4.3 Scientific Models: Building, Using, and Evaluating

Models Are the Framework's Central Cross-Cutting Tool

Competency 005 lists "evidence, models, and explanation" among the unifying concepts and then adds a separate expectation that teachers evaluate the strengths and limitations of various scientific models. That second expectation is the one candidates underprepare. The exam is less interested in whether you can define a model than in whether you can say what a specific model gets right, what it gets wrong, and whether it is fit for the instructional purpose at hand.

Four Families of Models

Every Model Trades Accuracy for Usability

Scale distortion in a solar-system model. If Earth is a 1 cm bead, then on the same scale the Sun would be a sphere about 1.1 m across and would sit roughly 118 m away — the length of a football field plus an end zone. A classroom model that fits on a table therefore has correct order of planets but wildly incorrect relative distances. A model that gets distances right (walking the solar system across a schoolyard) usually makes the planets invisible dots. Naming which of the two the model is preserving is the teaching move.

The water-flow analogy for circuits. Voltage is like pressure, current like flow rate, resistance like a narrow pipe. This analogy is genuinely useful for predicting that adding resistance reduces current. It fails when students infer that current is "used up" by a bulb the way water is consumed by a leak — charge is conserved, and it is energy that is transformed. A teacher who introduces the analogy should also plan the demonstration that breaks it: measure current before and after a bulb with an ammeter and show the readings are equal.

The Bohr model of the atom. It correctly conveys that electrons occupy discrete energy levels and that a jump between levels absorbs or emits a specific quantity of energy. It incorrectly implies that electrons travel in fixed circular orbits like planets, which the quantum-mechanical orbital model replaced. For grades 4-8 the Bohr model is still the right instructional choice; the professional judgment is to teach it as a useful picture rather than as literal truth, so that students who later encounter orbitals experience a refinement rather than a betrayal.

Weather and climate simulations. A computational climate model resolves the atmosphere into a grid and applies physical laws to each cell. It is powerful for exploring "what if" scenarios but cannot resolve features smaller than its grid, so it represents individual thunderstorms only through approximations. Students should learn that a simulation is a hypothesis-testing tool, not an oracle.

Scale and Proportional Reasoning

Model work is where grades 6-8 proportional reasoning gets exercised. If a stream table 1.2 m long represents a 6 km river reach, the scale is 1.2 m : 6,000 m = 1 : 5,000. Students can then compute that a 2 cm meander in the tray represents 100 m in the field. Two limitations follow immediately and are worth naming aloud: water does not scale — surface tension and viscosity behave the same in the tray as in nature, so the tray's flow is not dynamically similar to a real river — and time does not scale — a tray channel migrates in minutes what a real river takes decades to do.

Teaching Students to Evaluate Models

The routine that satisfies the framework is short and repeatable. After students use any model, ask for:

1. One feature the model represents well. ("The bead model shows that DNA has two strands held together in pairs.")
2. One feature the model distorts or omits. ("The beads are rigid and the same size; real DNA twists into a helix and the base pairs are different molecules.")
3. One question the model cannot answer. ("It cannot show how the strands separate during replication.")

Then ask what a better model for that specific question would look like. This sequence converts modeling from a craft activity into scientific reasoning, and it is precisely the behavior an exam item describes when it asks which teacher response best develops students' understanding of models.

Models Versus Theories and Laws

Students frequently confuse these terms, and the exam tests the distinction. A model is a representation used to explain or predict. A scientific law describes a consistent relationship observed in nature, often mathematically (the law of conservation of mass). A scientific theory is a well-substantiated, testable explanation of a broad range of phenomena supported by extensive evidence (the theory of plate tectonics). A theory does not "graduate" into a law with enough evidence — laws describe what happens and theories explain why — and a model may serve either one.