Density Day 3 or 4: Making Density Visual With Particle Model Drawings

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SWBAT: Represent the relative density of each cube in the PhET simulations with molecular drawings

Big Idea

Bringing learning alive with visuals is especially important when students are developing a deep understanding of density. In this lesson, students will draw molecules to help explain why some cubes sink or float, based on their density.


My goal this year is to make science more visual for my students through the use of particle modeling, so I asking my student in this lesson to make visual representation of what their data show.  In particular, I want students to demonstrate their understanding of density by drawing atoms/molecules to represent their interpretation of what might be causing their results.  Students will develop deeper understanding of MS-PS1-1 and SEP 6 (constructing explanations).

Up until now students have completed a series of lessons using the PhET density simulation.  They collected data and written CER responses for each of the 5 parts.  I now want to continue to support their ability to visually represent their understanding by implementing this lesson. 

Do Now

5 minutes

I want my students to refresh their memories about what they learned from each of the 5 PhET simulations by reading their C-E-R entries.  Students will use what they learned from each section of the simulation in a few minutes, so I want them to remember what they learned in each one.

I ask students to:

"Review your C-E-R for each of the 5 parts of the cube simulation."  

This gives me time to circulate around the room to quickly scan student work to see that everyone has completed their C-E-Rs.  You could have students type them up and submit them to you, but I want them to have them for today's lesson, so I do not require students to turn them in.  NGSS classrooms require students to constantly reflect on the outcomes of various activities, and this is no different.  My students will be utilizing their C-E-R responses to deepen their understanding.  This would not occur if I collected them and took a day or two to get them all back to them.

I will check for understanding when I circulate around to each group, asking questions that pertain to their C-E-R responses.  If I notice discrepancies, I will use that time together to help clarify their confusion.  

Visual Representation of Density Using Molecular Models

35 minutes

Visual representation is highly effective strategy to getting kids to make connections; scientists make models (SP 2) all the time, so promote that practice in your classroom!

To start the class, I explain how visualizing what we're learning about density will help us build deeper understanding of the topic.  

I then model how to perform a visual representation of the 'custom' part of the PhET simulation.  I find that modeling (the act of a teacher showing students how to perform a task) how to model (represent understanding using non-linguistic, or a combination of non- and linguistic-representations) scientific phenomena sets students up for success, so that's why I do it.

I start by speaking aloud about the meta-cognitive conversation that I am having in my mind about the simulation.  It sounds something like this:

"Okay, so in this simulation I learned that, regardless of the size of a sample of the same type of matter, such as aluminum, the density was the same.   What makes something more or less dense?  Oh, that's right--the amount of matter per unit volume, or how much matter is in a given volume.  If aluminum has a density of 2.7 g/cubic centimeter, then that means that regardless of the volume, the amount of matter is proportional to it.  That must mean the atoms or molecules that make up this type of matter--wait, it's an element, so it must be made of the same type of atoms, so I will draw atoms.  2.7 g/cm3 is relatively low, compared to Styrofoam which has a density of .75 g/cm3, so I will draw the molecules slightly closer together for models of aluminum.  My pics will look something like this.  

Once I model how to perform the task for students, I ask them to work in their groups and circulate around the room.  As I circulate, I help guide students who are struggling with either the C-E-R portion of the lesson or how to represent their C-E-R with molecular or atom models.  

Common connections that students sometimes struggle to make:

1) Same mass: Students struggle to how to explain how two objects can have the same mass but not the same density.  This comes down to their ability to understand the direct-proportional relationship of mass vs. volume.  In other words, it's not just about mass, we need to consider the volume associated with each sample.  This allows students to see that some samples are larger or smaller than others, affecting density.  Do the math with them, if it helps them see the light.  They should see that the higher the volume, the lower the density and the lower the volume, the higher the density (CCC: Patterns).  

2) Same Volume: The same is true for this, as it is for same volume, only now they see that it's not just volume that affects density, it is also the mass that is associated with the density.  Therefore, students should see that you must take into consideration both mass and volume when comparing densities.  That's the key take away from this section, so structure your questioning accordingly.  For example, you may ask: "What do you notice about the density of each samples that have the same volume?"  Follow-up question: What does this mean about the role that volume and mass play when determining density of objects?"

3) Same Density: The key here is that students see that all samples have the SAME density, yet they are not the same mass or volume, meaning that they are all made of the same type of matter.  Structure your questioning by asking students to reflect on what they learned during this lesson.  Students should see that if items have the same density, then it must mean that they are made of the same type of matter.  Density is a characteristic property of matter, meaning that it can be used to identify unknown objects.

Student Visual Representation of Density

4) Mystery:  Since density is a characteristic of matter, it can be used to identify unknown objects.  If students correctly calculated density, then they should be able to use the table to pick the correct object.  This is a good time to check for their ability to correctly calculate density.  

Review Molecular Models with Class

10 minutes

At the end of the day, I have some students go up to the board to draw their visual representations of one of the 5 simulations.  

Here is another sample. Students then compare their drawings to their peers and we have discussions. If there are errors, we use that as an opportunity to clarify their understanding.