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Dilution on a grid: adding solvent

Grade 7 pupils draw a 110 x 110 mm PLA frame with four horizontal lines and four solute tokens, place the grid on board W3 (25 cells of 22 mm, 1 cell = 3 g), and calculate mass percentage concentration: 16 %, 8 % after adding the entire second grid of water, and 10 % after adding three columns. The denominator is always the mass of the solution, not the water.

Age
12–14 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. Grid frame in one strand
  2. Four solute rings
  3. Four horizontal lines; finish pen work
  4. Set 1: four portions of solute
  5. Predict and add water
  6. Less water: three columns
  7. Check yourself without the grid
  8. For faster pupils: partner's grid

What gets built

A modular PLA solution grid (110 x 110 mm frame and four horizontal lines) with four solute tokens, placed on board W3 (25 cells of 22 mm). Worksheet W2 contains three completed sets (16 %, 8 %, 10 %), a prediction, rejection of the wrong denominator, and a task without the model (20 % -> 10 %).

What it practises
  • Calculating mass percentage concentration from solute mass and solution mass.
  • Distinguishing solution mass from solvent mass (the denominator).
  • Predicting and checking how adding solvent changes concentration.
  • Distinguishing an agreed mass portion (a grid cell) from volume and the structure of a solution.

Success criteria
  • The pupil calculates Cp = 16 %, 8 % and 10 % for three sets using the correct denominator (solution mass).
  • The pupil rejects 12 g : 63 g, identifying water mass instead of solution mass as the wrong denominator.
  • The pupil solves a task without the model (20 % -> 10 %) and explains in one sentence why adding water lowers concentration.

What you need
Filament
PLA
Colours
pupil's colour, partner's colour
Also
3D pen (one per pupil), transparent work mat, pen stand, W1: 1:1 grid frame and ring pattern (included printout), W2: calculation sheet (one A4 sheet, printed on both sides), W3: solution board with a second grid (included printout, A4 landscape), blank sheet (half an A4 sheet) for covering cells, pencil, ruler (one per pair)

Files

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Version 1ca36bc8-0547-4421-83b3-20127ae90f7f Updated 2026-10-12

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