Stop 6 of 12 · The path
Reactions — writing the change
A reaction rearranges atoms. The equation must balance: same atoms in, same atoms out.
Bench tool
Balance it: H₂ + O₂ → H₂O
Tap the steppers until each side holds the same atoms. Coefficients only — never touch the little numbers.
How it works
- Write formulas first. H₂, O₂, H₂O — subscripts never change while balancing.
- Balance by coefficients. Big numbers in front only.
- Check each element last. Metals, then non-metals, then H and O.
Worked examples
CH₄ + 2O₂ → CO₂ + 2H₂O. 1 C, 4 H, 4 O each side.
Zn + 2HCl → ZnCl₂ + H₂. Displacement: zinc pushes hydrogen out.
CaCO₃ → CaO + CO₂. Heat splits limestone — decomposition.
Where it goes wrong
Never change subscripts. H₂O to H₃O invents a new substance. Change coefficients only.
Try it at home: fizz balance
Weigh the fizz. Drop half a vitamin-C tablet into water with a pinch of bicarbonate of soda. Weigh the cup before and after (adult help) — the tiny loss is escaped gas; the atoms themselves are conserved.
For teachers
Learners change subscripts to force a balance — probe by asking what H₃O would even mean. Quickest check: hand them unbalanced H₂ + O₂ → H₂O and watch whether they touch only coefficients.