with 25 drops of blue food coloring 16 ounces of tap water combined with five level tablespoons (about 50 g) of kosher salt and 20-25 drops of green food coloring 16 ounces of tap water combined with 20-25 drops of yellow food coloring 16 ounces of alcohol combined with 30-35 drops of red food coloring for each group of students about 100-150 sheets of paper 5 plastic cups 4 medicine droppers 2-3 slices of potato, about 1" thick 4-6 clear plastic straw halves 2-3 paper towels 1 waste container 1 metric ruler 1 triple beam balance 1 graduated cylinder   for each student a copy of the "Finding the Density of Water (PDF)" handout a copy of the "Liquid Layers Data Sheet" (p. 15 in the Discovering Density GEMS guide a copy of the "Finding the Density of the Mystery Liquids (PDF)" handout Procedure: Part A (day 1): Challenge the students to measure the thickness of a single sheet of paper. After some discussion, the class should realize they can measure the thickness of a stack of several sheets of paper, and then divide the result by the number of sheets in the stack. Once they understand the process, have them determine the thickness of a single sheet. By comparing and averaging the result obtained by each group, they should come close to the "actual" measurement (make sure they use only metric units for this lesson). Next, ask the students the following question: "What other examples can you think of where you can measure one object by measuring many?" Next, have the students take a random number of sheets, and without counting, determine the number of sheets in the stack (measure the stack and divide by the thickness of a single sheet). Then have the students predict the thickness of a given number of sheets (the thickness of one sheet multiplied by the number of total sheets). Once students appear to be comfortable with the concept of unit rates, introduce the concept of density using a concrete example: for instance, you could hold up equal volumes of Rice Krispies and Rocks, and ask "Which is heavier?" The ensuing discussion can lead to a formal introduction of density as the mass of the molecules in a given object divided by the volume of that object. Next, distribute a copy of the "Water Density" data sheet, and have students compute the density of several different volumes of water. It is important that their results are discussed as a class, in order to emphasize the idea that the density of a substance does not change even when the amount of the substance does. It is also important for students to learn the density of water is equal to 1g/ml. Part B (day 2): see pp. 7-15 in the Discovering Density GEMS guide. Part C (day 3): (While this activity is optional, it does give the students one more chance to practice their measurement and calculation skills.) Distribute a copy of the "Finding the Density of Liquids" handout to each student. Working in groups of 3-4, have each group repeat the activity from Part A, computing the density of the four "mystery" liquids, instead of water. When they have finished, bring the class together to discuss what conclusions they can draw from the three day lesson. Assessment: "Finding the Density of Water" student handout "Liquid Layers Data Sheet" (p. 15 in the Discovering Density GEMS guide "Finding the Density the Mystery Liquids" student handout OUSD Science Content Standards (State of California Science Content Standards): #1-a through f, excepting c (#7-a through e). While density is not mentioned directly in the OUSD or State of California Science Standards, it is impossible to teach the OUSD Topics, "Energy in the Earth System" (#3) or "Plate Tectonics and Earth Structures" (#4), without a thorough understanding of density. References: Barber, Jacqueline, et. al. Discovering Density. Berkeley, CA.: Lawrence Hall of Science, University of California, Berkeley, pp. 5-16. Notes: Most 6th graders do not have experience dealing with unit rates, and might find it difficult to understand the meaning of a property of matter that is derived indirectly, as the quotient of two other direct measurements. Therefore, this lesson should begin only after students have gained some experience measuring the mass and volume of objects. Since density is an example of unit rate, it has a strong math connection. Conversely, when unit rate is taught in math, it is a good idea to reinforce the concept of density. Even though density is not mentioned specifically in the 6th grade state curriculum guide, it provides the basis for explaining most of the phenomena taught in 6th grade Earth Science. For this reason, it is crucial that students have a strong understanding of this crucial concept. Key Vocabulary: density: the mass of an object divided by the volume of an object. More generally, density refers to how massive and tightly packed together the molecules in an object are packed. density of water: because the metric system initially used water to define the basic units of measurement for volume and mass, the density of water is equal to 1 gram / 1 millimeter. Any object with a density less than 1g/ml will float in water, and any object with a density more than 1g/ml will sink in water. mass: the amount of matter in an object. volume: the space occupied by matter. Unit 1 Introduction amnd Overview Unit 1 Vocabulary Lesson 1: Laboratory Safety and Classroom Management Lesson 2: Observation and Inference Lesson 3: Introduction to Observation Lesson 4: Comparing H2O to CO2 Lesson 5: Matter and Energy Lesson 6: Exploring Dry Ice Lesson 7: Marge's Systematic Observation Lesson 8: Floating Bubbles Lesson 9: Marge's Experiment Lesson 10: Investigable Questions Lesson 11: Planning an Investigation Lesson 12: Conducting Investigations Lesson 13: Sharing Results Lesson 14: Layering Liquids Lesson 15: Layering Salt Solutions Lesson 16: Density in Everyday Life Lesson 17: Observing Convection Lesson 18: Understanding Convection Lesson 19: Convection in Air Lesson 20: Intro to Measurement Lesson 21: How Big is my Favorite Stuffed Animal? Lesson 22: Measuring Volume Lesson 23: Gummy Bear Lab Back to Grade 6 Units