To develop the idea that carbon dating is based on gathering evidence in the present and extrapolating it to the past. Students will use a simple graph to extrapolate data to its starting point.
This lesson is the third in a three-part series about the nucleus, isotopes, and radioactive decay. The first lesson, Isotopes of Pennies, deals with isotopes and atomic mass. The second lesson, Radioactive Decay: A Sweet Simulation of Half-life, introduces the idea of half-life.
By the end of the 8th grade, students should know that all matter is made up of atoms, which are far too small to see directly through a microscope. They should also understand that the atoms of any element are alike but are different from atoms of other elements. Atoms may stick together in well-defined molecules or they could be packed together in large arrays.
For students, understanding the general architecture of the atom and the roles played by the main constituents of the atom in determining the properties of materials now becomes relevant. Having learned earlier that all the atoms of an element are identical and are different from those of all other elements, students now come up against the idea that, on the contrary, atoms of the same element can differ in important ways. (Benchmarks for Science Literacy, p. 79.)
In this lesson, students will be asked to consider the case of when Frosty the Snowman met his demise (began to melt). The exercise they will go through of working backwards from measurements to age should help them understand how scientists use carbon dating to try to determine the age of fossils and other materials. To be able to do this lesson and understand the idea of half-life, students should understand ratios and the multiplication of fractions, and be somewhat comfortable with probability.
For the laboratory portion of this lesson, you will have to set up the ring stands, rings, funnels, and graduated cylinders. Fill the funnels with ice before the students arrive in the classroom. You can continue to fill the funnels as different classes arrive. Empty the graduated cylinders between classes if the volume is more than about 25 ml.
Begin by having students read the article The Story of Carbon Dating. The article briefly describes radio carbon dating.
To introduce the activity, ask students:
- How do you think archaeologists, when studying ancient pottery shards, determine how old their discoveries are?
- Have you ever heard of a technique called carbon dating, used to determine the ages of these archaeological samples?
Say to students:
"Carbon-14 undergoes beta decay with a half-life of 5720 years. The element carbon is an essential element in all living matter. Carbon-14 is produced constantly as our atmosphere is bombarded by cosmic rays. It is incorporated into the carbon cycle, so that all living things, including you, contain radioactive carbon-14.
Living things have about 15 disintegrations per minute per gram of carbon. Because living things constantly interchange carbon atoms, the amount of carbon-14 remains constant, but when organisms die, no new carbon-14 enters the organism. However, the carbon-14 that was in the organism at death continues to disintegrate.
By measuring how much carbon is left in a sample as well as its radioactivity, we can calculate when the organism died. It's a way of working backwards to solve a puzzle.
In this activity, you will work backwards to solve a puzzle, much like scientists work backwards to find the time that an organism died."
Give each student a copy of The Case of the Melting Ice student sheet. You may group them in any size group, but working in pairs is optimal for this exercise. The lab stations should have been set up already as described in the Planning Ahead section above. Students should complete the Analysis section of the lab sheet, which will be used as part of their assessment.
Advise students to read through the case first so that they understand what they should do. Written below is the case as it appears on The Case of the Melting Ice student sheet.
The Case of the Melting Ice
Frosty the Snowman lies melting in the funnels at your lab station. There were no eyewitnesses, but there are several suspects. All the suspects have holes in their alibis. You need to determine the exact time at which Frosty was put into the funnels to melt away, leaving no trace.
On a separate sheet of paper, immediately record the volume of Frosty's melted remains (water) in your graduated cylinder and note the time on the clock. Make a data table and, at regular intervals (you decide how long), record the time on the clock and the volume of water in the graduated cylinder. Stop after about 30 minutes, unless Frosty has completely melted earlier.
Students should answer the questions on their student sheet based on their graphs and the data they collected.
Students can check their answers by going to the Radiometric Dating page by Professor Pamela Gore from the Georgia Perimeter College. (This page has been archived and is found on the Internet Archive.)
In addition to using answers to students' Analysis questions and their graphs for evaluation, consider having them respond to the following in their science journals or as a homework essay:
Pretend you are on a month-long field trip to dig for artifacts that might have been left from the pre-colonial period in the United States. Write a letter to a friend explaining what radiocarbon dating is. Be sure to include how radiocarbon dating works backwards to solve a puzzle. Explain to your friend how you and other archaeologists, with the help of chemistry, determine how old your discoveries are. You can refer to How Carbon-14 Dating Works, from How Stuff Works, to help you answer the question.
The Nobel Prize in Chemistry 1960 Presentation Speech, given at the presentation of the Nobel Prize to professor Willard Libby for his use of carbon-14, highlights how the dating method works.
Willard Libby, from the Inventor of the Week Archive, profiles the career of the American chemist who created the carbon-14 dating method.