A chemical change makes a new substance, because atoms rearrange into new particles. A physical change only rearranges or spreads out the same particles. "Can it be reversed?" is not the test.
You can melt ice and freeze it again. You can't unburn toast. Is that how to tell a chemical change? Dissolved salt comes back when the water dries up…
In Year 6 you may have learned that changes you can reverse are physical. This lesson tests that rule, and replaces it with a better one.
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: is reversing the test?
ExploreTry it on the board and compare what happens with your prediction.
Explore: watch the particles
Hydrogen chloride is a new substance: it dissolves in water to make hydrochloric acid. Watch where its atoms come from.
Ice, water and steam. Watch the particles themselves.
Try it yourselfNothing is saved: start again as often as you like.
Let hydrogen and chlorine react until 10 hydrogen chloride particles have formed.
Use the board. This ticks when you get there.
Heat the water from ice to steam.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: the new substance test
In your book: explain the new substance test, then explain why dissolving salt shows that "can it be reversed?" is not the test.
Physical or chemical? Use the new substance test.
Ice meltingStill water.
Salt dissolving in waterThe salt is still there: evaporate the water and you get it back.
Paper burningNew substances: carbon dioxide, water and ash.
An iron nail rustingRust (iron oxide) is a new substance.
Cooking an eggThe egg's proteins change into new substances: it can't go back to runny.
Water boilingThe bubbles are steam: still water.
Baking soda fizzing in vinegarThe bubbles are carbon dioxide, a new gas.
Chocolate melting
Elaborate TypicalUse the same idea in a new context.
Elaborate: is dissolving chemical?
Try it yourselfNothing is saved: start again as often as you like.
Dissolve the salt in the water.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: burning
Try it yourselfNothing is saved: start again as often as you like.
Burn the methane.
Use the board. This ticks when you get there.
When methane burns, what happens to its atoms?
The carbon atoms end up in carbon dioxide
The hydrogen atoms end up in water
The methane is destroyed and turns into energyThe atoms are all still there, in new particles. The energy came from the rearrangement.
The gas gets hotterBurning releases energy: watch the temperature in the panel.
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
A chemical change makes a new substance: atoms separate and join into new particles.
A physical change (melting, boiling, dissolving) keeps the same particles.
"Is there a new substance?" is the test. "Can it be reversed?" is only a clue: dissolving can be reversed and is physical.
In a chemical change no atoms are made or destroyed; they are rearranged.
For teachers
Students state the new substance test for chemical change, explain it as atoms rearranging into new particles, and use it in place of the reversibility rule (AC9S8U07, node 2, a hinge node). About 25 minutes.
I can state that a chemical change makes a new substance.
I can classify changes as physical or chemical with the new substance test.
I can explain why "can it be reversed?" is not a reliable test, with a counterexample.
Before class
A concept tutorial for introducing or revising the idea; it does not replace practical work. Set the width with ?width=minimum, typical (default) or wider.
The hinge risk is the reversibility rule from AC9S6U04. Present the new substance test as a replacement, not an addition, and name the counterexamples (dissolving; recharging a battery).
Running the lesson
Engage: take a show of hands for 'yes, it always works'. Come back to them at Evaluate.
Explore: on the reaction board, point to the atom tally: same atoms before and now.
Explain: the sort's 'why' notes appear once each item is right; ask students to say the test aloud for two of them.
In your own words: the dissolving counterexample is the evidence that the old rule has gone.
The hard part (on purpose)
Using 'is it a new substance?' instead of 'can it be reversed?'. When a student calls dissolving chemical, ask how they could get the salt back.
When students get stuck
A student says boiling is chemical because of the bubbles. Ask: What are the bubbles made of? Fix: Steam: water as a gas. Same substance, so physical. Bubbles are a clue, not proof.
A student says dissolving is chemical because the salt disappears. Ask: How could you get the salt back? Fix: Evaporate the water. The salt particles were there all along, spread out.
A student says burning destroys the fuel. Ask: Where did the carbon atoms go on the board? Fix: Into carbon dioxide. Atoms are rearranged, never destroyed.
Stretch and support
Stretch: Is cooking pasta a chemical change? What would you look for to decide?
Support: Give the test as a sentence frame: 'Is there a new substance? I think … because …'.
Where it leads
Next in the unit: indicators of chemical change (node 3) and energy in reactions (nodes 4 and 5).
In class: steel wool in vinegar (rust over a few days), and dissolving then evaporating salt water, side by side.
What each step is for
Engage: is reversing the test?: Most students hold the reversibility rule from primary school; committing to it first makes its failure (dissolving) memorable.
Explore: watch the particles: Watching the particles in a chemical change and a physical change, side by side, shows the difference the new substance criterion names.
Explain: the new substance test: Stating the new substance criterion as a replacement for reversibility, then applying it to cases that break the old rule, is the hinge.
Elaborate: is dissolving chemical?: Dissolving is the classic ambiguous case: the solid seems to disappear, but the particles are unchanged.
Elaborate: burning: Burning is the change students most often describe as 'destroying' the fuel; watching the atoms go into new products counters that.
Now you can
State that a chemical change makes a new substance.
Classify changes as physical or chemical with the new substance test.
Explain why "can it be reversed?" is not a reliable test, with a counterexample.