Density- An Intrinsic Property

Objective: Students will develop an understanding of the nature of density while fostering creative scientific thought and presentation skills.

Students may agree that density is an intrinsic property, a property that is independent of the quantity of a substance. Asking students to prove this in the lab leads to an inquiry-based lab design of their own.

After discussing the formula for density, Density=Mass/Volume, and sighting examples of other intrinsic properties, such as melting point and boiling point, students are given 50-60 minutes to collect data in the lab to prove that density is an intrinsic property.

Materials provided include the following:
1. All Standard Lab Drawer Equipment (the only item they truly need from the drawer is a small graduated cylinder, but giving students all their options leads to a more challenging experience)
2. triple-beam balance or electronic scale
3. various amounts of metal shot (zinc, tin, aluminum, and copper work well)
4. Weigh boats
5. Standard densities for the metal shot are written on the board for student reference.

Note- strainers, paper towels, and sink stoppers are helpful in controlling the amount of metal shot that "gets out of hand." The shot can be dried after use and reused.

After some pre-thought and discussion, lab partners eventually come to the conclusion that they must find differing mass quantities and volumes of metal shot to prove that density is not dependent on the amount of material. Students come to the main table to select a metal. I supervise students as they pour out a reasonable quantity of their choice of any metal shot (they may decide to select more than one type of metal shot, but caution them not to mix them together!). Students carry the shot in a weigh boat to their stations. Additional weigh boats are made available for ease of transfer on the scales.

At the station, the mass of each quantity of metal is found using the scales, and the volume of each quantity is found using a graduated cylinder. Many students will mistakenly use the larger 100ml graduated cylinder from their drawers first. As they build a data table of corresponding mass and volume measurements, students begin to determine the density of each mass/volume relationship. If the larger cylinder was used, students will question why their density measurements do not closely match the given density of their metal. I circulate around the lab, explaining the advantage of using the smaller 10ml cylinder (a more precise measurement!) and assisting, in general.

As students perfect their technique in making mass and volume measurements, they are asked to document each step of the procedures they are writing by taking a digital image properly showing their lab technique. These images are used by the students to create a PowerPoint presentation, so that they may later showcase their procedures to the class via the class computer and digital projector.

Due to time constraints, I randomly pick three presentations out of all the presentations turned in for homework. Students are asked to briefly present them to the class. Students in the audience are allowed to critique the presentations by scoring a rubric that highlights the following: integrity of lab procedures, image composition and quality, proper labeling of images, presentation technique, and overall clarity.

Sometimes, students will inadvertently prove that density is an extrinsic property! If students discuss at great length in the required formal lab report what errors might have caused their data to show this, they are not penalized. The goal is to expand the students' creativity of scientific thought, lab skills, and understanding of the intrinsic nature of density.