## Eureka!

March 30, 2010

by: Cynthia House

I sponsor an after school Science Club in a K-5 elementary school. The club is organized into two-week-long sessions, each session focusing on a specific topic. One of this year’s most successful sessions involved the Archimedes Balance from Educational Innovations.

For week one, I prepared:

• calculators
• answer sheet, listing the sample materials and their densities
• fill-in table to record findings:

The table I provide includes simple, step-by-step instructions for students so they can easily determine measurements such as volume, mass and density.  For complete instructions including all charts, use this link.

Students worked in pairs with first and second grade children teamed with a fourth or fifth grade student. We introduced the topic with a brief Power Point biography of Archimedes and his accomplishments, focusing on the story of King Hieron’s crown. Then students practiced determining the density of materials using the Archimedes balance and the samples supplied in the sets (all directions are included in the kit).

The Archimedes Balance relies on Archimedes’ principle which states that a floating object displaces its own weight of fluid.  The balance consists of a graduated cylinder partially filled with water and a tube that fits inside the cylinder and can float in the water.  By placing an object inside the inner tube and measuring the amount of water displaced, you can easily determine the objects weight.

The fill-in table helped them remember what measurements to take when, and how to calculate results. I was surprised at how completely engaged all of the students were, and the accuracy of their results.

To prepare for the second week I assembled the following;

1. control samples: strips of zinc, copper, aluminum, iron, and  nickel metal (battery electrodes), and the brass and nylon cylinders from the Class Set of Six Archimedes Balances Classroom Set and Classroom Density Assortment.
2. samples of unknown composition including coins, screws, bolts and other fasteners, furniture hardware, machine parts, plumbing fixtures, etc. We had 18 different samples. For very small items, students used enough pieces to obtain accurate volume and weight measurements, for example, twenty pennies instead of one.
3. metric rulers, electronic balance, and micrometer
4. calculators
5. electrical conductivity tester made from a battery holder, tiny light bulb in a socket, wire, and alligator clips
6. magnets
7. graduated cylinders from the Class Set of Six Archimedes Balances Classroom Set
8. cylinder protectors (see drawing below)
9. fill-in tables to record findings

We started the second day by telling the students that the local library needed to obtain samples of items made out of zinc for a display of the chemical elements. Our Science Club had been contacted to find those zinc items.

Students began by determining the electrical conductivity, magnetic characteristics, and density of the control samples. Since many of the samples did not fit into the floating tubes provided with the Archimedes Balance sets, and to save time, we determined weight using an electronic balance. If volume could be easily calculated using measurements with ruler and micrometer we did so.

Having finished characterizing the controls, students now examined the assortment of items of unknown composition, choosing for themselves which items to investigate.  They were cautioned that some items may be lacquered, effecting conductivity results, or plated, effecting the color. Students determined weight using the electronic balance, and volume using the graduated cylinders. I thoroughly enjoyed listening to each team’s reasoning for selecting promising candidates. (The zinc sample was post 1982 United States pennies.)

## High School Density

July 22, 2009

by: Ron Perkins

Whether teaching general science, chemistry or physics, one of the first experiments I assigned was to determine the density of a metal using a set of different sized cylinders of aluminum in a tray.

Each Student:

• Determined both the mass and volume of a single assigned sample.
• Recorded their data point on a large classroom Mass vs. Volume Graph.
• Participated in a class discussion on: determining volume by different methods; drawing a straight line through the data points (including the origin); and calculating the slope of the line (rise over run)

Ron’s suggestions:
1. The set of aluminum cylinders (DEN-102) or PVC (DEN-120) are ideal beginning sets. The brass set (DEN-110) is interesting as one can determine the percentage of brass and zinc from the density using a CRC Handbook. The Polypropylene set (DEN-132) is interesting because the specimens float.

2. The Density Mystery Set (DEN-202) uses the element of surprise to teach students to trust their data. The set is made of two different black polymers, each with its own density. When the data is plotted, two different straight lines are produced, each with its own slope or density. Typically students will assume that the material is all the same and start questioning their own measurements. About half of the samples sink in salt water and half float

3. Our most popular set provides samples of 12 different substances (DEN-212). Also popular are the cubes of 6 metals (DEN-220). Once students have mastered mass and volume measurements, they find it interesting to be able to identify a substance by determining its density.

Advantages of using our Density Kits:

• Students learn that the density depends upon the ratio of mass to volume and not upon size of the sample.
• Students observe that some methods for determining volume are more accurate than others.
• Students discover that the slope of the “best” straight line usually gives a more accurate density value than calculating from a single piece of data.
• The teacher can immediately tell from the data points if a student needs help in measuring.
• The teacher can quickly see if all of the samples have been returned at the end of class.