Models, Claims, Evidence & Reasoning · Grade 7 Science
Chapter: Scientific Inquiry & Diversity of LifeModels, Claims, Evidence & Reasoning
Learning goals
- I can explain models, claims, evidence & reasoning.
- I can use evidence, a model, and stated assumptions to support a biological explanation.
Before this unit
Use the supplied vocabulary, distinguish an observation from an explanation, and identify any units or model assumptions. Revisit a connected earlier unit if needed; new terms are explained here.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
In 1847 Ignaz Semmelweis was working in a Vienna maternity hospital with two wards. In one, staffed by doctors and students, far more mothers died of childbed fever than in the other, staffed by midwives. The mothers knew it and begged to be admitted to the second ward. Semmelweis noticed that the doctors came to deliveries straight from dissections and the midwives did not, and he required everyone to wash their hands in a chlorinated lime solution. The death rate in the doctors' ward fell sharply within months. His evidence was strong and his colleagues largely rejected it, partly because he could not say what was being transferred — germ theory was still decades away. The episode shows both halves of this unit: how powerful a well-supported claim is, and how much depends on the reasoning that connects it to its evidence.
A scientific explanation has three parts and they are worth naming separately. The claim is the statement you are asserting — what you think is true. The evidence is the specific observations or measurements that support it, with numbers and units where they exist. The reasoning is the connection: why those particular observations support that particular claim, usually by way of a mechanism that explains how one thing would produce the other. Semmelweis had a strong claim and strong evidence; the reasoning is where he was stuck, and it is where most student answers are weakest too.
Alongside the explanation sits the model — a simplified representation used to think with. A diagram of a cell, an equation for photosynthesis, a food web, a physical apparatus standing in for a river: all are models, and all leave things out deliberately. That is what makes them useful. It also means every model has a limitation, a respect in which it does not match the thing it represents, and a good scientific statement names the main one rather than waiting to be caught out by it. "This food web shows who eats whom but not how much energy passes at each step" is a stronger sentence than the diagram alone.
Build your understanding
Where this fits
You can design a comparison, measure it, and summarise what came out. The last step is turning that into a statement somebody else should accept — and knowing what would make them right to refuse. This unit puts claim, evidence and reasoning together, and it closes the methods half of the chapter before the final unit turns to living things themselves.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
Part 1 · Separating what you saw from what you concluded
An observation is what anybody present could have recorded: the covered soil lost 5 g, the uncovered soil lost 12 g, the readings were taken twelve hours apart. An inference is what you concluded from it: that the cover reduced evaporation. The first can be checked by repeating the measurement; the second is an argument about what the measurement means.
Mixing them is the commonest flaw in a write-up, and it usually shows up as a sentence with no numbers in it. "The cover stopped the water escaping" reads like a finding and is actually an interpretation with the evidence left out. "The covered sample lost 5 g against the uncovered sample's 12 g over the same twelve hours, so the cover was associated with 7 g less loss" keeps them apart and lets a reader judge.
A useful discipline is to write the observations first, as a list with units, before writing a single interpretive sentence. Then every claim you make afterwards has to point back to a line on that list. If it cannot, you have found something you believe for reasons that are not in your data.
Pause and explain
Classify each as observation or inference: (a) the uncovered soil lost 12 g; (b) covering soil reduces evaporation.
Check your reasoning after answering
(a) is an observation — a measured value anybody could check by repeating the weighing. (b) is an inference: it generalises from the comparison to a claim about what covering does, which goes beyond the single measurement and depends on the conditions having been otherwise matched.
Part 2 · Reasoning is the mechanism, not a restatement
The weakest reasoning is the kind that just says the evidence again. "The cover reduced evaporation because the covered soil lost less water" has a because in it and explains nothing — it restates the measurement. Reasoning has to supply the link: water leaves soil as vapour from the exposed surface, a cover reduces the area in contact with moving air and traps humid air above the soil, so fewer molecules escape per minute.
Once you state a mechanism you get something valuable: a prediction you can test. If the mechanism is about exposed surface and air movement, then a cover with holes in it should perform somewhere between the two, a cover in still air should matter less than one in a draught, and a wider container should lose more than a narrow one. Any of these could fail, and that is what makes the mechanism a scientific proposal rather than a story.
