Density Part 3: Calculating Density of Cubes

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SWBAT measure the mass and volume of objects to calculate density.

Big Idea

Now that students can visually represent and explain the basic molecular factors within a certain volume that determine density, it is time for them to measure mass and volume to calculate density.

Do Now

5 minutes

In this lesson students are deepening their understanding of MS-PS1-1 and developing mathematical reasoning skills (MP 2).  I utilize the Do Now to have kids recall the definition that we started to develop yesterday, asking students to continue to define density, based on our previous classroom activities' learning outcomes.  Even if they cannot define density clearly, I want them to at least acknowledge that mass and density are involved.

We will then review as a class and I introduce today's station activity and how it will run.

Students Measure Mass and Volume of Various Unknown Cubes

30 minutes

I set up 6 stations (that's the number of groups that I have established in my classes) with one cube at each station.  Each station also has a triple beam balance and a metric ruler.  Students are asked to determine the cube's mass and measure it to calculate the volume of each cube.  They use the information to calculate the density of the cube present at each station.

Station Rotation and Overview

Prior to setting students free, I lead a short discussion to review how to use a triple beam balance and calculating the volume of a prism.

I then state that each group has approximately 5 minutes at each station, and I will let them know when it is time to transition to the next group.  As they are generally slower on the first station rotation, I tend to give them a bit more time as they are getting started. I tell them that their signal to let me know that their group is done is to sit quietly reading their independent reading book.  At the very least, their goal is to measure the 3 sides and mass of the cube.  Later, they can use that information to calculate the density of each cube.

Students are expected to complete pages 164-165 from the What is Density? Activity Sheet.  Note: I pick 6 of the 8 cubes.

Students Calculate Density

15 minutes

Now that students have the required data to calculate density, I ask them to recall what the activity where they constructed fictitious matter inside of a cube.  I then ask, "Based on the mass and volume measurements that you have on your paper, why do you think that some objects will be more or less dense than others?"

I am trying to elicit student thinking to connect that the amount of matter in a volume determines the density of an object.  To help them to make that connection, I have them predict which cubes will be more dense than the other before they actually calculate density.

I then tell them that to use the formula D=M/V to calculate density and model the calculation steps as an example.  

This is one way to help your students remember the formula for calculating density:

Help Remembering Density Formula

Mass=10 grams and Volume=2 cm cubed

10/2=5 g/cm cubed

To help connect to yesterday's lesson, I demonstrate how you can visually model what our calculations show with models.  I proceed to state that if each cube is 1 cubic centimeter then there would be 5 grams worth of matter in each cube.  

Check for Understanding

5 minutes

As students are wrapping up their lab and beginning to clean up, I am circulating around the room and checking student progress. I want to begin hearing them discuss the relationship of volume and mass in determining density, in addition to hearing them explain what the matter might look like at the molecular level. I ask questions that get them thinking about the relationship between mass and volume for the different samples.  

I may ask: "What might explain why cube 1 is more dense than cube 3?" In their response, I can quickly determine where they may be faulting--are they struggling with the density calculation itself? Are they struggling with the visual representation aspect of explaining density? Some students are still struggling to grasp volume versus mass, so this is a great time to have discussions with groups or individuals within a group who may need some help.  

This is a great opportunity to tie in the Crosscutting Concepts of patterns and structure and function. As you walk around the room ask groups to rank the cubes from highest to lowest mass. They can then see if there are any patterns with high mass to higher density. They can then explain this from a structure and function standpoint. In that, students will see that the higher the density, the more mass it will have, which most likely means the matter that makes it up either has a lot of mass per unit and/or the matter is tightly packed.