Learning About Density: A Better Way?
A good understanding of density is important in many courses in science. Earth Science, Biology, Chemistry and Physics all have applications for this concept. Learning about density usually involves a lecture based lesson plan where students are mathematically shown how to solve problems using the density formula. I find that this method of teaching the idea of density to students, has them eventually good at the mathematical processes involved in problem solving, but will usually not give students an understanding of the nature of density itself.
A few years ago I started to teach density in a much more open-ended, hands on manner.(shown below) This has proven to be a much more successful manner for students to acquire a firm working knowledge about the nature of density.
Materials:
Plastic film canister with top
250-mL beaker of play sand
Spoon or scoop
Metric ruler
The Challenge:
“Who can create the heaviest plastic film canister, filled with sand, that will still float in water?”
The Catch
Students may NOT use water at any time during the lab experiment.
This open-ended experiment immediately excites, scares and motivates the class. Students are allowed to do some research about how they will proceed. To achieve success, students will need to find out the following information:
The density of water
That objects with a density less than that of water will float.
That objects with a density greater than that of water will sink.
To mathematically and experimentally determine the volume of the plastic film canister. The film canister is in the shape of a cylinder.
That the mass of an object is equal to its density multiplied to its volume, from manipulation of the density formula.
Since the density of water is 1.0 g/mL, then the mathematically calculated volume of the plastic cylinder is also going to be equal to the heaviest mass of the plastic cylinder and sand that will still float in water.
For example: If the volume of the plastic cylinder is calculated to be 45.0 mL, then the plastic cylinder and sand should have a mass of 45.0 grams.
The students will submit a lab report showing 1) The materials used in the experiment, 2) The procedures they believed would solve the problem, 3) A calculation section showing their work, and then finally 4) A conclusion of their results.
The students will also submit a labeled plastic film canister, filled with what they believe to be the appropriate amount of sand.
Once all reports and canisters have been submitted the “live” test results are done in front of the class. I fill a small fish tank with water. I place the label on the front of the fish tank called “The Pool of Judgement”. In order, I call the group with the lightest canister to the group with the heaviest. Each group will drop their experimental into the fish tank of water. The heaviest canister that still floats, is the winner.
The excitement for students to be able to test out their idea in front of an audience, is a big selling point of this activity. I have also found that even for students who do not have successful “test outs” with their canister, usually see very quickly the errors they have made. I highly recommend giving this activity a try.
