In a chemical reaction atoms are rearranged into new particles, never made or destroyed. Counting atoms before and after shows why mass is conserved and why equations must balance.
A log burns down to a handful of ash. Where did the rest of the log go?
You know a chemical change makes a new substance. This lesson follows 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.
Practise: A few on your own, to check you have it.
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: burning methane
ExploreTry it on the board and compare what happens with your prediction.
Explore: count the atoms
Watch the line under the counts: atoms at the start and now. Do the numbers ever change?
Try it yourselfNothing is saved: start again as often as you like.
Burn the methane until the target is reached.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: rearranged, not destroyed
In your book: draw CH₄ + 2O₂ → CO₂ + 2H₂O as particles before and after, with each atom as a labelled circle. Then explain, using atoms, why the total mass is the same before and after a reaction.
In CH₄ + 2O₂ → CO₂ + 2H₂O, how many oxygen atoms are on the left side?
How many oxygen atoms are on the right side?
PractiseA few on your own, to check you have it.
Balanced or not?
Is each equation balanced (the same atoms on both sides)?
H₂ + Cl₂ → 2HCl2 H and 2 Cl on each side.
H₂ + O₂ → H₂O2 O on the left, 1 on the right.
2H₂ + O₂ → 2H₂O4 H and 2 O on each side.
C + O₂ → CO₂
N₂ + H₂ → NH₃2 N on the left, 1 on the right; 2 H on the left, 3 on the right.
CH₄ + O₂ → CO₂ + H₂O4 H on the left, 2 on the right; 2 O on the left, 3 on the right.
Elaborate TypicalUse the same idea in a new context.
Elaborate: making water
Try it yourselfNothing is saved: start again as often as you like.
React the hydrogen and oxygen.
Use the board. This ticks when you get there.
In 2H₂ + O₂ → 2H₂O, 12 hydrogen particles react with 6 oxygen particles. How many water particles can form?
Elaborate WiderUse the same idea in a new context.
Elaborate: open and closed
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 chemical reaction, atoms are rearranged: bonds break and new bonds form. No atom is made or destroyed.
The same atoms means the same mass: the law of conservation of mass.
A balanced equation has the same number of each type of atom on both sides. Balance it by changing the numbers in front, never the formulas.
Mass seems to change only when a gas escapes or joins from the air.
For teachers
Students model a reaction as atoms rearranging, count atoms on both sides, and connect the unchanged atoms to conservation of mass (AC9S9U07, node 2, a hinge node). About 25 minutes.
I can use a model to show a reaction with the same atoms before and after.
I can count the atoms on each side of an equation and check they match.
I can explain that atoms are rearranged, not made or destroyed, so mass is conserved.
Before class
A concept tutorial for introducing or revising the idea; it does not replace a closed-system mass experiment. Set the width with ?width=minimum, typical (default) or wider.
The hinge risk is balancing as a procedure without meaning. Keep returning to the atom tally on the board: balancing describes what the atoms actually do.
Model note: in the simulation, CH₄ + 2O₂ happens as one event, taking the second oxygen from nearby. Real combustion goes through several quick steps; the atoms and energy are the same.
Running the lesson
Engage: ask who chose 'turn into heat and light'. The tally answers them.
Explore: pause the board at any moment and count one atom type on screen against the panel.
Explain: the two number checks are the counting skill; the last question links it to mass.
Practise: the H₂O₂ question is the commonest balancing error; discuss it aloud.
The hard part (on purpose)
Tracking every atom from before to after. When a student says the fuel was used up, ask them to find one of its carbon atoms now.
When students get stuck
A student says the methane was used up, so its atoms are gone. Ask: Find a carbon atom on the board now. Where is it? Fix: In carbon dioxide. The particle changed; the atom didn't.
A student balances by changing subscripts. Ask: Is H₂O₂ still water? Fix: No: it is hydrogen peroxide. Change the number of particles (in front), not the formula.
A student says gases weigh nothing. Ask: What happens to the mass of a balloon when you blow it up? Fix: It increases a little: gas particles are atoms, and atoms have mass.
Stretch and support
Stretch: Balance N₂ + H₂ → NH₃, then check it on paper by drawing the particles.
Support: Give an atom-count table (atom type, left side, right side) to fill in for each equation.
Where it leads
Next: balanced symbolic equations (node 4) and conservation of mass with evidence (node 5).
In class: steel wool burned on a balance (mass goes up), and vinegar with baking soda in a sealed bottle with a balloon, weighed before and after.
What each step is for
Engage: burning methane: Students who think burning destroys matter or turns it into energy will pick those options; the atom tally answers them directly.
Explore: count the atoms: The live atom tally makes 'same atoms, new arrangement' visible while the reaction runs.
Explain: rearranged, not destroyed: Counting atoms on both sides, then connecting the count to mass, makes conservation a consequence rather than a rule to memorise.
balanced or not?: Checking balance by counting atoms is the skill equations depend on; doing it for several reactions builds the habit.
Elaborate: making water: A second reaction, with a ratio to reason about, extends counting to predicting how much product forms.
Elaborate: open and closed: Apparent mass loss in open containers is the main evidence students cite against conservation; explaining it with the atom model completes the idea.
Now you can
Use a model to show a reaction with the same atoms before and after.
Count the atoms on each side of an equation and check they match.
Explain that atoms are rearranged, not made or destroyed, so mass is conserved.