Eating Nails for Breakfast
Lesson 5 of 11
Objective: Students will learn how to use magnetism extract iron from a mixture. They will be able to share data and make an argument from the evidence.
I do this lab annually because it helps to reinforce the idea that many substances are mixtures. The idea that there is metal in cereal is a discrepant event for many students, but they cannot deny the evidence that the magnet offers. This lesson is also the first time in class that I ask students to aggregate class data in order to reach a conclusion, and as such, develops an important skill with an accessible task.
This lesson reinforces the NGSS Crosscutting Concept of Structure and Function by giving students the opportunity to think about how substances are comprised. Students will be challenged with the Science Practice of Engaging in Argument using Evidence to defend their analysis of the various breakfast cereals, and because they are only using qualitative evidence, they will be introduced to the need to quantify results.
Today students will show how a physical property can be used to separate a mixture. To start, students read the Extracting Iron From Cereal Procedure. With a partner, they then use stick figure cartoons to create a series of steps necessary to conduct the mini-lab. So that we can easily compare everyone's procedure, I model how the cartoon should be set up by projecting the Extracting Iron From Cereal Cartoon graphic organizer.
I frame this part of class by explaining that understanding procedures is important for their success and safety in the chemistry classroom. We compare procedures by projecting them with a document projector. The document Extracting Iron From Cereal Cartoon Suggested Steps has some suggested steps, sans pictures.
This student's magnet cartoon shows a typical student response. I really emphasize that the cartoon is not meant to capture every detail, but more it is a way for us to focus on the details; the game we play with the cartoon is to try to ifnd all of the details that were not captured by the cartoon.
We look for as much detail and understanding of each step as possible. If we see omissions, I ask the class what would make this step more clear. I wrap this section of class up with a reminder that following each step carefully, and asking questions when in doubt, is an important part of our year together.
I then release them to begin the lab.
Mini-lab: As students conduct the lab I walk around and monitor for safety and adherence to the procedure. I also engage students in conversation. The actual extraction does not take very long--about 10 minutes. What will be interesting is to see how students share data. I have left this part of the lab purposefully vague. Note: I typically use 3 different kinds of cereal. Total has the most iron, and then I pick two other cereals off the shelf that are inexpensive and that have less than 100% of the total USRDA for iron.
Students may develop a clever mechanism for comparing their iron amounts. It should be obvious from just looking that Total has more iron in it than the other cereals. However, during debrief I will revisit their "data" and talk about the need in science to quantify results, which we will do for the rest of the year.
When students are done cleaning up they spend some time answering the questions found at the end of the lab procedure.
I start this part out by holding up a piece of iron and asking students what they think about the idea of eating iron. When that sharing has run its course, I ask for volunteers to share their answers for the first question from the bottom of the procedure.
Based on your data, and the data from your classmates, which breakfast cereal provides the most iron per serving?
Students will probably say Total had the most iron, but there may be some discrepancies. If so, I try to create some tension around these disagreements. This should launch nicely into question 2:
Check the Nutrition Facts Panel on the box or bag from the cereal. Do the ingredients support your findings?
This leads us nicely into the idea of qualitative vs. quantitative data. Students probably gathered qualitative data, but the USRDA data is reported in quantitative data. I explain that while it was probably easy to categorize our levels of iron in each cereal qualitatively, scientists rely more on quantitative data with agreed upon units of measurement to compare their data. Students were clearly uncomfortable with what data to enter when we finish this lab, which means they have an appreciation for units and quantities. The photo of extracted iron gives an example of what they were wrestling with.
The Iron in Breakfast Cereal Class Data reflects their uncertainty in terms of quantifying the iron they extracted. I explain to them that if we had more time we could remove the iron from the bag, dry it, and find its mass. However, this procedure would only be useful if everyone was able to extract all of the iron from their sample. Here is a photo of what students were trying to quantify.
I gauge the success of this lesson informally based on how students respond to these questions and they types of conversations that occur during the lab. If they are able to extract iron and use evidence in their claims then I consider the lesson a success.