Interpret particle rearrangement

Supplied materials

• All measurements and observations below are supplied classroom examples. They are not data from actual learners or a claim about a particular real site.
• Paper or a browser document for a labeled drawing, table or explanation.
• Worked problem inputs: A reaction drawing has four oxygen atoms before but only three after. Diagnose the model rather than invent a chemical exception.
• Assessment case 1: A model starts with 4 H atoms and 2 O atoms, then bonds them as two H₂O molecules. What is accounted for? Which inference or prediction is best supported?
• Assessment case 2: Which scientific label fits the specifically described step or feature? Supplied case: Old links break and new links form.
• Assessment case 3: Which scientific label fits the specifically described step or feature? Supplied case: Water molecules form from different reactant molecules.
• Assessment case 4: Which scientific label fits the specifically described step or feature? Supplied case: Diagram loses one oxygen atom.
• Assessment case 5: Which scientific label fits the specifically described step or feature? Supplied case: Carbon count is unchanged.
• Assessment case 6: Products have different connections but match every reactant element count. What follows? Which inference or prediction is best supported?

Learn the idea

In ordinary chemical reactions atoms rearrange into new combinations. Element identities and atom counts are conserved; molecules can change. A chemical reaction is different from a nuclear transformation.

Atom conservation means same element counts before and after. Bond rearrangement means atom connections change. Molecule change means products have different molecular groupings. Model error means an element appears or disappears without explanation.

For the supplied mechanism problem, A reaction drawing has four oxygen atoms before but only three after. Diagnose the model rather than invent a chemical exception. An oxygen atom is omitted or the boundary is incomplete. Ordinary chemical rearrangement does not destroy an oxygen atom. Correct the representation or identify where the atom went.

1. Read the supplied problem and mark the structures, processes or quantities named: A reaction drawing has four oxygen atoms before but only three after. Diagnose the model rather than invent a chemical exception.
2. Draw the relevant boundary, pathway or comparison and label it using the scientific ideas above.
3. Write a causal explanation or calculation, showing how each observation supports the result.
4. Compare your reasoning with the worked solution; name a condition that would change or limit the inference.

Check your reasoning

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Take it further

New diagram: two nitrogen atoms and six hydrogen atoms initially form two NH₃ molecules. Expected: atom counts are conserved; label two nitrogen and six hydrogen atoms on both sides, although molecular grouping changes.

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