Mystery Mixtures: Reading the Evidence for Chemical vs. Physical Change
Shared by A STEM Educator
Students will distinguish chemical reactions from physical changes by analyzing evidence (temperature change, gas production, color shift, new substance formation) and explain how thermal energy affects particle motion and bond breaking/forming.
Lesson Overview
Students launch with a teacher-led mystery substance demo, then spend 30 minutes testing two unknown paper-and-pencil 'mixtures' (thought experiments + data tables) and pooling class evidence to define the indicators of a chemical reaction. Thermal energy effects on particle motion and bonding are woven throughout.
Materials
- Whiteboard and markers (teacher use)
- Paper and pencils (student use)
- One box of baking soda from the teacher's desk drawer (for demo — no measuring needed)
- One bottle of white vinegar (common in school kitchens; teacher retrieves before class)
- Any disposable cup or the teacher's own coffee mug for the demo
- Students' own pencils for data tables drawn on blank paper
Scaffolded Task Progression
Draw this table on your paper:
| Observation | Physical Change? | Chemical Reaction? | Indicator (if reaction) |
|---|---|---|---|
| Ice melts into water | |||
| Steel wool turns orange-red and feels warm | |||
| A log is split into smaller pieces | |||
| Milk left out turns sour and smells bad | |||
| Sugar dissolves in tea | |||
| Baking soda + vinegar fizzes |
For each row:
(a) Check the correct column.(b) If it is a chemical reaction, name the specific indicator(s) you used to decide.
Draw two boxes side by side on your paper. Label the left box 'Before Reaction' and the right box 'After Exothermic Reaction.'
In each box, draw 6 dots to represent particles.
- Left box: draw the dots close together, barely moving (add tiny short lines as motion marks).
- Right box: draw the dots more spread out with longer motion marks.
Then answer these questions in writing:
1. What happened to the SPEED of the particles after the reaction?
2. Where did the extra energy come from?
3. If this reaction makes the container feel HOT, is energy being released or absorbed by the reaction?
4. What would the particle diagram look like for an ENDOTHERMIC reaction (one that makes the container feel cold)?
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