Transcript for Making Sense of Density – Kent Treadgold

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Killer tracks under “Kent Treadgold”

Script
Hi, I'm Kent Treadgold. I teach 7th grade science at Explorer Middle School in Everett, WA. Today we're going to be exploring density in my classroom.
“How many people think water is more dense? How many people think oil is more dense?”

The challenge of teaching density to middle school students is that it's a very abstract concept and most of what they're used to dealing with is concrete and real and things they can feel and see.

Prior to today, the students had only been introduced to the idea of density from sitting in their seats in the classroom and having me right up on the board density equals mass divided by volume. For 98% of them, that equation was meaningless. Today they're going to have hands on experience finding the density of these different objects and this is when they're going realize what the concept of density is really all about.
Today were going to continue with the solid density activity that you started yesterday.
Today when the students come in they will go in to their lab areas with their partners. They will then start measuring the mass of the four cylinders, they will then measure the volume of the four cylinders and they will start calculating the density.
When you’re working with the cylinders we’ve got 4 different sizes, we’re going to have small, were gonna have medium, were gonna have large, and were gonna have Mr. T. Where would Mr. T go in this arrangement? Ok the biggest one is Mr. T I agree with that. Let’s go to work.
(Classroom interaction)

Today the students are going to be using computers to collect data. They open excel and they create the file. I used to create templates in excel where they would enter the data. Well I found it to be much more valuable to have the students create their own spreadsheet where they collect their data-- for them to enter the formulas and for them to create the chart. The single most important item that I want students to take away from today's activity is that density will not change as the size and shape of an object change as long as the material is the same material.

After they've calculated the density, they call me over. This is a great time to just check to make sure the groups are in fact making accurate measurements and you can do a quick scan within a second or two of a group's data and see if they made a mistake or not. If the data looks good, at this point the students will then graph the data.
“So the volume keeps on increasing, the mass keeps on increasing but the density stays the same because they are made of the same material.”
When I first designed this lesson I was hoping that the graph itself would be the culminating point where it would be realized that density doesn't change as the sample size changes. I found as I was doing it with students that I needed to ask them some probing questions after they created the graph that would cause them to go back and look at their data, to look at the graph and figure out the answers to those questions.
“The four cylinders are all made up from the same material what do you notice on the graph about the mass as the cylinders get bigger?”
“The volume and mass both get bigger, but the density stays the same”
After that one of the questions I pose involves slicing a cylinder lengthwise so let’s revisit number 6 before we do number 7, pick one of the cylinders. You take Mr. T and you slice it that way what size will it be? It’d be half, if we sliced it in half hollow what would it’s volume be? Half of what it was, excellent. So now we need to calculate the density. 1.14 which means? It would stay the same. You got it!
And one of those questions will trigger that realization in almost every case and when that happens they are able to do the last step, which is just write a general rule for the density of any material.

One of the great things about this lesson is that students are able to be successful no matter what their academic level is. I am hoping that students will realize the concept of density, and I’m hoping they arrive at it not through listening to me but through working on their own and with their fellow group members to arrive at measurements and arrive at answers to questions.