High School Biology-Chemistry SMILE Meeting
23 September 2003
Notes Prepared by Porter Johnson

Therese Donatello [ST Edwards, Elmwood Park]       Density  + Archimedes Principle
Terri
led us through some simple experiments that are designed to help students take accurate measurements and write proper laboratory reports. [In her classes she asks students to develop and write a procedure for a laboratory experiment using part or all of the materials on a list that she provides.]  She began by placing the following equipment and materials on the table:

Balance   ...  Spring scales   ...  Weights   ...  Graduated Cylinders   ... Water
Then Terri asked us what information we could obtain about water and other substances using these materials.  [Terri had asked her students to write a procedure for an experiment of their choosing, using these materials.] There were various suggestions from SMILE participants, such as these:
Mass ...  Volume ...  Density ...    Length  ...
Terri then asked us the following specific questions regarding density measurement:
• If I want to find the density of a body, what equipment or materials should I use?
• How would I find the density using them?
We then divided into groups, and began developing and writing down a protocol to determine the density of a body, such as one of the weights in the set.
We developed the following successful scheme:
1. Use the balance to determine the mass of the body, M.
2. Partially fill the graduated cylinder with water to a specified (or measured) volume, and record that volume, Vwater.
3. Put the body into the cylinder (completely submerged), and re-measure the volume of the body plus the water: Vtotal.
4. Subtract to determine the volume of the body, V = Vtotal - Vwater .
5. Calculate the density,  D = M / V.

Terri then asked us to determine the density of water using these materials (a simpler question), and we developed the following procedure:

1. Determine the mass of the empty graduated cylinder, M0.
2. Add a measured volume of water, V, to the cylinder, and determine the total mass of cylinder plus water: Mtotal.
3. The density of water is Dwater = Mwater / V = [ Mtotal  - M0 ] / V.
Next Terri asked us to develop an experiment illustrating Archimedes' Principle using the materials at hand, as listed above. That is, we wish to show that objects fully submerged in water will displace a weight of water that is equal to the "apparent loss of weight" of the submerged object. Through group discussion, we developed and then followed this procedure:
We used  two steel cylinders,  each about 1.5 cm in diameter and 3.0 cm long.
1. Fill a plastic cup up to the rim with water.
2. Suspend both cylinders from a string, and determine the total mass using a spring scale. (We got  140 grams).
3. Lower the cylinders into the cup until they are completely submerged, and again measure the "apparent mass" of these objects while they are submerged. (We got 100 grams).
4. Collect the water overflowing from the cup, and determine its mass. [We got about 40 grams.]
5. Eureka! Archimedes' principle works!

A wonderful combination of ideas, Terri! Thanks.

Ben Stark [IIT] took the group on a tour of his research laboratory, to show what tools are used in modern research in molecular biology, biochemistry, and microbiology. Fascinating, Ben!

Schedule of Future Presentations:

Notes taken by Ben Stark.