Teacher-Student Interaction

A goal of this laboratory activity is to provide an opportunity for students to develop a formula from data. The students collect and graph the data. The expected result is a straight line with a zero intercept. Because the line should intersect the origin, the general mathematical function (y = mx + b) is reduced to y = mx. By substituting the axis labels, it becomes apparent that y = mx can be rewritten as:

Mass = Slope(m) x Volume

Mass/ volume = m = Slope

The amount of mass packed into the volume is shown mathematically by the slope and corresponds to the physical property density. The formula is D = m /V . Students may encounter difficulty with the mathematical relationship and it might be necessary for the teacher to provide a classroom discussion.

This laboratory activity can be readily transformed into a specific gravity exercise using a weight displacement technique. Plot the mass in air versus the mass difference for specific gravity.

The activity can make use of the volume displacement technique. Students can weigh the dried metallic objects on the balance pan. (Caution students not to drop the metallic object into a glass graduated cylinder since the graduated cylinder might break.) If time is short, have students measure six objects instead of eight. If you choose the weight displacement method, demonstrate how to tare the thread.

Many hardware items such as bolts and screws are not made of pure iron but are alloys of iron. In order to achieve a straight line, select objects known to have the same densityÑitems manufactured from the same material. The physics teacher may have iron cylinders of various sizes that you may use. In some schools the metals (industrial arts) teacher may be able to assist you in making variously sized iron objects.

If possible, have a computer set up in the room so students can record their data and perform the calculations. The data table provided in this exercise can be modified to include density. In addition the computer can graph the data.

This activity also provides an opportunity to discuss extensive and intensive properties, closest packing of ions or atoms in solids, and density and specific gravity problems.

Anticipated Student Results