As an introduction to density, I do a demonstration/discussion/group activity using density bottles. They are small sports drink bottles that I estimated to have a volume of approximately 400 mL. There are 5 bottles, each filled with a different item: cotton, air, sand, rice, and colored water.
These are some of the questions I used for our discussion:
- “Do these bottles have the same volume?” There is some uncertainty at first, but then they quickly say “yes”.
- “Do these bottles have the same mass?” No
- “Why don’t they have the same masses?” Variety of answers
- “Which one do you think is the heaviest?” We do a survey with a show of hands then have the kids give some reasons for their answers
- “Which one do you think has the most ‘stuff’ crammed into the bottle?” It’s interesting, there is a wide variety of answers and it usually doesn’t match the answer to the question of which is the heaviest
I tell them that they will find out the answers in a minute! We watch the BrainPOP movie for Measuring Matter. After the movie, I give the analogy of standing and waiting for an elevator. Two identical elevators open up: one has 2 people in it and the other has 15 people in it. “Which elevator would you choose and why?” Naturally, they say the one with only two people, there is more room in that one. I ask them, which elevator is denser? The one with 15 people, of course. We then discuss that there is less empty space available in the elevator with 15 people in it. I then relate molecules to the people in the elevator, matter that has a lot of molecules, or atoms, crammed into a given space are denser than objects whose molecules or atoms have a lot of empty space between them.
I hand one bottle to each group and have them find the mass. We collect the data and I write it on the board. I re-ask the following questions:
- Which one is the heaviest?
- Which one has the most ‘stuff’ crammed into the bottle? (Variety of responses)
- Which one is the densest? (Variety of responses)
Now that we have the mass and the volume, we calculate the densities for each bottle. After we collect the data, I have the kids come over to the dunk tank. One at a time, we predict which bottles will float. We do a survey and raise our hands if we think the bottle will float. I have one student place the bottle into the tank and we see if it floats or not. We continue until all 5 are in the tank.
The cotton, rice, water, and air filled bottles floated, the bottle with the sand, sank to the bottom. I then ask the kids “Why did the bottle of sand sink?” They usually say it was the heaviest. I then say, “But a cruise ship is a lot heavier, and it doesn’t sink? Why?” I give them a hint, “Look at our data, what do the bottles that floated have in common?” After a while, they figure out that the bottles that floated, all had numbers that were decimals, or less than one. The sand was over 1, and sank. I tell them the density of water is 1, so objects with a density greater than 1 will sink.
We talked about how the bottle of sand is the densest b/c it has the most amount of “stuff” crammed into the same space, and that there is less empty space between the atoms. I tell them that the density of gold is 19.3 g/cm3, and that if this bottle was filled with gold, it would be about 19 times denser, meaning that there would be 19 times more “stuff” crammed into the same space. The next day we talked about the story of Archimedes. We calculated how much mass the same bottle would have if it was filled with pure gold - it would be 7,720 grams!!
After the dunk tank, we did a small group acitivty using the graphic organizer from BrainPOP. It shows a ring, balloon, yo-yo, and pillow. We have to categorize them according to mass, volume, and density, from highest to lowest. We do one category at a time and I give them a minute for each, going over the answers between each category. I liked this graphic organizer b/c it really made them think about each item and their properties.
pg. 34 - BrainPOP – Archimedes
pg. 35 - BrainPOP – Mass, Volume, Density Graphic Organzier