## Mineral density teaching accuracy, slope, and percent error in the earth science classroom year 6 electricity

Prior to doing the lab, I use ideas from a Science Scope article (Peterson-Chin, 2004) to introduce density in a qualitative way during a class discussion. The discussion also includes examples of measuring density and the units for density. I specifically go over how to measure the volume of an irregularly shaped object by water displacement.

Each student receives a lab handout with the procedure. I then give each group five pieces of the same mineral, but of different sizes. Make sure the samples will fit in the graduated cylinders you have. I use seven different minerals so that each lab station has a unique mineral (and students can’t copy). The minerals I use are: quartz, hematite, muscovite, orthoclase feldspar, plagioclase feldspar, gypsum, garnet, or pyrite. Students predict the value of their mineral’s density, knowing that the density of water is 1 gram/cubic centimeter. Students make a data table in their notebook (with teacher guidance as needed) and make their mass/volume measurements.

Students first calculate the density of each mineral piece (density=mass/volume). Students then plot their volume-mass data on an x-y graph and then enter their data onto a spread sheet and make a line of best fit. I talk with the groups that their line should have a y-intercept of zero and ask them why that should be. I also discuss with groups that the data should be linear. I expect there to be a lot of scatter in data as students tend to be sloppy in **their measurements**.

I help the students see that the *slope* of the line is the relationship between mass and volume of a substance and is therefore the density of that material. Students compare the **slope** of the line (density) with the known density value and calculate the **percent error** in **their measurements**. The students’ grade in the lab is a function of how low their **percent error** is, so if a group wants to redo __their measurements__, they can. I have not done this lab before, so at this time I don’t know what a reasonable *percent error* is for the 13-15 year old age group. On average, I would expect older students to be able to make better measurements than younger students can.

When I am confident each group is doing measurements and calculations correctly, I have the students find the densities of two unknown minerals, which may look somewhat similar but density is a distinguishing property (apatite/beryl or topaz/corundum or halite*/calcite). Students make their mineral identifications based on density.

Even though the concept of density is introduced in 6th grade, 8th graders usually have little experience and understanding regarding the concept. I usually try to teach density again at 8th grade because the students NEED to review the ideas with a mind that is starting to handle abstract concepts. Because density differences drive convection in both the atmosphere and mantle, understanding density is fundamental to grasping some basic earth science processes.

This lab requires that the teacher circulate among the lab groups to check on and refine the students’ measurement techniques. I expect the students to measure to the nearest tenth of a gram. When the students **calculate density**, it is a good time to discuss significant figures because students WILL ASK how many decimal places they should have.

I have never taught density before with such a direct math connection. Usually I have the students consider a mineral’s density by asking themselves if a mineral feels heavier for its size or lighter for its size. Most students have an idea of how heavy a hand-sized rock should be. After all, they are accustomed to picking up rocks with an average density of 2.65 grams/cubic centimeter and actually have a good feel for a sample being heavier than it should be. This is a perfectly fine way to estimate a mineral’s heft. However, I want my students to develop a more sophisticated understanding of density that includes how density can be expressed mathematically. Assessment