The Disappearing Act: Conservation of Mass in Chemical Reactions
Shared by A STEM Educator
Students will develop and use visual models (atom-counting diagrams) to demonstrate that total mass is conserved in chemical reactions by balancing chemical equations, connecting to NGSS MS-PS1-5: developing and using a model to describe how the total number of atoms does not change in a chemical reaction.
Lesson Overview
Students investigate the puzzling observation that burning steel wool gains mass while burning wood loses mass, using this contradiction to build understanding of conservation of mass and balanced chemical equations in open vs. closed systems.
Materials
- Steel wool pads (1 per group)
- 9V batteries (1 per group — for igniting steel wool)
- Digital balance / scale (accurate to 0.1g)
- Zipper-seal plastic bags (2 per group)
- Birthday candle + clay base
- Matches or lighter (teacher only)
- Molecular model kits OR printed atom-tile cutouts (H, O, C, Fe tiles)
- Student investigation handouts
- Colored pencils (red, blue, black)
- Whiteboard markers + small whiteboards or laminated mats
- Projector for class discussion
Scaffolded Task Progression
Using the class demo data recorded on the board, complete the table below and answer the questions.
| Substance | Mass Before (g) | Mass After (g) | Change in Mass (g) | Gained or Lost? |
|---|---|---|---|---|
| Steel Wool | 2.3 | 3.2 | ___ | ___ |
| Candle | 4.1 | 3.6 | ___ | ___ |
(a) Calculate the change in mass for each substance.(b) Both substances are burning — a chemical reaction is happening. If matter cannot be created or destroyed, how is it possible that one gained mass and one lost mass?(c) What do you think is entering or leaving each system during burning?
(a) For the iron reaction, count the Fe and O atoms on each side of the unbalanced equation Fe + O₂ → Fe₂O₃. Are they equal?(b) For the carbon reaction C + O₂ → CO₂, count atoms on each side. Are they equal?(c) Which reaction is already balanced as written? Which is not?
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