Chemistry — Investigation · Physical Science: Matter, Forces & Energy, Grade 8
Use evidence and models to explain chemical change, conserve atoms, and track energy transfer.
Account for matter in a reaction
Investigate
Can a lower measured mass in an open container mean that matter was destroyed?
Use three sessions: plan the question, boundary and predicted mechanism; collect safe records or analyze the supplied data; then graph, explain, compare with the initial model and propose a revision. The data option needs no special equipment. Time is a suggested allocation, not a measured completion claim.
Materials
- Provided open- and closed-system data
- Paper or modeling counters for particles
- Notebook
Before you begin
This is a data and model investigation. Do not perform a sealed gas-producing reaction at home; pressure can build up. No chemical mixing is required.
Plan an evidence comparison
Compare whether matter can cross the boundary. Other reaction conditions and measurement methods should match.
Method
- Make a notice-and-wonder record. State one observation, one possible explanation and a question the investigation can address. Agree on the measuring rule before seeing the results.
- Draw the boundary of each system. Decide what the balance measures before interpreting the data.
- Calculate the measured mass change in each trial.
- Represent reactants and products with labeled counters. Rearrange the same atoms rather than adding or removing them.
- Explain how gas leaving an open system affects what a balance records. Identify evidence that would test your explanation.
- Write a claim supported by both mass data and your particle model.
- Revise the initial model, label the specific evidence that changed it and state a remaining question. Propose a changed condition, a predicted result and evidence that would challenge your mechanism.
Example data — for analysis when materials are unavailable
Illustrative example data for analysis, not measurements collected by the learner.
| System | Initial mass (g) | Final measured mass (g) |
|---|---|---|
| Closed, trial 1 | 150 | 150 |
| Closed, trial 2 | 151.5 | 151.5 |
| Open, trial 1 | 150 | 148.4 |
| Open, trial 2 | 151.5 | 149.9 |
Analyze and explain
- What pattern differs between open and closed systems?
- What does your particle model conserve?
- Could a balance error also affect the result? What comparison helps distinguish explanations?
- Predict what a complete matter boundary should include. Distinguish a gas escaping the measured boundary from atoms being destroyed.
What a strong explanation includes
- Calculate at least one mass difference with units.
- Make the system boundary explicit.
- Use conservation of atoms to connect the data to the claim.
- Identify the system, relevant variables and measurement method.
- Explain a model revision and a specific uncertainty.
Parent / teacher guide: discuss after the investigation
Each illustrative open trial lost 1.6 g from the measured system, while each closed trial retained its initial mass. Gas moving out of the open boundary can explain the lower measured mass without atoms being destroyed. Repeat measurements and measurement uncertainty should also be considered. Predict what a complete matter boundary should include. Distinguish a gas escaping the measured boundary from atoms being destroyed. A complete report separates observation from inference. Preserve all planned trials, including a result that does not fit the prediction, and distinguish missing readings from zeros. The supplied numbers are illustrative, not records the learner collected. If observations differ, check the method, units and conditions before deciding which claim is supported. A schematic model should connect parts through a mechanism and explicitly name its limits. Describe a follow-up that changes one intended condition, keeps influential others comparable, and states the possible result that would challenge the explanation. A proposed improvement is not confirmed until it is retested. Different defensible conclusions can receive credit when they use the actual evidence and meet the stated criteria.
Compare this model with the learner’s actual evidence. Different measurements can support a different conclusion; check the reasoning and limitations, not matching wording.