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Isotopic decay of chewing gum

Kyle Nichols
Skidmore College
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This page first made public: Jul 11, 2008


This activity allows students to see (and feel) the decay of chewing gum as it relates to ½ lives. One or two students chew gum during class and each minute they weigh the gum. At the end of the experiment students calculate the half-live of chewing gum.

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I use this as an in-class activity during an introductory 'lecture' on isotopic dating.

Skills and concepts that students must have mastered

Students do not require mastery of any skills or concepts. They must have completed the appropriate reading before class and be comfortable reading and understanding equations.

How the activity is situated in the course

I use this demonstration during class time after I introduce the basics of isotopic decay. I take several minutes to explain that the demonstration is an analog to isotopic decay. I make sure to point out that decay results in a different isotope, not a loss of material. In the gum demonstration the resultant product is just taken up by the body. As the students are collecting the data, I explain and define the equation and also open discussion about error and uncertainty. I graph the data real-time so students can see the trend.


Content/concepts goals for this activity

The goal of this activity is for the students to understand isotopic decay and half-lives.

Higher order thinking skills goals for this activity

Depending on how the activity is designed, students will be exposed to hypothesis testing (if different types of gum are used), quantification of data and model fitting, spread sheet models, and identifying sources of error.

Other skills goals for this activity

None really...unless it adopted as a mini-laboratory exercise.

Description of the activity/assignment

Student preparation for this assignment is the same as any other class. They are responsible to complete the reading before class. After I have introduced the concept of isotopic decay I have three student volunteers conduct the 'experiment'. One student is the time keeper while the other two students chew gum and weigh the gum at one minute intervals. The only instructions to the students are to chew consistently and to make sure they try to weigh a 'dry' piece of gum. The time keeper is responsible for making sure the gum chewers weigh the sample after each minute of chewing. I enter the data into a spread sheet as it is collected. That way I can discuss the data, explain the equations and ask questions. The entire experiment takes about 25 minutes to complete. After all of the data are collected, we determine half-lives through an iterative process by minimizing the RMS error. This also allows me to introduce error and one way to quantify it.

This demonstration could be expanded into a component of a laboratory. Multiple students could chew the same gum to increase the sample size, the could chew different types of gum and measure a different decay constant, they could plot their own graphs, calculate their own decay constants and half-lives, and use a type of candy that does not follow the from exponential decay form. In this form students could form their own hypotheses and test them.

There is no additional information that you need to know. I have included a spreadsheet of data from Fall 2007. You can use the same spreadsheet and just adjust for the data from your students. The only thing to keep in mind is that gum will not decay to almost 0 because there is a portion of the gum that will remain. We are only interested in the portion of the gum that does 'decay'. Therefore, I have included a 'mass correction' calculation based on the mass of the gum after the 10 minute mark.

Determining whether students have met the goals

No real evaluation as it is designed, other than asking questions during the exercise. This exercise would work well with clickers or as a lab exercise.

More information about assessment tools and techniques.

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