Density

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Objective

Students will be able to conceptually and mathematically explain the concept of density.

Big Idea

Density, the ratio of mass to volume, is a tool that can be used to describe and identify substances.

Introduction

Density is one of the basic physical properties that can be used to identify unknown substances. In this lesson students are able to look at density from a macroscopic perspective using a density column. They must also think about density at the nanoscale by looking at the factors that affect density. Finally, students will calculate densities using mass and volume measurements. 

This lesson highlights the NGSS Crosscutting Concept of Structure and Function because it deals with density. Density describes how much matter occupies a given amount of space, and things that are more dense act differently than things that are less; more dense objects tend to sink compared with object that are less dense. Density is the result of two nanoscale properties--the mass of the individual particles, and how tightly they are bonded. This lesson also involves the NGSS Practice of the Scientist Using Mathematics and Computational Thinking by using the density formula of mass divided by volume to create the derived complex unit of g/cm3.

Do Now/Activator

10 minutes

Students read the density column directions and summarize them in a sentence or two. They will identify two important pieces of advice in the directions.

As a whole class we discuss the directions. I use cold call for this dialogue. Questions I will ask are: 

What safety comment is the directions? (safety goggles)

What is something you need to know to make a successful density column? (add slowly)

What's another one? (Aim for the center of the test tube)

How should you handle your test tube once you begin adding materials? (gently, and keep it stationary if possible)

Once these steps are established I point out where the equipment and supplies are and students work in groups to create their column.

To save time or resources, this step in the lesson can be omitted and the teacher can make one large column that looks like the one in this density column from class

Mini-lesson and Guided Practice

15 minutes

Mini-lesson: After students have set up their density columns I show them the density presentation that describes the factors that effect density, the tools and units associated with density, and the formula for calculating density. During this time students take notes. In order to proceed quickly through the presentation, I provide a Density Notes Template for students to record information. After the presentation I post the Density Notes Answered to assist students who did not get all of the information in the presentation. This sample of student work was typical; most students are comfortable with the pace of the notes presentation.

I then review the directions for the density mini-lab, which is designed to help them become more comfortable with finding the density of objects. 

Guided practice: During the presentation I give students a moment to calculate the density of the substance in slide 6 before advancing to the answer in slide 7. Students sometimes do not know how to do this simple division, so I remind them that in a fraction the numerator is the divisor and the denominator is the dividend. While most students were comfortable with using the formula, some students feel the need to abandon the formula in favor of putting the larger number on top. I remind them that the formula rules this process.

Application

25 minutes

Students use the information from the mini-lesson to find the densities of various objects. I like to use kits like this one from Frey, where students have to identify the different substances by figuring out the density of the object. I label each item with a letter so that the whole class can compare data with ease at the end of the lesson. I also project or copy the  Density Data Table for the same purpose. Because I do not have time to teach significant figures in this lesson, I instruct students to round to two decimal places. During this time I work the room and comment on measuring techniques and help students with calculations.

The biggest challenge for students is using the displacement method. Students  have a hard time reading at the meniscus, and they have a hard time remembering what two volumes are necessary in order to use this method. I remind them that it is the volume before they submerge the item subtracted from the volume after.

While it does not make sense for me to provide an answer key because they are linked to the specific samples of material that I have, I do have this list of Common Densities that my students use to identify the substances that I have in my kit from Frey.

This density lab data table typifies the types of data students were generating from this measurement lab. This film clip shows how I encourage students to use evidence to support their substance identifications after they have completed their measurements.

Debrief

10 minutes

During this time the whole class compare answers. I emphasize that the goal here is to see how we are doing with measurement and with calculations. I do a quick fist-to-five, in which I ask all students to show me where they are with density by raising their hands with a self-assessment, with 5 being expert and fist being significantly lost. 

I then pass out the Density Exit Ticket, which I use to assess conceptual understanding of density.

My hope is that students will recognize that that at the macroscale some objects float on others, and that the items at the top have a lower density. Students should note that molecular size and compactness are the two factors that affect density. A sketch that captures one or both of these would show a strong understanding.

Based on this typical density exit ticket student response, I feel that students have a reasonable grasp of the concept of density.