Conserve mass in a closed model

Supplied materials

• All measurements and observations below are supplied classroom examples. They are not data from actual learners or a claim about a particular real site.
• Paper or browser document; an ordinary calculator is optional.
• Rule: Total mass equals the two supplied material masses. Formula: a+b. Variable key: a = water mass (g), b = solute mass (g)
• Distinct worked inputs and result: A distinct sealed model has 12 g water and 3 g solute: 12+3=15 g total.
• Assessment case 1: Supplied case 1: water mass (g)=20; solute mass (g)=5, using the explicitly named quantities. Total mass equals the two supplied material masses. What result follows?
• Assessment case 2: Supplied case 2: water mass (g)=30; solute mass (g)=8, using the explicitly named quantities. Total mass equals the two supplied material masses. What result follows?
• Assessment case 3: Supplied case 3: water mass (g)=40; solute mass (g)=10, using the explicitly named quantities. Total mass equals the two supplied material masses. What result follows?
• Assessment case 4: Supplied case 4: water mass (g)=15; solute mass (g)=6, using the explicitly named quantities. Total mass equals the two supplied material masses. What result follows?
• Assessment case 5: Supplied case 5: water mass (g)=25; solute mass (g)=7, using the explicitly named quantities. Total mass equals the two supplied material masses. What result follows?
• Assessment case 6: Supplied case 6: water mass (g)=50; solute mass (g)=12, using the explicitly named quantities. Total mass equals the two supplied material masses. What result follows?

Learn the idea

A closed material system exchanges no matter with its surroundings. Dissolving or changing state does not destroy the material. Use grams, not a visual guess about volume.

Total mass equals the two supplied material masses. The formula in the supplied model is a+b. Variable key: a = water mass (g), b = solute mass (g). Add masses in grams. A closed boundary is essential to this claim.

A distinct sealed model has 12 g water and 3 g solute: 12+3=15 g total. Dissolved matter remains present even when invisible. Track the quantity and unit before calculating. Explain the meaning of a positive, negative or fractional result when relevant. A numerical answer supports the stated model calculation; broader claims require the additional measurements and assumptions described here.

1. Name the measured quantities and their units before substituting values.
2. Write the stated rule and use the variable key to match each symbol to its explicitly named quantity.
3. Calculate each supplied case separately, keeping intermediate arithmetic and the result unit.
4. Explain the result in context and identify the assumption that would invalidate direct use of the rule.

Check your reasoning

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Take it further

Changed-input task: use the same stated model with water mass (g)=21; solute mass (g)=5. Expected result: 26 g. Explain what changes and keep this assumption: Dissolved matter remains present even when invisible.

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