In a physical change (melting, boiling, dissolving) the particles move, spread out or pack together, but every particle stays the same: no new substance is made. Steam is still made of water particles.
A puddle disappears on a hot day. A sugar cube vanishes in tea. Has anything actually gone?
You know particles are atoms, alone or joined. This lesson asks what happens to them when something melts, boils or dissolves.
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: the bubbles in boiling water
ExploreTry it on the board and compare what happens with your prediction.
Explore: melt, boil, dissolve
Drag the temperature slider. Watch how the particles are arranged and how they move. Do any new kinds of particle appear?
Add salt. Where do the salt particles go?
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.
Dissolve some salt in the water.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: what a physical change is
In your book: draw the particles of ice, liquid water and steam in three boxes, using the same symbol for each water particle. Under them, explain why melting and boiling are physical changes.
Physical change: what changes, and what stays the same?
How the particles are arranged
How fast the particles move
The spaces between the particles
The atoms inside each particle
The number of particles
The substanceIce, water and steam are all water.
Physical change or not?
Butter melting
Water vapour forming drops on a cold window
Sugar dissolving in teaThe sugar particles are still there, spread through the tea.
Cutting paper into pieces
Toast burning
An iron nail rusting
Elaborate TypicalUse the same idea in a new context.
Elaborate: inside the steam
Try it yourselfNothing is saved: start again as often as you like.
Inspect the particles in the steam.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: does mass change?
Try it yourselfNothing is saved: start again as often as you like.
Heat the ice to steam again. Does the number of particles change?
Use the board. This ticks when you get there.
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
In a physical change (melting, freezing, boiling, condensing, dissolving, cutting) the particles change how they are arranged, how far apart they are and how fast they move.
The particles themselves stay the same, so no new substance is made. Steam is still water; dissolved salt is still salt.
The number of particles stays the same, so the mass stays the same.
Next lesson: a chemical change, where atoms rearrange into new particles.
For teachers
Students explain physical change as the same particles rearranged, spread out or packed together, with no new substance formed, and represent it with the particle model (AC9S8U07, node 1). About 24 minutes.
I can explain a physical change at the particle level.
I can say what changes and what stays the same in a physical change.
I can draw particle diagrams of a physical change.
Before class
A concept tutorial for introducing or revising the idea. Set the width with ?width=minimum, typical (default) or wider. It comes after Elements and compounds and before Is it a new substance? (chemical change, node 2), which uses the same boards to compare.
Not a hinge node, but its misconceptions return in chemical change: boiling splits water into hydrogen and oxygen; particles expand when heated; dissolved substances are destroyed. The prediction tests the first.
Board notes: the heat board shows water's particles as single dots (not their atoms); the steam board shows each particle's atoms. Real steam is invisible; the white cloud above a kettle is tiny drops of liquid water. The dissolve board's salt particles stand for sodium and chloride ions.
Running the lesson
Engage: tally the predictions. 'Hydrogen and oxygen' is the one to come back to in the Typical step.
Explore: ask whether any new kind of particle appeared, and what did change.
Explain: the 'what changes' sort and the physical-or-not sort are the success criteria.
In your own words: the three boxes drawn with the same symbol for water are the evidence.
The hard part (on purpose)
Keeping particles the same size. When a student draws bigger particles in steam, ask what grew: the particles or the gaps.
When students get stuck
A student says the particles get bigger when water boils. Ask: On the board, did the dots get bigger or the gaps? Fix: The gaps. The particles stay the same size; they move faster and spread out.
A student says dissolving is a chemical change because the salt disappears. Ask: If you evaporate the water, what is left? Fix: The salt. Its particles were spread out, not changed.
A student says bubbles always mean a new substance. Ask: What is inside a bubble in boiling water? Fix: Steam: water particles. Bubbles are only a clue to a chemical change, not proof.
Stretch and support
Stretch: Why does a puddle dry up on a warm day without boiling? (Some particles at the surface have enough energy to escape: evaporation. Still water particles, still a physical change.)
Support: Give the frame: 'In … the particles … but each particle is still … , so no new substance is made.'
Where it leads
Next: Is it a new substance? (chemical change, node 2), which uses the melting and dissolving boards again to compare with a reaction.
In class: melting chocolate or wax, condensation on a cold can, dissolving and recovering salt by evaporation; weigh ice in a sealed bag before and after melting.
What each step is for
Engage: the bubbles in boiling water: Students who think boiling breaks water into hydrogen and oxygen, or that the particles themselves expand, pick those options; the board keeps every particle the same.
Explore: melt, boil, dissolve: Watching the same particles rearrange through three changes builds the particle test for a physical change: same particles, new arrangement.
Explain: what a physical change is: Naming what changes (arrangement, spacing, motion) and what doesn't (the particles themselves) makes 'no new substance' a particle-level statement, ready for chemical change.
Elaborate: inside the steam: Inspecting the atoms of steam particles answers the bubbles question directly: still H₂O, not hydrogen and oxygen.
Elaborate: does mass change?: Keeping track of the particles explains why mass stays the same in a physical change, even when volume changes.
Now you can
Explain a physical change at the particle level.
Say what changes and what stays the same in a physical change.