"You're so dense!" Is it an insult or is it a compliment? Density is a rich physical science concept because understanding of density not only builds on students' prior knowledge of mass (Measurement: Mass) and volume (Measurement: Volume) as fundamental properties of matter, but density helps us leap from quantifying matter to being able to identify types of matter or how matter might interact with other matter.
Throughout this exploration of density, students access learning objectives related to the Matter and Its Interactions Disciplinary Core Idea (MS-PS1). This core idea requires students to be able to analyze and interpret data on the properties of substances. Density data also relates to Common Core Mathematics Standards in the Ratios and Proportional Relationships and Measurement and Data domains. As an understanding of density develops, students are better able to understand proportional relationships, which provide information about the magnitude of properties and processes and recognize that scientific relationships can be represented through the use of algebraic expressions and equations (CCC). While students conduct investigations to produce data to serve as the basis for evidence that meet the goals of the investigation (SP3), they apply mathematical concepts and processes (SP5) and develop models to describe phenomena (SP2).
In order to ENGAGE students in this lesson, students observe a simple density column made of water and oil in a plastic bottle. Students follow the Predict - Observe - Explain (POE) strategy (Predict - Observe - Explain: A Protocol for Demonstrations) to access prior knowledge about density. The POE cycle is based on this prompt:
What happens to the matter if I tip the bottle over?
This simple demonstration leads to great discussion since the reasons why the liquids stay separated and in the same order are complex. If additional demonstrations are desired to further explore student ideas, the Density Demonstration Notes offer some alternatives. At some point, students generate the idea of density during the discussion, which is the signal to introduce the investigations using the Measurement Density Student Handout.
The EXPLORE stage of the lesson is to get students involved in the topic so that they start to build their own understanding of the concept of density as it relates to the mass and volume of matter. They will understand how to calculate density using accurate measurements and metric units and apply the concept of density to real-world experiences such as sinking and floating. To help students explore the concept of density, students use a variety of resources to express the concept of density in three different ways using the Measurement Density Student Handout:
1) Words: complete the data table.
2) Mathematically: write the equation for density using words and abbreviations.
3) Visually: draw dots to represent the particles in each object.
In order for these activities to go smoothly, it is important to provide resources that will help students independently find the information. Develop a resource list for each table or listed on a web-site that provides quick access to the resources available. Examples of some helpful resources are:
1) Science Textbook
2) Density Simulation (pHET)
3) Density of Different Types of Matter Multimedia (American Chemical Society)
4) What is Density (video) (The Science Classroom)
While students EXPLORE, they will need support accessing the resources and clarifying their understanding. Density is a complex concept, so rotation through groups to ask students probing questions helps students develop their understanding. As students continue to explore density, it may be helpful to conduct a mini-lesson modeling how to calculate density. This video is an example of a student modeling how to measure mass/volume and calculate density:
Once students complete the background information (page 1) of Measurement Density Student Handout, a review of the information as a class or in small groups can solidify the foundation of basic understanding. Students may benefit from having a copy of the Measurement Density Student Handout Notes to refer to.
The EXPLAIN stage provides students with an opportunity to communicate what they have learned so far and figure out what it means and the EXTEND stage allows students to apply their new knowledge to a novel situation. The novel situation is called the "Density Logging Lab". In this investigation, students are given this problem:
You are given the problem of transporting different construction materials from your location to the manufacturing facility by using a nearby river. Unfortunately, you aren’t sure what the materials are or whether they will float. By using density, identify the materials and prove you can float them to the facility.
Teacher Note: The materials students will use are Density Cubes that have the same volume but are made of nine different materials (Link to Density Cubes). I have one set of cubes for each lab group.
To engage students in this investigation, we view Old River Log Driving, "the world's oldest video".
Using the video, ask students:
Why is it possible to use the river to move the logs from the logging site to the sawmill?
This discussion should be driven toward these concepts: floating, sinking and density. When students start connecting the idea that objects that are more dense than water will sink and objects that are less dense than water will float, set them loose to complete the investigation. Following the procedure, students:
1) Find at least six cubes of different types of matter.
2) Predict which of the objects will float or sink in water. Record in data table. Answer question: Of the six objects, which one do you think is the most dense and why?
3) Measure the mass of each object. Record in data table.
4) Calculate the volume of each object. Record in data table.
5) Calculate the density of each object. Record in data table.
6) Using the list of densities (Logging Lab Mystery Matter Investigation), identify each type of matter. Record in data table.
7) Determine if each object will float by comparing its density to the density of water. Record in data table.
Teacher Note: Do not let students test the density cubes using water. Before this happens, students should decide what they think each cube is made of and compare their calculated densities to the density of water (they can research or test the water to find the density of water). There are two reasons students need to wait: 1) If students test the sinking and floating using water, they often skip the process of calculating density (which is the objective); and 2) The density cubes that are made of wood absorb water, which causes their densities to increase! To test sinking and floating, it is fun to do an interactive "game show" style demonstration to sink and float the objects. During this process, students check their work to find out how accurate they were.
Teacher Note: Check student's calculations of volume. Remind students that the volume of each cube doesn't change, which means volume is a controlled variable. Asking questions like:
If volume doesn't change, what is changing for each cube?
If the mass increases while the volume stays the same, what happens to the density of the object?
What is the independent variable and what is the dependent variable in this investigation?
As students complete their data tables, it is important to check their math. This is the first time students have calculated density; it is common to find errors due to measurement, calculation or units. Once the data is reviewed, students explain their understanding (Writing Arguments from Evidence) using these two questions:
1) Which object was the densest? Describe and/or draw what it means to be the densest object.
2) Can density help us determine the type of matter an object is made of? Why or why not?
The last step in this part of the lesson is to provide students with additional practice with calculating density. Students complete the page 3 of Measurement Density Student Handout or Density Practice Student Handout. Examples of student work for these activities can be viewed here: Measurement Density Student Work.
The EVALUATION stage is for both students and teachers to determine how much learning and understanding has taken place. While I don't expect that students have a complete understanding of density by the end of this lesson, completing a formative assessment is an important way to identify specific challenges. The primary way I evaluate student learning is by giving students an assessment that focuses on the "remember", "understand" and "apply" levels of understanding: Density Check Out Quiz or Density Quiz or Measurement Density Retest.
Prior to giving the assessment, students participate in several days of review as part of their daily warm-up to evaluate the level of understanding. For warm up ideas, use this resource: Density Concept Warm Ups. Additionally, students use their new understanding of density to explore why objects sink and float in a related inquiry investigation: Dunking for Density Investigation.