Passport of a Carbon Atom: Where Do the Atoms Pile Up?

Investigation question: When carbon atoms move among the air, water, living things, soil and fossil fuels by chance at each step, which reservoirs end up holding the most atoms, and what changes when people start burning fossil fuels?

Scientific purpose

Students become carbon atoms in a room-sized model of the carbon cycle. Six stations stand for reservoirs, and a die roll at each station decides where the atom goes next, following rules that mirror the real processes. Each student records a passport of their own journey, and the class pools everyone's final positions into a bar chart. A second run changes one rule, fossil fuels can now be burned, and the class compares the two charts. The lab turns a static cycle diagram into a system with rates, randomness, storage and a human-caused change.

Why this helps students with this chapter

A cycle diagram on a page looks like one loop every atom follows in order. Walking the cycle with a die makes three things visible that the page hides: atoms take different routes, some reservoirs hold atoms for many rounds while others pass them on at once, and the number of atoms in the whole system never changes no matter how they shuffle. Changing a single rule and re-running shows how people shift a cycle without creating or destroying anything.

Standards connections

MS-LS2-3 (assesses, developing and using a model of matter cycling among living and nonliving parts of an ecosystem, with the boundary of the system defined): Students run, record and then revise a model of carbon cycling among producers, consumers, decomposers, soil, water, air and fossil fuels, state the boundary of the modeled system, and use the pooled data to explain conservation of matter and the effect of fossil-fuel burning. No chemical reactions are used, which respects the assessment boundary.

MS-LS1-6 (supports, photosynthesis as the entry point for carbon into producers): The producers station's incoming arrow is photosynthesis; students use it as a process name without being asked to explain its mechanism.

Roles (rotate each session)

• Atom (every student): Rolls the die at each station, reads the rule, records the round in their own passport and moves to the next station. Every student is an atom; the group roles below are extra jobs.
• Station keeper: One member of each group also keeps a station: reads the rule aloud for anyone unsure, keeps the die at the station, and counts the atoms present at the end of each run.
• Tally recorder: One member of each group collects the group's three passports after each run, checks they have ten rounds, and reports the group's final stations to the class tally on the board.
• Grapher: One member of each group copies the class tally into the recording sheet and draws the two bar charts for the group to analyze.

Materials

Estimated cost: 6 to 10 per class, reusable (an estimate; A set of eight large foam dice costs about $6; station cards and passports are printed; tape and chart paper are classroom stock.)

Safety

• Students walk between stations; clear bags and chairs from the paths and set a walking-only rule. Mark station areas with tape so groups do not crowd a corner.
• Foam or large plastic dice only; no throwing dice across the room.
• No chemicals, heat or sharp tools are used.

Teacher preparation and setup

1. Print and post six station cards around the room with space for students to stand: Atmosphere, Water, Producers, Consumers, Soil and decomposers, Fossil fuels. Put the natural rules on the front and the fossil-fuel-era rules on the back.
2. Decide the starting spread: divide the class evenly across the six stations for run 1 (for 30 students, 5 per station). Keep the same spread for run 2 so the two runs can be compared fairly.
3. Draw the class tally table on the board before class: six station rows, two run columns, and a row for the total that should equal the class size every time.
4. Run three rounds yourself with a die to check that the rules read clearly; most confusion comes from students moving before recording.

Investigation design

Student procedure

1. Stand at your starting station and write it in round 0 of your passport. The class should be spread evenly across all six stations. Agree on the boundary of the model: the six stations are the whole system, and no atom may leave the room or join from outside.
2. Round 1: roll the die at your station once. Read the rule for your roll on the station card. In your passport write the round number, the station you are at, the number you rolled, the process named on the card and the station you are going to. If the card says stay, write 'stays' as the process and your current station as the destination.
3. Move to your destination station only after you have written the row. Wait there until the teacher calls the next round. Everyone rolls once per round.
4. Repeat for ten rounds. At the end of round 10 stay where you are. Station keepers count the atoms at their station and report the count; the tally recorder writes the group's three final stations on the board tally. Check that the class total equals the number of students.
5. Return to your run 1 starting station. The teacher flips every station card to the fossil-fuel-era side. The only rule that changes is at the Fossil fuels station, where rolls of 4, 5 or 6 now send the atom to the Atmosphere by combustion. Run ten more rounds exactly as before and record them in the run 2 passport table.
6. At the end of run 2, count and tally again. Check the total.
7. In your group, copy both tallies into the recording sheet and draw two bar charts, one per run, with the same vertical scale so they can be compared by eye.
8. Count how many rounds of your own passport you spent in each reservoir across both runs and fill in the personal summary table.
9. Answer the analysis questions on your own, then compare answers in your group and revise anything you now disagree with.

