Introduction to Density (Without Calling It Density)

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Objective

SWBAT explain how two different cubes of the same volume can have different masses.

Big Idea

Students have a general sense of how matter changes forms but their understanding of other properties of matter need more development. This lesson gets students thinking about density as a property of matter.

Section 3 Overview

In the lessons on volume, I wanted my students to develop a deep understanding about volume, not just telling me that volume is length x width x height.  My focus on developing a deeper conceptual understanding of density will be similar to my approach with rectangular volume. In particular, I want students to explain:

1) Why objects are more or less dense from a molecular standpoint, considering size, arrangement and mass of different types of matter

2) Use density as a characteristic property to identify unknown liquids and solids 

3) Explain the role that temperature has on density of matter and the implications of this cause and effect relationship

Do Now: Teacher Demo, Preconceptions & Wonderings

10 minutes

Students fill in a KLEWS Chart for this lesson, which serves a few purposes: 1) find out what they already know about density, 2) challenge their preconceptions about density, and 3) to give them a space to reflect on what they learned, supported by evidence. At the end of section 3, each student will have a document that helps them better track their understanding of density, helping develop their conceptual understanding (SP6 - Constructing Explanations and Designing Solutions) through metacognitive reflection.  

As an introduction to density, I demonstrate how two objects having the same volume can have different masses. I do this by measuring the mass of copper and aluminum on a balance in front of the class.

As opposed to simply telling students that the size, arrangement and mass of the atoms may affect the density of different types of matter, I give them the opportunity to record questions that they may have in their 'wondering' section of their KLEWS chart for this lesson. They also fill in column one with their present conceptions of the demonstration. Later, they can reflect on their findings from various investigations to determine any cause and effect (CCC) relationships to deepen their conceptual understanding of density. 

Here is a sample of a KLEWS chart that I recorded on the board.  It represents the findings of the entire class.

Finding the Mass of Various Cubes

15 minutes

Telling kids how to calculate density using an equation and expecting them to understand the intricacies that determine the density of matter is too teacher-centered, resulting in a shallow conceptual understanding of the important property of matter that is grounded in so many fundamental principles of science.

Teachers may be tempted to show students how to calculate density at this point, which we will get to later, but I am going to take the opportunity to have students develop their understanding through inquiry.

As such, I am not even introducing volume any more than I already have during the Do Now. Instead, I have students weigh various cubes to see if they all have different masses.  

Teacher Note: They do have different masses. My hope is that this investigation leads to other questions, such as:

  • "Why do the cubes all appear to have the same volume but different masses?"  
  • "How can something be the same size but have different masses?"
  • "Are the cubes made of different types of matter?"  

More advanced students may notice that the objects all look different and ask:

  • "Does what an object is made of affect how it looks and acts?"

At this point, I want students to look at other pictures of matter to find cause and effect relationships.

Students Observe Molecular Models of Matter and Find Patterns

10 minutes

Now that students have masses of various cubes, I want to present molecular model images to them so that they can find some patterns between the mass of each cube and the relative molecular structure of the different types of matter.

I ask, "Are there any patterns that we see between the mass of each cube and our observations recorded in each picture?"

Structure of activity:

1) Ask students to observe the molecular structure images that are labeled by material (see resources) and record their observations in their notebooks for each type of matter.

2) What do we notice about the molecules in terms of size, space between molecules, and the mass from the previous activity?

3) Answer question: Why do you think that some cubes are heavier than others, despite being the same size?

Here is a sample of a student response.

Wrap-up

3 minutes

Students will not leave this lesson with a clear understanding of density. As educators, we need to accept that NGSS-based lessons are messy and not conveniently wrapped up at the end. Lesson wrap-ups for NGSS lessons need to recap what what discovered, giving students ample time to reflect and process their findings so that they can accurately explain this phenomenon in the future. Scientists don't have answers to complex questions handed to them, nor should our students!  

During this time, you can have students fill in their KLEWS chart for this lesson together as we review our findings. Ask for volunteers to share what they discovered during today's class.  This helps all students reflect on what they learned.