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A reaction without a missing atom

Grade 7 pupils use PLA to draw nine flat, single-layer pieces (4 empty hydrogen H discs, 2 oxygen O discs with a bar, 3 molecule frames, diameters 14 and 40 mm). On the printed reaction board they arrange reactants and products, choosing coefficients in 2H2 + O2 -> 2H2O and 2H2O -> 2H2 + O2 so that no atom is lost.

Age
12–14 years
Length
approx. 45 min
Level
Easy
Where you teach
School
Purpose
Educational
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How to run this session

Stages and timing
  1. Hydrogen H discs: four empty rings
  2. Oxygen O discs: two rings with a bar
  3. Three frames and pen work finished
  4. First attempt: H2 + O2
  5. Choose a coefficient: no atom disappears
  6. Count atoms on both sides
  7. Reverse the reaction: water decomposition
  8. Coefficient or subscript?
  9. Optional: partner puzzle

What gets built

A personal set of nine PLA pieces (4 H, 2 O, 3 frames) for moving atoms between the reactant and product fields. W2 records the first unsuccessful attempt, two balanced equations with atom counts on both sides (formation and decomposition of water), rejection of a subscript change, the transfer task H2 + Cl2 -> 2HCl and a prediction for 4 H2 molecules.

What it practises
  • Choosing coefficients for a simple reaction equation while preserving the number of atoms of each element.
  • Distinguishing coefficients from subscripts in a chemical formula.
  • Identifying reactants and products in molecular notation.
  • Distinguishing a notation model from a real chemical reaction (masses, energy, safety).

Success criteria
  • Pupils choose coefficients and write 2H2 + O2 -> 2H2O and 2H2O -> 2H2 + O2 with H and O atom counts on both sides (4 and 2).
  • Pupils identify reactants and products in both equations.
  • Pupils explain that changing a subscript (H2O2) gives a different substance, whereas the coefficient only states how many identical molecules there are.
  • Pupils solve the transfer task H2 + Cl2 -> 2HCl.

What you need
Filament
PLA
Colours
colour shown in the illustration
Also
3D pen (one per pupil), PLA filament (one colour per pupil, without changing during the lesson), transparent mat, pen stand, W1: 1:1 piece outlines and reaction board (package printout, 2 pages), W2: observation sheet (package printout), pencil, ruler (one per pair, to check the printout)

Files

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Version cbbe688d-5198-4004-afa8-91603257f208 Updated 2026-10-14

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