For today’s class we did two different labs. The first one I found while digging for materials for the chemistry class I’l be teaching this fall. This website, Middle School Science, has a great list of resources for middle school chemistry, including this activity for ‘bottles of stuff.’ I liked this activity because the kids get to bring in a bottle of stuff, they have to guess which ones are more massive and then actually estimate the mass of each bottle. Since few Americans are familiar with grams, its a bit of challenge to guess the mass of a bottle of legos in grams so I gave them a big bottle that had a mass of 500 grams, a 50 gram brass mass and 5 gram cubes (each cube is a gram). This helped them at least get the right order of magnitude. Once everybody had their estimates the kids learned how to use a triple beam balance to measure the actual mass of each container. There were a few surprises, like the bottle of yarn actually had a smaller mass than the bottle of air, but the kids were quick to realize the bottle of air was bigger and made of thicker plastic and it had a larger cap.
When they were done making their measurements and ranking the bottles from lightest (least massive) to heaviest (most massive) we talked about their density a bit. I asked them if they thought any of them would float and they thought they woud since they still had air in them. One kid asked if we could test it out so I pulled out a big bowl and filled it with water. They noticed the bottle full of cotton was almost entirely above the surface of the water but the very massive bottle of screws was almost completely under the water. This happens because the weight of the bottle displaces a volume of water that would equal the weight of the bottle. The bottle of cotton only weighed around 20 grams so it only displaces 20 grams of water but the bottle of screws was over 500 grams and so it displaces over 500 grams of water.
The second lab we did was from the Real Science Odyssey Physics curriculum, Water Displacement: It’ll Float Your Boat! Each kid was given a ball of sculpey clay about 1 inch in diameter and a cup of water. They marked the level of the water on the outside of the cup with a whiteboard marker. Then they dropped their ball of clay in the water and marked the new level of the water. The water line went up since the clay was taking up space in the cup. Then they made different shapes and tried again. The water always came back to the same level. After doing that a few times they started making boats that float and saw that the water line was still the same!
My son, Charlie, who’s helping with the class shared this video by Steve Splanger where he floats an Aluminum foil boat on what looks like regular air, but its actually a very heavy clear gas called sulfur-hexafluroide.
We didn’t show this in class but here’s a Bill Nye video on buoyancy where they do a similar activity with clay boat vs ball and measure the actual amount of water displaced and show its the exact same for both.