Balancing equations is where many chemistry students get stuck. The free PhET simulation Balancing Chemical Equations shows the atoms on each side of the arrow, so students can see why coefficients matter. This lab gives you a 50-minute lesson, worksheet questions and an answer key.
In this guide8 sections
Lab at a glance
- Grade level: 8–11
- Time: 45–50 minutes
- Objectives: explain conservation of mass; balance simple equations using coefficients; distinguish coefficients from subscripts.
Know the simulation
- Introduction screen: three equations to balance: make ammonia, separate water and combust methane.
- Tools: a balance-scale view and a bar-chart view that compare atoms on each side.
- Balancing Game: three levels of increasing difficulty with scoring.
Step-by-step lesson plan
- Hook (5 min). Burn a candle in a sealed jar demonstration or show a photo; ask “Where did the wax go?”
- Review (5 min). Subscripts are part of a molecule; coefficients count molecules. Connect to our Build a Molecule lab.
- Introduction screen (15 min). Students balance all three equations using the bar chart or scale view and record their coefficients.
- Pattern discussion (5 min). What changes and what never changes when you balance?
- Balancing Game (12 min). Levels 1 and 2; strong students try level 3.
- Exit ticket (3 min). Balance one new equation on paper.
Worksheet questions
- Balance: N₂ + H₂ → NH₃
- Balance: H₂O → H₂ + O₂
- Balance: CH₄ + O₂ → CO₂ + H₂O
- Why can’t you change the subscript in H₂O to balance an equation?
- What does “conservation of mass” mean in your own words?
- Which view (scale or bar chart) helped you most, and why?
Answer key
| Equation | Balanced form |
|---|---|
| Make ammonia | N₂ + 3H₂ → 2NH₃ |
| Separate water | 2H₂O → 2H₂ + O₂ |
| Combust methane | CH₄ + 2O₂ → CO₂ + 2H₂O |
| Subscripts | Changing a subscript changes the substance itself (H₂O₂ is not water). |
| Conservation of mass | Atoms are not created or destroyed, so each element has the same number of atoms on both sides. |
Common misconceptions
- Changing subscripts to balance. Emphasize that subscripts define the molecule.
- Balancing molecules instead of atoms. Students must count atoms of each element.
- Thinking coefficients can be fractions in final answers. Use the smallest whole numbers.
Balancing strategy for students
- Count atoms of each element on both sides.
- Balance elements that appear in only one compound on each side first.
- Balance hydrogen and oxygen last.
- Recount everything.
Frequently asked questions
Is the PhET Balancing Chemical Equations simulation free?
Yes. It is free to use online from PhET at the University of Colorado Boulder.
What is the balanced equation for methane combustion?
CH₄ + 2O₂ → CO₂ + 2H₂O.
How many levels does the Balancing Game have?
The game has three levels that increase in difficulty.
Seeing the atoms makes balancing click for most students. For more simulation lessons, see our states of matter simulation lab and science lab guides.