Expected observations

• In run 1 the Fossil fuels station gains atoms slowly and never loses them, so its count rises over ten rounds while the Water station holds a steady group; Atmosphere and Producers trade atoms back and forth every round.
• In run 2 the Atmosphere count after round 10 is higher than in run 1 and the Fossil fuels count is lower, because half of each round's fossil-fuel rolls now release atoms by combustion. With a class of about 30, expect a difference of several atoms rather than a dramatic one; pooling across periods sharpens it.
• Every total equals the class size. Any mismatch means an atom was miscounted or moved before recording, which is itself a good conservation check.

Likely difficulties

• Students move before writing the row, then cannot remember their roll. Call rounds slowly and require pencils down before anyone walks.
• Students at a stay rule sometimes think they rolled wrong or do not record the round. A stay is a legitimate row: the process is 'stays' and the destination is the same station.
• Small classes give noisy tallies, and run 2 may by chance look no different. Have students compute the expected direction from the rules (q5) and compare with the actual, which is how scientists judge a noisy result.
• Some students argue that the Soil-to-Fossil fuels rule (one in six) is 'wrong' because real burial is rare. They are right; use it to discuss why a model exaggerates a rate to make a slow process visible in ten rounds.

Structured debrief

1. Put both class bar charts side by side. Ask what the one changed rule did to the Atmosphere bar, and why the Fossil fuels bar fell. Then ask the question that matters: did the total change? It cannot; every student is still in the room. That is conservation of matter, and the planet works the same way with its atoms.
2. Ask which reservoirs were 'sticky' in run 1 (Fossil fuels, where nobody could leave; Water, with a one-in-two stay rule) and which were 'fast' (Consumers, where half the rolls go straight to the air). Connect stickiness to residence time: real fossil fuels hold carbon for hundreds of millions of years, and a mouse holds it for months.
3. Lay a few passports next to each other and show that no two journeys match. The cycle diagram is a map of possible routes, not a track. Ask who skipped a reservoir entirely in ten rounds, and who bounced between two stations.
4. Name the model's limits in students' own words: real chances differ by season and by place, real reservoirs are vastly different in size (the ocean holds far more carbon than the air), the model has no rocks and no energy, and the fossil-fuel burial rule is far too fast. Then ask why the model is still useful: it shows direction of change correctly when one rule changes, which is what we needed to know.

Recording sheet: what students fill in

Lab 1 recording sheet: Passport of a carbon atom

You are a carbon atom. At each station, roll once, find your number on the station card, write the row, and only then move. The rules below are a model: the chances are invented for teaching, but every arrow is a real process in the carbon cycle.

Analysis questions: answer key

Individual evidence of learning

Each student submits their own two passports, personal summary table and written answers to q1 through q9. The passports show the student can record a model's state and the process on every move; q6 and q7 are the evidence for conservation of matter and system boundary; q9 is the evidence for using the model to predict the effect of a change.

Assessment criteria

Accessibility adaptations

• Use large foam dice and large-print station cards; a station keeper can read the rule aloud for any atom who asks.
• A student who cannot move between stations can stay seated as a station keeper and run an atom on a tabletop version with a small token, or roll for a partner who walks.
• Provide a passport with rounds 1 and 2 filled in as a worked example and a word bank of the six process names.
• Accept a spoken explanation recorded on a device for q8 and q9, and allow the bar charts to be built with stacked sticky notes on the board instead of drawn.

Low-cost alternative

Run it at the tables instead of around the room: each group lays out six paper station cards, uses one die and three coins as atoms, and rolls for each coin in turn. The class tally then pools the groups' coin positions. One die per group and a sheet of paper is the entire cost.

Model limitations

The chances on the cards are invented so that every reservoir is visited within ten rounds; real rates differ by season, by ecosystem and by orders of magnitude (burial that leads to fossil fuels is extremely rare, and the ocean holds far more carbon than the air). The model has equal-sized reservoirs, leaves out rocks such as limestone, shows no energy flow, and treats each atom as moving independently. Combustion of wood and wildfire are absent from the natural rules. The model is good for direction of change and for conservation, not for how large a real change would be. This activity has not been physically tested by the authors; the expected tallies are estimates from the rules.

Teacher notes

Check q4 as two fractions read from the cards, not as a probability calculation. For q3 and q5 grade against your class's actual tally; if chance produced an odd result, a correct explanation of what the rules predict and why the sample was small earns full credit. Keep both tallies on the board for the misconception slide: the unchanged total is the best answer to 'fire destroys matter.' If you run several periods, add the tallies together and show students how the pooled chart is smoother than any one class's chart.