A balanced equation has the same number of each kind of atom on both sides, because atoms are only rearranged. You balance it by changing the coefficients in front of formulas, never the formulas themselves.
Chemists write reactions in a shorthand that has to obey one rule: no atom may appear or vanish.
Last lesson: atoms are rearranged, never made or destroyed. This lesson writes that down as an equation.
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: water from hydrogen
ExploreTry it on the board and compare what happens with your prediction.
Explore: balance it
Use + and − to change how many of each molecule there are. Watch the drawing and the atom tally: balanced means every row says "same".
Try it yourselfYour edits are saved in this browser.
Balance the equation with the smallest whole numbers.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: why only the coefficients
In your book: balance H₂ + Cl₂ → HCl. Draw the molecules on each side and count the atoms of each element to show it balances.
How many hydrogen atoms are in 2 H₂O?
In CH₄ + 2 O₂ → CO₂ + 2 H₂O, how many oxygen atoms are on the products side?
PractiseA few on your own, to check you have it.
Three more
Start with the atom that appears in only one substance on each side.
Try it yourselfYour edits are saved in this browser.
Balance the burning of methane: CH₄ + O₂ → CO₂ + H₂O.
Use the board. This ticks when you get there.
Balance the making of ammonia: N₂ + H₂ → NH₃.
Use the board. This ticks when you get there.
Balance magnesium burning: Mg + O₂ → MgO.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: the heavier ash
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 balanced equation has the same number of each kind of atom on both sides.
Balance by changing coefficients (the numbers in front). Never change subscripts: they define the substance.
Use the smallest whole numbers, and check by counting atoms of each element.
A balanced equation shows conservation of mass: the same atoms, so the same mass.
For teachers
Students balance simple symbolic equations by changing coefficients (never subscripts), verify by counting atoms, and explain that a balanced equation shows conservation of mass (AC9S9U07, node 4, a hinge node). About 28 minutes.
I can balance a simple equation by changing only the coefficients.
I can check an equation by counting the atoms of each element on both sides.
I can explain why a balanced equation shows conservation of mass.
Before class
A concept tutorial for introducing or revising the idea, after Where do the atoms go? (node 2). Set the width with ?width=minimum, typical (default) or wider.
The hinge errors are systematic: changing subscripts (H₂O to H₂O₂) and miscounting atoms in a coefficient (2 H₂O as 2 hydrogen). The board makes both visible: formulas can't be edited, and every molecule is drawn, so students can count rather than compute.
Board notes: formulas without brackets only; coefficients 1 to 6. The board accepts a balance only with the smallest whole numbers (4 H₂ + 2 O₂ → 4 H₂O balances but is not accepted). Word equations (node 3) and open and closed systems (node 5) are class work; the Wider step is one open-system case.
Running the lesson
Engage: count who chose H₂O₂. Ask them what H₂O₂ is.
Explore: ask what changed in the drawing when they pressed + on H₂O (a whole molecule, two H and one O).
Practise: the three equations are the node's evidence; methane is the hardest, so give the 'atom that appears once' hint aloud.
The hard part (on purpose)
Changing coefficients, never formulas, and counting atoms each time. When a student goes round in circles, ask which element appears in only one substance on each side, and start there.
When students get stuck
A student wants to change a subscript. Ask: What substance would H₂O₂ be? Fix: A different one. Only the number of molecules (coefficient) can change.
A student counts 2 H₂O as 3 atoms. Ask: How many molecules is that, and how many atoms in each? Fix: Two molecules of three atoms: 4 hydrogen and 2 oxygen.
A student goes round in circles on methane. Ask: Which element appears in only one substance on each side? Fix: Balance carbon, then hydrogen, then oxygen last (it appears in two products).
Support: Give an atom-tally table to fill in for each attempt: element, left, right.
Where it leads
Before: Where do the atoms go? (node 2). Node 5 (conservation of mass in open and closed systems) is a class task: weigh a reaction in a sealed bag, then open it.
In class: molecular model kits to build both sides of 2 H₂ + O₂ → 2 H₂O.
What each step is for
Engage: water from hydrogen: The prediction contrasts the legal move (coefficients) with the commonest slip (changing a formula), before students try either.
Explore: balance it: Seeing whole molecules appear and the tally change with each coefficient makes balancing a count of real atoms, not a number puzzle.
Explain: why only the coefficients: Rejecting the subscript change and linking the tally to conservation of mass makes balancing a scientific constraint, not a rule.
three more: Balancing is a precision skill (practise here): three equations with different starting moves build fluency with the same check each time.
Elaborate: the heavier ash: An open-system result that seems to break conservation of mass is explained by the balanced equation: oxygen came in from the air.
Now you can
Balance a simple equation by changing only the coefficients.
Check an equation by counting the atoms of each element on both sides.
Explain why a balanced equation shows conservation of mass.