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Half-Life : Licorice

Description: With the Half-Life Laboratory, students gain a better understanding of radioactive dating and half-lives. Students are able to visualize and model what is meant by the half-life of a reaction. By extension, this experiment is a useful analogy to  radioactive decay and carbon dating. Students use licorice to demonstrate the idea of radioactive decay. This experiment is best used by students working in pairs.

licorce

Grade Level
5-12

Disciplinary Core Ideas (DCI)
3-5ETS1-2, MS-ESS1-4, HS-ESS1-6

Time for Teacher Preparation
40-60 minutes – To gather materials

Activity Time:
40-60 minutes (1 Class Period)

Materials

Safety

Science and Engineering Practices

Cross Cutting Concepts


Objectives
Students try to model radioactive decay by using the scientific thought process of creating a hypothesis, then testing it through inference. It is a great introduction to the scientific process of deducing, forming scientific theories, and communicating with
peers. It is also useful in the mathematics classroom by the process of graphing the data.

Students should begin to see the pattern that each time they “take a half-life,” about half of the surrogate radioactive material becomes stable. Students then should be able to see the connection between the M&M’s and Puzzle Pieces and radioactive elements in archaeological samples. Seeing this connection will help students to understand how scientists can determine the age of a sample by looking at the amount of radioactive material in the sample.

Background
Half-Life
If two nuclei have different masses, but the same atomic number, those nuclei are considered to be isotopes. Isotopes have the same chemical properties, but different physical properties. An example of isotopes is carbon, which has three main isotopes, carbon-12, carbon-13 and carbon-14. All three isotopes have the same atomic number of 6, but have different numbers of neutrons. Carbon-14 has 2 more neutrons than carbon-12 and 1 more than carbon-13, both of which are stable. Carbon-14 is radioactive and undergoes radioactive decay.

Radioactive materials contain some nuclei that are stable and other nuclei that are unstable. Not all of the atoms of a radioactive isotope (radioisotope) decay at the same time. Rather, the atoms decay at a rate that is characteristic to the isotope. The rate of decay is a fixed rate called a half-life.

The half-life of a radioactive isotope refers to the amount of time required for half of a quantity of a radioactive isotope to decay. Carbon-14 has a half-life of 5730 years, which means that if you take one gram of carbon-14, half of it will decay in 5730 years. Different isotopes have different half-lives.

The ratio of the amounts of carbon-12 to carbon-14 in a human is the same as in every other living thing. After death, the carbon-14 decays and is not replaced. The carbon-14 decays, with its half-life of 5,730 years, while the amount of carbon-12 remains constant in the sample. By looking at the ratio of carbon-12 to carbon-14 in the sample and comparing it to the ratio in a living organism, it is possible to determine the age of a formerly living thing. Radiocarbon dates do not tell archaeologists exactly how old an artifact is, but they can date the sample within a few hundred years of the age.


Teacher Lesson Plan

Licorice

  1. Instruct the students to label the horizontal axis of the graph paper “Time (seconds)” and the vertical axis “Radioactive Licorice (%)”. Show them how to calibrate the horizontal axes so that one block equals 5 seconds and two blocks equal 10 seconds. Instruct them to mark the horizontal axis at 10-second intervals.
  2. Give each student one piece of licorice to place onto the graph paper. Tell them to stretch the full length of the licorice vertically over the time “zero” mark and to make a mark on the paper at the top of the licorice. This mark represents 100% of the  radioactive material at time zero.
  3. Call out “GO” or “HALF-LIFE” at 10-second intervals for up to 90 seconds. When you say “GO” or “HALF-LIFE,” the students will have ten seconds to remove one-half of their licorice and set it aside. They place the remaining piece of licorice on the 10 seconds line and mark its current height. At 20 seconds, they should again remove half of the licorice and set it aside, then mark the height of the remaining portion on their graphs at the 20 second line. Repeat this process until 90 seconds have gone by.
  4. Now, the students should connect all the height marks with a “best fit” line, completing a graph of the “Half-Life of Licorice.”

 

NOTE: The original strip of licorice represents radioactive material; the portion which is “set aside” during the activity represents the material that has “decayed” and is no longer radioactive.

NGSS Guided Inquiry
Explain about radiation and half-lives of isotopes. Tell students to design their own experiment, using paper, M&M’s®, Pennies, other 2 sided material or licorice as a radioactive material undergoing decay to discover the nature of the half-life of that material.
You might suggest that the students experiment with their graphing results to see if trends begin to form.


Student Procedure

Licorice

  1. Label the horizontal axis of the graph paper “Time (seconds)” and the vertical axis “Radioactive Licorice (%)”. Calibrate the horizontal axes so that one block equals 5 seconds and two blocks equal 10 seconds. Mark the axis at 10-second intervals.
  2. Start with one piece of licorice to place onto the graph paper. Stretch the full length of the licorice vertically over the time “zero” mark, which is the same as the vertical axis. Make a mark on the graph paper at the top of the licorice. This mark represents 100% of the radioactive material at time zero.
  3. Your teacher will call out “GO” or “HALF-LIFE” at 10-second intervals up to 90 seconds. When your teacher says “GO” or “HALF-LIFE” you will have ten seconds to remove one-half of your licorice and set it aside. Place the remaining piece of licorice on the 10 seconds line and mark its current height. At 20 seconds, you should again remove half of the licorice and set it aside, then mark the height of the remaining portion on your graph at the 20-second line. Repeat this process until 90 seconds have gone by.
  4. Now, connect all the height marks with a “best fit” line, completing a graph of the “Half-Life of Licorice.”

 

NOTE: The original strip of licorice represents radioactive material. The portion which is “set aside” during the activity represents the material that has “decayed” and is no longer radioactive.

Data Collection
Student Data Collection Sheets

Post Discussion/Effective Teaching Strategies
Questions provided on the Student Data Collection Sheets

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