Science · Original worked example
Test the point where two scientific explanations disagree
Compare two proposed explanations, identify their different predictions, and choose evidence that distinguishes them in original ACT-style science passages.
Before you begin
Useful foundation: Separate an observation from a proposed explanation and track each viewpoint's claims.
Your goal: Choose a measurement that tests a specific disagreement rather than something both explanations already predict.
List what each explanation predicts under the same conditions. Evidence is most useful for distinguishing them when their predictions differ. Supporting one prediction does not automatically prove every part of an explanation.
Work through an example
Two students explain why a pond's surface water became cooler overnight. Student A proposes that heat transfer to cooler air was sufficient, even with no evaporation. Student B proposes that evaporation was necessary and that cooling would not occur if evaporation were prevented. Both accept that the temperature fell.
- Identify the shared observation: the surface water became cooler. Measuring that again alone would not distinguish the explanations.
- Identify the disagreement: A allows cooling without evaporation; B says evaporation is necessary.
- In an ideal controlled test, prevent evaporation while preserving the other relevant heat-transfer conditions. Measure whether the water still cools.
- Cooling under those conditions would contradict B's necessity claim and be consistent with A. A real experiment would also need to check whether the method of preventing evaporation changed other heat-transfer processes.
Try a question yourself
In a hypothetical experiment, seeds are kept moist at the same temperature. Model 1 predicts that light is required for these seeds to germinate, so none will germinate in complete darkness. Model 2 predicts that they can germinate without light, so at least some will germinate in complete darkness. Both predict that at least some will germinate in light.
Choose an answer, then check your reasoning. This public practice is not saved.
Read all answer explanations
Some seeds germinate in light.: Both models predict this observation, so it does not distinguish them.
Some seeds germinate in verified complete darkness. — correct answer: Model 2 predicts this result, while Model 1 predicts none in darkness. Under the stated conditions, the observation supports Model 2 over Model 1 without proving every possible claim about germination.
The seeds are kept at a constant temperature.: Constant temperature is a controlled condition, not an outcome on which the models disagree.
The seeds are given equal amounts of water.: Equal water helps keep the comparison fair, but it does not show whether light is required.
What to do next
Make a two-column prediction table before choosing evidence. Mark the row where the explanations disagree and look for a test of that row.
Revisit the idea in a week using a different question. Repeating this example can help you understand it, but remembering its answer is not evidence of learning a new problem.
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