A Story of Two Substances

Overview

Students will experience a short narrative that contrasts physical vs chemical changes and properties, then investigate two “substance stories” to reason about evidence and conservation of mass. The lesson builds toward explaining how mass is conserved while atoms are rearranged.

Learning intentions

Students will be able to:

• Distinguish between physical and chemical properties and changes using observable evidence.
• Classify a change as physical or chemical based on what happens to substances.
• Explain conservation of mass by tracking matter before and after a change.
• Use the mole concept appropriately at a basic level as “counting particles,” without doing complex calculations.

Success criteria

• I can identify evidence that a change is chemical (new substances formed) or physical (substance identity stays the same).
• I can fill in an observation table for both investigations (what I saw/measured).
• I can represent particles/models before and after using simple diagrams.
• I can write a short conservation-of-mass reasoning statement using “atoms/matter are conserved” and “mass stays the same.”

Curriculum links

• SC.912.P.8.2: Differentiate between physical and chemical properties and physical and chemical changes of matter.
• SC.912.P.8.9: Apply the mole concept and the law of conservation of mass to calculate quantities participating in reactions (qualitative conservation reasoning in this lesson).
• SC.8.P.9.1: Demonstrate and conclude mass is conserved when substances undergo physical and chemical changes.
• SC.8.P.9.2: Differentiate between physical changes and chemical changes.
• SC.912.N.1.1.2: Conduct systematic observations, use clear procedures, identify variables, and follow safety guidelines.

Lesson structure (60 minutes)

1. 0–7 min · Hook narrative (teacher script). Teacher reads a brief “two substances” story while showing a single container labeled “Story A” and “Story B.” Students predict: “Which story changes into something new?” and “Will mass change?”

• Teacher script (read aloud): “Once there were two travelers: Salt and Water, and later Vinegar and Baking Soda. Salt had a ‘quiet power’—it dissolves without becoming a new substance. When salt goes into water, you still have salt particles and water particles, just mixed differently. The book of matter says: when you only rearrange, the story is physical. But then Vinegar met Baking Soda. You can hear it fizz, you can see bubbles, and soon the materials look different. That’s not just rearranging—that’s forming new substances. The matter story changes chemically. Now here’s the rule that the universe keeps: even when the story changes, the number of atoms doesn’t vanish. If we weigh before and after, mass should stay the same. So, Salt-and-Water is Story A (physical), Vinegar-and-Baking Soda is Story B (chemical). Let’s prove it with observations.”

2. 7–12 min · Mini-lesson: evidence + classification. Teacher charts “Physical = identity stays the same; Chemical = new substances.” Students do a quick, whole-class “thumbs” response to two teacher prompts: (a) melting ice, (b) rusting iron.
3. 12–20 min · Safety + procedure briefing (demo setup). Teacher reviews safety (goggles, gloves optional, tie hair back, no tasting) and points out observables and measurements; students copy a simplified procedure flow and identify independent/dependent variables (type of change; evidence/observations).
4. 20–30 min · Demo 1 (physical change: dissolving or melting). Teacher demonstrates salt dissolving in water (or melting ice if preferred) in a clear cup; emphasizes that the substance remains the same even if it disperses. Students record observations: before/after appearance, any temperature change, and where the “substance” is now.
5. 30–43 min · Demo 2 (chemical change: vinegar + baking soda). Teacher demonstrates vinegar + baking soda in a sealed balloon setup (or cup with gas collection if available). Students observe evidence: fizzing, balloon inflation (or bubbling), and whether a new substance forms (e.g., CO₂ gas evidence).

• Alternative chemical option (if vinegar/baking soda not available): iron wool + vinegar in a covered jar with a measurable mass before/after; emphasize rust formation as new substance evidence. (Only run if materials are safe/available.)

6. 43–53 min · Student two-part investigation. Students work in pairs at lab stations with pre-portioned materials and complete the two-part observation table (Part A physical, Part B chemical). They add simple mass checks qualitatively (e.g., “same scale reading before/after” when scale allows) and draw particle/model diagrams before and after for both parts.
7. 53–58 min · Whole-class sensemaking. Teacher asks: “What evidence made it chemical?” and “Why should conservation of mass still hold even though appearance changed?” Students share one key phrase from their reasoning.
8. 58–60 min · Exit ticket (individual). Students answer two prompts: (1) classify each change as physical or chemical with one reason; (2) write a conservation-of-mass reasoning statement.

Resources

• Goggles for every student
• Safety gloves optional
• Paper towels, tray liners
• Digital scale (preferred) or balance/scale demonstration
• Clear cups/beakers (at least 2 per group)
• Stirring rods/spoons
• Measuring spoons/cups for consistency
• For Part A (choose one set):
• Salt + water, or ice + cup
• For Part B:
• Vinegar + baking soda
• Balloon (optional) or tubing/materials for gas observation
• Observation table handout (two-part)
• Particle/model representation worksheet (simple circles/atoms template)
• Exit ticket slips
• “Glencoe Chemistry 2008” as the classroom reference text for student-friendly background on matter changes

Assessment

• Formative checkpoint 1 (during Hook): Students predict which story is physical vs chemical and justify with a single sentence stem (“I think it’s chemical because…”).
• Formative checkpoint 2 (during investigations): Teacher circulates to check observation table completeness and that classification uses evidence, not just “it looked different.”
• Exit ticket (summative, short checklist + reasoning): accuracy of classification and conservation-of-mass statement.

Exit ticket rubric/criteria (checklist)

• Correctly identifies physical change and provides one evidence-based reason.
• Correctly identifies chemical change and provides one evidence-based reason.
• Conservation-of-mass reasoning statement includes:
• matter/atoms are not created or destroyed, and
• mass stays the same (or “before and after should match on a scale”).

Differentiation

• Support for ELL/struggling learners:
• Provide sentence starters: “Physical means… / Chemical means…,” “Evidence: ___,” “Conservation of mass: atoms…”
• Offer a partially completed observation table and a particle diagram template with labeled “before” and “after.”
• Allow oral response first, then student transcribes for the exit ticket.
• Extension for advanced students:
• Ask: “How could you design a better evidence test to confirm a new substance forms (not just a mixture)?” Include a claim-evidence-reasoning (CER) response.
• SEN/processing support:
• Reduce variables: same cup size, same amounts, same timing; assign clear roles (measurer, recorder, model drawer).

Notes on safety (built into steps)

• Wear goggles during all liquid mixing.
• Do not taste any materials.
• Handle vinegar/baking soda carefully to avoid splashes into eyes.
• Clean spills immediately; dispose of mixtures per classroom policy.