Chemistry Lab · Free lesson · 22 minutes
Balance atoms, preserve the substances
Balance an equation by changing coefficients, count every atom and distinguish an amount ratio from a mass ratio.
SS1–SS2 · chemical equations01 · Think first
Start with a question.
Two hydrogen molecules and one oxygen molecule can rearrange into two water molecules. The atoms are conserved; the molecules are rearranged. This lesson uses models, not an instruction to mix gases.
What you already know
- Read a chemical formula
- Count atoms including subscripts
- Use small whole-number ratios
02 · See it
An atom-count audit
Coefficients multiply whole formulae. The balanced hydrogen equation has equal atom totals on the two sides.
- Reactants
- 2H₂ + O₂
- Products
- 2H₂O
- Hydrogen atoms
- 4 on each side
- Oxygen atoms
- 2 on each side
Use the justified worked steps below to explore this idea.
03 · Show steps · explain why
One justified step at a time.
- 1
Start with H₂ + O₂ → H₂O
Formulae must identify the correct substances before balancing. At this point H balances but O does not.
04 · Mistake clinic
Find the first wrong idea.
H₂ + O₂ → H₂O₂ is a balanced water equation.
Compare the correction and its reason
2H₂ + O₂ → 2H₂O conserves atoms while retaining water's formula.
Changing the subscript changes water into hydrogen peroxide. An atom-balanced equation for the wrong substances does not describe the intended reaction.
Check: Check substance identities first, then count every element after each coefficient change.
A mistake is evidence about the next idea to repair. It is not a label for the learner.
05 · Your turn
Try first. Then use a hint.
06 · Check & reflect → next step
Understanding includes explaining.
Which changes preserve the substance and which change its identity? Why must mass conversions use molar masses?
Use mass divided by molar mass to connect measurable grams to chemical amounts.
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JEM-CH-C05-003 · Phase 5B Chemical Language Mole Stoichiometry Solutions Quantitative Analysis · Lab 03 · printed pp. 17–18
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