You can't see atoms rearrange, but you can see the signs: a colour change, a gas given off, a solid forming in a liquid, light, a smell, or a temperature change. Each is a clue that a new substance formed, not proof: boiling water bubbles too. The test is still a new substance.
Bake a cake and it rises, browns and smells wonderful. Leave a nail outside and it turns orange. In each case, how do you know something new was made?
Last lesson: a chemical change makes a new substance. This lesson: how to spot one without seeing the atoms.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
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: fizz or no fizz
ExploreTry it on the board and compare what happens with your prediction.
Explore: signs on the board
Try it yourselfNothing is saved: start again as often as you like.
Burn the methane. Note the temperature at the start and the end.
Use the board. This ticks when you get there.
Now dissolve some salt. Any new kinds of particle?
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: signs and the test
In your book: list five signs of a chemical change. Next to each, give one example where the sign appears without a chemical change, if you can.
Is it a sign of a chemical change?
A colour change when two substances mix
Bubbles of a new gas
A solid forming when two clear liquids mix (a precipitate)
Light or heat given out, or the mixture getting colder
A new smell
The substance changing shapeCutting or bending changes the shape, not the substance.
Chemical change, or a sign that misleads?
A cake browning in the oven
Vinegar and bicarb fizzing
Water boiling and bubblingThe bubbles are steam: still water.
Blue and yellow paint mixing to greenThe paints are mixed, not changed.
A nail rusting orange
A hot drink cooling down
Why does each sign suggest a new substance has formed?
A colour change
Bubbles of gas
A precipitate
Heat or light given out, or getting colder
A new smell
Elaborate TypicalUse the same idea in a new context.
Elaborate: melting gives signs too
Try it yourselfNothing is saved: start again as often as you like.
Heat the water from ice to steam.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: a precipitate
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
Signs of a chemical change: a colour change, a gas given off, a precipitate, light, a smell, a temperature change.
Each sign is a clue, not proof: boiling water bubbles and paint mixing changes colour, with no new substance.
The test is whether a new substance formed, with new properties.
For teachers
Students list observable signs of chemical change and explain that each is a clue to a new substance, not proof, distinguishing misleading signs in physical changes (AC9S8U07, node 3). About 22 minutes.
I can spot a sign that misleads in a physical change.
Before class
A concept tutorial for introducing or revising the idea; it is best paired with a circus of quick reactions (vinegar and bicarb, lead-free precipitates, steel wool burning). Set the width with ?width=minimum, typical (default) or wider. It follows Is it a new substance? (node 2) and comes before Hotter or colder? (nodes 4 and 5).
Not a hinge node. Students tend to treat any one sign as proof; the prediction and the "clue or trap" sort target it.
Board notes: the board can't show colour, smell or a precipitate; it shows new particles and the temperature change. The dissolve board shows a case with no sign at all.
Running the lesson
Engage: tally the predictions. Any single sign chosen means 'sign = proof'.
Explore: ask which observation from the burning board would also happen without a reaction.
Explain: the two sorts are the success criteria.
In your own words: five signs, each with a misleading example where possible.
The hard part (on purpose)
Holding 'clue' and 'test' apart. When a student says 'it bubbled, so it reacted', ask what is inside the bubbles before agreeing.
When students get stuck
A student says boiling is a chemical change because of the bubbles. Ask: What are the bubbles made of? Fix: Steam: water particles. No new substance, so physical.
A student says any colour change means a reaction. Ask: What happens when you mix blue and yellow paint? Fix: It looks green, but both paints are still there. A colour change is a clue, not proof.
Stretch and support
Stretch: Why is 'it can't be reversed' a poor test for a chemical change? (Some physical changes are hard to reverse, like shattering a glass; some reactions can be reversed, like charging a battery.)
Support: Give the signs as picture cards (bubbles, colour, cloudy solid, flame, thermometer, nose) to sort into 'sign' and 'proof needed'.
Where it leads
Next: Hotter or colder? (exothermic and endothermic, nodes 4 and 5). Comparing physical and chemical changes with all the criteria (node 6) is a class task.
In class: a reaction circus with an observation table (what you saw, which sign, new substance?).
What each step is for
Engage: fizz or no fizz: Students often treat any bubbles or any temperature change as proof of a reaction; boiling water separates 'sign' from 'proof'.
Explore: signs on the board: Comparing a reaction (temperature rises, new particles) with dissolving (no new particles) shows which observations point to a new substance.
Explain: signs and the test: Listing the signs, then sorting cases where a sign misleads, keeps 'new substance' as the test and the signs as evidence for it.
Elaborate: melting gives signs too: Melting and boiling show a temperature change and bubbles with no new substance, the clearest misleading signs.
Elaborate: a precipitate: A precipitate is a sign students meet less often; reasoning about it with particles checks the idea transfers.