Density Part 2: The Role of Volume in Determining the Density of Objects

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SWBAT explain the relationship between mass and volume when discussing density.

Big Idea

Students yelling, "It sinks because it's heavy" isn't good enough! Students will learn the relationship that mass and volume have when explaining how matter can have different densities.

Do Now

5 minutes

Students learned a lot about mass and structure of different molecules yesterday, so give them an opportunity to refresh their memories by looking back at their notes and then ask them to fill in this sheet. I also ask students to review how to calculate cubic volume, which they have learned earlier in the year.  

The purpose of the initial sheet is to get my students thinking about how two objects can have the same mass--which at this point they know to mean the amount of matter in an object; loosely translated as the amount of atoms or stuff--and different volumes.  

Note: the ACS suggests teaching atoms later, so I follow their suggestion. I find that doing so results in a more meaningful experience for my students, as they have a context for what they're learning.

Additionally, after reviewing the new standards, students don't necessarily need to know the parts of the atom to be successful with any of these topics. In that, the inquiry process could lead them to develop an understanding that opposites attract, and positive charged protons and negatively charged electrons make that possible. NGSS is about conceptual understanding, not memorizing information for the sake of memorizing information. Likewise, calculating the number of protons, neutrons and electrons can be held off until high school.  

Do Now Cubes

Tying Mass and Volume Together with Manipulative Models

15 minutes

In the Do Now, most students are flabbergasted that two objects can be the same mass but have different volumes.  The importance of visual thinking, deployed as a strategy in the NGSS-based classroom, cannot be understated.  As educators, we need to bring the invisible to life, if we are to expect students to develop a deep conceptual understanding of matter.  

In this section students are given a dry erase marker and a transparency with a cube photocopied on it.  Students then must create fictitious matter by drawing circles to represent atoms in any arrangement desired.  I don't expect students to know how atoms attract to make elements; I just want them to see that the arrangement of atoms determines how much matter (mass) can fit within a certain volume. This sets the stage for a meaningful discussion about the definition of the word density.  I want students to know that density is how much matter is in a given volume.  I state that there are many reasons for atoms and molecules to arrange in certain ways, but our main emphasis is developing a relationship between mass and volume.

Modeling Density

I start by asking students to draw matter that has a low density.  As I circulate around the room, I am looking for students to do any of the following: draw matter that is spread out and/or draw atoms and explain that the mass of each one is low.  If I notice misunderstandings surrounding this I will step in and ask students questions, like: "How is wood different from copper, based on what we learned yesterday?" or "If you draw the atoms close together, does that mean that it will have more mass per volume or less?"

I then transition to asking students to draw matter having a high density.  Students need to then think about what it means to have a high density and should represent their understanding by drawing molecules relatively close together or note that they have a high mass/atom which, even if they are fairly spread out, could explain the higher density.

Student Reflections and Checks for Understanding

10 minutes

Making connections is an important part of learning certain scientific phenomena.  In this case students are tying together what they already know about mass, volume, and atoms/molecules to explain a difficult phenomenon.  To get kids thinking about what they are learning in terms of molecular structure and the relationship to density, I ask them to answer the following question:

How does the amount of matter in a specific volume affect the density of a cube?

Students are encouraged to discuss their answers first to allow other students to critique their ideas before writing their response sin their notebooks.  

Student Density Explanations

Here are a few responses:

Density question response 1

This student is making connections to the amount of matter in a certain volume.  I would like to see more discussion about the difference between chains and typical patterns, like in copper and aluminum.  Since there is more empty space in the chains and the matter that comprises plastics have low atomic masses, there is less mass/volume (low density).

Density question response 2

This student is making good connections between mass vs. volume affecting density but is only considering mass, not volume.  It's important that students discuss the relationship between mass AND volume.

Density question response 3

This student is contemplating the relationship between mass and volume, but isn't necessarily discussing the role of the structure.  

Clearly, students have more work to do and that is fine!  More connections and deeper understanding will occur when they are encouraged to analyze each factor of density in the PhET simulation lesson.