Ten Grams of Foil: Confronting the Floating and Sinking Misconception in One Period
A Class VII physical science teacher demonstrates floating and sinking using a 200 g iron block, which sinks, and a 2 kg wooden log, which floats. She then writes on the board: 'Objects float if they are light and sink if they are heavy.' In a follow-up question, thirty-one of thirty-eight students explain that a large steel ship floats 'because it is made of light metal'. She wants to confront the idea in a single 40-minute period. Available equipment: 10 g of aluminium foil, two 500 ml beakers, water, salt, an iron nail, a wooden block, and a spring balance. A colleague advises her to simply state the rule 'density = mass ÷ volume' and set ten numerical problems.
1. Explain why the original demonstration produced or reinforced the misconception, and why stating the density formula would not dislodge it.
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The demonstration was designed so that it could not fail to confirm what students already believed:
- Two variables changed at once. The iron block and the wooden log differ in both mass and material. Nothing in the pair isolates mass, so students can attribute sinking to weight and floating to the wood being 'light stuff' - and both readings survive.
- The awkward case was explained away. The 2 kg log floated while the lighter 200 g block sank, which should have contradicted the rule. Instead the class reassigned floating to material ('wood is light'), which is precisely the move that later produces 'the ship is light metal'.
- No prediction was elicited. With two data points and no commitment made in advance, there is nothing for the students to be wrong about. The demonstration illustrated the teacher's conclusion rather than testing the students'.
Stating density = mass ÷ volume supplies a third verbal rule to memorise. It requires comparing two quantities that students cannot yet measure for a hollow steel hull, so it never comes into contact with the intuition it is supposed to replace. The misconception lives at the level of why things float; a formula answers a different question.
2. Design the key discrepant event using only the listed equipment. State the prediction you would elicit first and the question you would ask afterwards.
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The discrepant event: 10 g of foil, twice.
- Elicit prediction first, in writing. Show the sheet of foil and ask: 'I am going to put exactly this much foil into water twice. Will it float or sink the first time, and the second time? Write your answer and your reason.' Every student commits before anything is tested - this is what makes the later conflict personal rather than the teacher's.
- Crumple the foil into a tight ball and drop it in. It sinks. Confirm the mass on the spring balance if students suspect something changed.
- Take an identical 10 g sheet, shape it into an open boat, and float it. The mass is identical, the material is identical, the water is identical. Only the shape differs.
- Intensify the conflict. Place the iron nail in the foil boat; it still floats. Add a second object until it sinks - showing that the same boat carries extra mass and still floats, then fails at some point.
- The question to ask: 'The ball and the boat weigh exactly the same and are made of the same thing. What is different about them?' Steer the answer towards the space the boat encloses and the water it pushes aside, not towards lightness.
Sequence: predict → test → record the surprise → discuss → then name the idea. Naming after the conflict, not before, is what distinguishes this from the original demonstration.
3. How would you assess whether the misconception has shifted? Include a transfer item and say what a residual misconception looks like.
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Assess with predict–justify items on unfamiliar objects, not with definitions:
- A ball of plasticine versus the same plasticine shaped as a boat.
- A sealed full bottle versus an identical bottle emptied and capped - same shape, different mass.
- A steel ship and a steel nail, side by side.
For each, the student writes a prediction and a one-sentence reason. The reason, not the prediction, is the evidence.
What counts as progress: 'The same amount of plasticine spread out pushes aside more water, so it floats.' The student now explains floating by displaced water rather than by lightness.
What a residual misconception looks like: 'The boat has air inside, so it is lighter.' This is a partial idea, not simply wrong - the student has noticed that something about the boat differs and is groping towards average density. The teacher should refine it: air is not weightless, but spreading the same mass over a larger volume lowers the average density of the object. Treating this answer as a failure would discard a nearly correct intuition.
Clear residual misconception: predicting that a larger or heavier boat must sink regardless of shape, or that aluminium 'floats because it is light' when the foil ball has just sunk in front of them. That student has memorised the lesson's events but not revised the rule.