To classify an unfamiliar change, ask the main question first: is a new substance made? Then check the clues: colour, gas, solid forming, light, and temperature or energy change. One clue can mislead (boiling makes bubbles and ice melting takes in heat, yet both are physical), so several clues pointing the same way give more confidence.
A pot of water boils: bubbles, steam, heat. A pot of milk 'boils' and then turns to a skin. Both look dramatic. Only one is a chemical change. How do you tell?
A short lesson with no simulation. You have the new-substance test, the clues and energy change. Now use them together.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: two kinds of bubbles
ExplainUse the particle model to explain what you saw.
Explain: classify with evidence
In your book: choose one change from your kitchen. Write what you see, say whether it is physical or chemical, and give two clues and the new-substance reason.
Physical or chemical change?
Butter melting in a panSame substance, new state.
An egg cooking and turning solid and whiteThe proteins change into new substances.
Sugar dissolving in teaThe sugar can be recovered by evaporating the water.
Iron nails going orange-brown and flaky in damp airRust is a new substance.
Dry ice (solid carbon dioxide) turning straight into a gasStill carbon dioxide, now a gas. (The white fog is water from the air, condensed by the cold.)
Milk going sourNew substances (acids) are made.
Elaborate TypicalUse the same idea in a new context.
Elaborate: a whole recipe
Elaborate WiderUse the same idea in a new context.
Elaborate: testing
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
First ask: is a new substance made? That is what makes a change chemical.
The clues (colour, gas, solid, light, temperature or energy change) support the answer, but one clue can mislead.
Several clues pointing the same way give more confidence.
Real processes, such as cooking, often include both physical and chemical changes.
For teachers
Students apply the new-substance criterion, observable clues and energy change together to classify unfamiliar changes, and explain why several clues are better than one (AC9S8U07, node 6, an integration node). About 16 minutes.
I can classify an unfamiliar change using all the criteria.
I can say why several clues are better than one.
I can give an example where one clue misleads.
Before class
A short questions-led lesson with no simulation, closing the unit on physical and chemical change. Set the width with ?width=minimum, typical (default) or wider. It follows the energy lesson.
The first prediction targets the bubbles misconception. Real cases are the best material: bring a few unlabelled changes (a lit match, an effervescent tablet, melting wax) and have groups argue for a classification before you reveal.
Board notes: none; the lesson has no board. Cases are simplified: cooking involves many reactions.
Running the lesson
Engage: tally the predictions. Ask the 'both' group what substance is the gas in the pot.
Explain: the sort and the 'cold reaction' case are the success criteria.
In your own words: a kitchen change with two clues and the new-substance reason.
The hard part (on purpose)
Deciding on the evidence, not the first clue. When a student says 'it bubbled so it's chemical', ask what the bubbles are and where they came from.
When students get stuck
A student says a temperature change means a chemical change. Ask: What happens to the temperature when ice melts? Fix: It takes in energy, but it's still water. A temperature change is a clue, not a test.
A student says dissolving is chemical because the sugar disappears. Ask: How could you get the sugar back? Fix: By evaporating the water. The sugar is still there, so it's physical.
Stretch and support
Stretch: Is burning a candle physical, chemical or both? Say which part is which. (The wax melting is physical; the wax vapour burning is chemical.)
Support: Give a three-box organiser (What I see; Is there a new substance?; My decision) for each case, with the new-substance question printed in bold.
Where it leads
Next: Year 9 atoms and the periodic table, and balancing equations.
In class: a 'mystery change' station with five tubes and cards; vinegar and baking soda, steel wool in vinegar, ice in a beaker with a thermometer.
What each step is for
Engage: two kinds of bubbles: Students treat bubbles as proof of a chemical change. The new-substance test comes first; indicators only support it.
Explain: classify with evidence: Applying the new-substance criterion, indicators and energy change to unfamiliar cases, and spotting a misleading clue, meets all three success criteria.
Elaborate: a whole recipe: A multi-step process makes students classify each stage separately and see that one process can include both kinds of change.
Elaborate: testing: Indicators are used in the real world for detection, and false positives are a design problem.
Now you can
Classify an unfamiliar change using all the criteria.