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Grade 9 · lesson 2 of 4

Check Mass with a System Boundary

Explain apparent mass changes by tracking material across a boundary.

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Supplied materials

  • Use the supplied fictional observations or model. Paper and pencil are optional; spoken answers are welcome. No equipment or real experiment is needed.

Learn the idea

A balanced equation conserves atoms in an ordinary chemical reaction. A mass ledger also needs a boundary: what container and material are included in the total? In a closed model where no material enters or leaves, total mass before and after stays the same at the precision used here. A reaction producing gas does not make the gas mass disappear.

Fictional supplied record: a sealed container and contents total 160 g before a reaction and 160 g after. In a separate open-container record, the starting container and contents total 160 g; afterward the remaining container and contents total 152 g, and 8 g of gas crossed out of that boundary. Both ledgers account for 160 g. These are reading exercises; do not mix substances or seal a real reaction.

  1. State the closed and open measurement boundaries.
  2. Compare the closed totals.
  3. Add remaining mass and escaped gas for the open record.
  4. Explain why 152 g alone is not the mass of all the original material.
Worked example

See the method in action

Closed total stays 160 g. Open ledger: 152 g remaining + 8 g transferred out = 160 g. The apparent 8 g loss is material outside the measured boundary.

Your turn

Check your reasoning

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An open model begins at 75 g and ends with 69 g inside; no matter enters. How much mass must have crossed out if these are complete records?

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See the answer and explanation

6 g

75 − 69 = 6 g crossed out under the complete no-entry model. This is a boundary transfer, not destruction of matter.

Take it further

Change the case to 2 g entering as well. Starting 75 + entering 2 − remaining 69 = 8 g must have crossed out. Include both boundary directions.

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These LifeStarter passages, scenarios and exercises are authored for this Academy with AI assistance and review. External resources keep their publishers’ terms. A grade label suggests a starting level; it does not establish alignment with your school’s complete requirements.