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Paired ions: the neutral charge of salts

Grade 8 pupils draw six PLA ion tiles with tabs and notches representing charge, add paper labels and use the “Neutralisation field” board to assemble NaCl, CaCl2 and Na2SO4. They reject the incorrect formula NaSO4 so that the sum of cation and anion charges is zero.

Age
13–15 years
Length
approx. 45 min
Level
Medium
Where you teach
School
Purpose
Educational
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How to run this session

Stages and timing
  1. Cations: two 1+ tiles and one 2+ tile
  2. Anions: two 1− plates and one 2− plate
  3. Labels: which charge belongs to which ion?
  4. NaCl and CaCl2 on the neutralisation field
  5. Na2SO4 and the NaSO4 trap
  6. Check your understanding; for faster pupils: CaSO4

What gets built

Six labelled PLA ion tiles, including one complete sulfate(VI) ion tile. On the board, the balanced set forms a closed rectangle with no unused tabs or notches. The W2 record strip contains three formulae, names and charge balances, and the rejected formula NaSO4.

What it practises
  • Writing formulae and names of simple salts by balancing cation and anion charges to zero.
  • Determining the charges of Na, Ca, Cl (and Al) ions from the periodic table, and SO4 from the acid formula H2SO4.
  • Treating the polyatomic ion SO4²- as an indivisible unit when writing a formula.
  • Distinguishing a salt formula (ion ratio) from a molecule and from the tile model.

Success criteria
  • The pupil assembles NaCl, CaCl2 and Na2SO4 with no unused tabs or notches, and records each salt’s charge balance, formula and name on the W2 strip.
  • The pupil rejects NaSO4, identifying the unused notch as an uncompensated charge, and corrects the formula to Na2SO4.
  • Without tiles, the pupil predicts AlCl3 and explains why the sum of the charges in a salt formula is zero.

What you need
Filament
PLA
Colours
pupil’s colour
Also
3D pen (one per pupil), PLA filament (one colour per pupil, unchanged during the lesson), transparent work mat, pen stand, W1: 1:1 pattern with four ion tile outlines (print supplied in the package), W2: board, six labels and record sheet pack (printouts supplied), pencil, ruler, scissors (one pair per pair of pupils)

Files

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Version f9ac8885-47f0-47bd-b994-a71b7d0c8da8 Updated 2026-10-14

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