Density can be described as a way of comparing the "heaviness" of materials. I gave the students an example of a basketball and a rock that weigh the same. The rock is smaller than the basketball but it is heavier. Since the rock has its weight held in a smaller package, the rock is denser than the basketball.
The first activity today was a demonstration lab - "Mover." For this lab, we had a plastic water bottle that I cut the top off to make a funnel. The bottom of the bottle was filled with cold tap water that we added four ice cubes to. We then filled a measuring cup with warm tap water and added blue food coloring to the warm water. Using the funnel, I poured the warm blue water into the ice cold water while the students observed what happened. We noticed the blue water rise to the top of the bottle.
This happens because cold water is denser than warm water. This is due to the cold water contracting while the warm water expands. A drop of cold water, then, is more dense than a drop of warm water. The dense cold water settled in the bottom of the bottle while the less dense warm water rises. This also explains why the surface of the ocean feels warmer than deeper water in the same section of ocean.
I then dropped a nail in a glass of water and asked the students why, if the small nail sank, can large cruise ships and aircraft carriers float? The students provided some suggestions and we completed the lab, "Floating Boat," to find out.
The students each placed their boats and foil balls into a bucket of water. The boats floated as they were supposed to. The balls were supposed to sink but ours all floated!
Even though the ball and boat have the same weight, the ball takes up a smaller space making it the denser object. The ball pushes less water out of the way than the boat so there is not (should not be) enough upward force to cause the ball to float.
Large ships are able to float, despite their great weight, because they have hollow compartments that are filled with air. This air increases their buoyancy.
Continuing our discussion of buoyancy and buoyant force, we moved on to discuss differences in salt water and fresh water. I asked the students if there is a difference in the buoyant force of fresh water and salt water. I also asked which one they thought had a greater buoyant force. To test their hypotheses, the students completed a hydrometer lab. For this, we filled a pen cap with modeling clay and then dropped it in a jar of tap (fresh) water. The students then added salt (1 tablespoon at a time) and observed the changes in the pen cap. We found that adding salt caused the pen cap to gradually float to the top of the jar.
The hydrometer lab led into a discussion of the Dead Sea. The Dead Sea is the lowest point on earth. Its elevation is 1,300 feet below sea level! We compared this to the elevation of Vista which is 563 feet above sea level. The Dead Sea is also the saltiest body of water on earth - it is almost 10 times saltier than any of earth's oceans! The minerals in the Dead Sea make the water so dense that people are able to bob on the surface like a piece of cork.
We finished the class session with two last lab projects: "Risers" and "Subs."
For "Risers" the students were each given a clear plastic cup, club soda, and some modeling clay. They divided the modeling clay into five rice-sized pieces and added club soda to the cups. The students immediately dropped the pieces of clay into the cups of soda and watched what happened. The carbon dioxide bubbles in the soda stuck to the clay causing the balls of clay to be light enough to rise to the surface of the cup. Once the clay reaches the top, the bubbles are knocked off and the clay sinks back to the bottom of the cup.
To complete the "Subs" lab, each student was given a plastic cup and a flexible drinking straw. Each student had a turn placing the cup on its side and pushing it beneath the surface of a bowl full of water. They then turned the cup so it sat upside down on the bottom of the bowl. Each student slipped the end of the straw under the rim of the glass and blew through the straw while supporting the glass with one hand (while trying not to restrict the movement of the glass). The students observed that the glass tried to rise to the surface of the water while they were blowing into the straw.
In order for submarines to sink, special tanks on the subs are filled with water. This is similar to the students filling their cups with water. To allow the sub to surface, the water in the tanks is replaced with air. We demonstrated that by blowing into the cups. The air makes the submarines more buoyant, allowing them to float to the surface.
To look forward to next week: Friction!
The labs Floating Boat and Risers are from Physics for Every Kid.
VanCleave, J. (1991). Physics for Every Kid: 101 Easy Experiments in Motion, Heat, Light, Machines, and Sound. San Francisco: Jossey-Bass.
The labs Mover, Hydrometer, and Subs are from 200 Gooey, Slippery, Slimy, Weird, and Fun Experiments.
VanCleave, J. (1993). 200 Gooey, Slippery, Slimy, Weird, and Fun Experiments. New York: John Wiley and Sons, Inc.
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