Conservation of Mass: Atoms Don't Disappear — They Rearrange
Shared by A STEM Educator
Students will demonstrate understanding that mass is conserved in chemical reactions by counting atoms in balanced equations, analyzing open vs. closed systems, and designing experiments or arguments using multiple modalities and entry points.
Lesson Overview
Students investigate conservation of mass in chemical reactions through the context of industrial catalytic converters and chemical manufacturing, discovering that atoms are rearranged — never created or destroyed — even when gases seem to vanish.
Materials
- Unopened and opened carbonated drink bottles (warm-up)
- Digital or triple-beam balance (grams)
- Colored interlocking cubes or snap-together atom blocks (2 colors minimum — e.g., red for oxygen, black for carbon)
- Printed atom-counting worksheets with blank equation templates
- Ziploc bags, baking soda, vinegar, small cups (sealed-system experiment)
- Sticky notes (3 colors) for sorting/labeling
- Markers and large chart paper or VNPS whiteboards
- Printed card sets: Atom Inventory Cards and Reaction Scenario Cards
- Optional: tablet/laptop with PhET 'Balancing Chemical Equations' simulation
- Sentence frames and vocabulary anchor charts (ELL/language support)
- Highlighters for annotating equations
Scaffolded Task Progression
Your teacher has two identical bottles of carbonated soda: one sealed (unopened) and one that has been open for 10 minutes.
Predict first: Circle your prediction below.
- The sealed bottle is HEAVIER than the open bottle.
- The open bottle is HEAVIER than the sealed bottle.
- They weigh EXACTLY the same.
Then observe: Watch as your teacher places each bottle on the balance. Record the masses below.
| Bottle | Mass (g) |
|---|---|
| Sealed (unopened) | _______ |
| Open (10 min) | _______ |
Discuss with a partner: Where did the missing mass GO? Did matter disappear, or did it go somewhere? Use the sentence frame if helpful:
*'I think the mass difference happened because...'*
Connection: Factories use catalytic converters to change exhaust gases (like carbon monoxide) into less harmful gases. If gases escape, does mass disappear from the universe? Hold that question — we'll answer it today.
Chemical reactions rearrange atoms — they NEVER destroy them. Let's prove it.
Your materials: Colored snap-cube blocks. Use BLACK cubes for Carbon (C) and RED cubes for Oxygen (O).
Reaction in a catalytic converter:
Carbon monoxide + Oxygen → Carbon dioxide
Step 1 — Build the REACTANTS (left side):
- Build 2 molecules of CO: each is 1 black + 1 red cube snapped together.
- Build 1 molecule of O2: 2 red cubes snapped together.
Step 2 — Count and record:
| Atom | Reactant Side | Product Side |
|---|---|---|
| Carbon (C) | _______ | _______ |
| Oxygen (O) | _______ | _______ |
| TOTAL atoms | _______ | _______ |
Step 3 — Rearrange (do NOT add or remove cubes) to build the PRODUCTS:
- Build 2 molecules of CO2: each is 1 black + 2 red cubes.
Step 4 — Recount the product side in the table above.
Check: Did the total number of each atom change? _______
If your counts match on both sides, you have just proven conservation of mass with your hands!
[ Interactive graph / diagram, available in the full lesson ]
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