Semmelweis is the cautionary case. His mechanism — some material carried on hands from dissections — was essentially right, but he could not name it or show it, so colleagues who disliked the implication could dismiss the reasoning while the evidence sat in front of them. A mechanism you can state and test is what turns a correlation into an explanation people can act on.
Pause and explain
Why is "the cover reduced loss because less water was lost" not reasoning?
Check your reasoning after answering
Because it restates the evidence instead of connecting it to the claim. Reasoning has to supply a mechanism: water leaves as vapour from the exposed surface, and a cover reduces that exposed area and traps humid air above the soil, so fewer molecules escape. That version also predicts what a perforated cover or a draught would do, which the restatement cannot.
Part 3 · What a model is for, and what it leaves out
A model is a deliberate simplification built for a purpose, and the purpose decides what can be left out. A food web drawn as arrows between species is excellent for showing who depends on whom and useless for showing how much energy moves. A diagram of a cell shows the arrangement of organelles and misrepresents their crowding badly — real cytoplasm is packed, not spacious. Neither is wrong; each is answering a different question from the one it fails at.
So judging a model means asking what it was for. "Is this model accurate?" is the wrong question; "is this model adequate for the question I am asking?" is the right one. A globe is a poor model of Earth's interior and a good one of the arrangement of continents, and nothing about the globe changes between those two judgements.
State the main limitation when you use one. It costs a sentence and it prevents the error the model invites: that a cell is roomy, that a food chain is a single line, that an equation for photosynthesis describes one event rather than a long sequence of reactions. Naming the simplification is part of using the model honestly.
Pause and explain
A diagram shows a cell's organelles spaced neatly apart. Name its purpose and its main limitation.
Check your reasoning after answering
Its purpose is to show which organelles a typical cell contains and roughly where they sit, which it does clearly. Its main limitation is the spacing: a real cell is extremely crowded, with contents behaving more like a gel than a fluid, so the diagram suggests room and free movement that are not there.
Part 4 · Testing a competing explanation, and revising
Before accepting an explanation, write down a rival. If the covered soil lost less water, the rival might be that the covered container started with less water, or was cooler, or was smaller in surface area to begin with. Each rival suggests a check: weigh the starting water, record both temperatures, measure both surfaces. Working through rivals is what separates a conclusion from a first impression.
Then be specific about what the evidence can carry. The covered sample still lost 5 g, so a claim that a cover prevents all evaporation is contradicted by the investigator's own data — a surprisingly common kind of error, where the claim is stronger than the result the person just wrote down. "Reduced by 7 g under these conditions" is supported; "prevents" is not.
Revision is the normal end of this process, not a failure of it. A claim narrowed to the conditions it was tested under, with its mechanism stated and its main rival addressed, is a better piece of work than a bold claim with the same data behind it. The habit to build is writing the limitation yourself, before anyone else has to point it out.
Pause and explain
The covered soil still lost 5 g. What does that rule out?
Check your reasoning after answering
It rules out any claim that the cover prevents evaporation entirely, since the covered sample demonstrably lost water. The supported claim is that the cover was associated with 7 g less loss than the uncovered sample under these conditions — a reduction, not a prevention, and one measured over a single stated interval.
Apply the whole unit · Evaluate an explanation
Illustrative observation, not a measured experiment: covered soil loses 5 g of water over a day, while uncovered soil loses 12 g in the same setting. Starting water, container and temperature are stated to be comparable. A draft claim says that a cover always prevents all evaporation.
Your task
Write a better claim, cite the numerical evidence, explain a plausible mechanism, and identify a limit on what the record supports.
What your response should show
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Response guidance — after your attempt
Calculate the difference the record shows: 12 − 5 = 7 g less water lost from the covered sample over the stated day.
Correct the claim against the investigator's own data: the covered sample lost 5 g, so "prevents all evaporation" is contradicted; the supported word is reduced.
Supply the mechanism: water leaves as vapour from the exposed surface, and a cover reduces that exposed area and traps humid air above the soil, so fewer molecules escape per unit time.
Name a limit and a rival worth checking: one comparison over one day in one setting, and the record does not confirm that the two containers had identical surface areas or air movement.
A better claim is that covering the soil was associated with 7 g less water loss over a day under these conditions — 12 g uncovered against 5 g covered — rather than that a cover prevents evaporation. The investigator's own figure refutes the stronger wording, because the covered sample still lost 5 g. The mechanism that would explain the reduction is straightforward: water leaves soil as vapour from the exposed surface, and the rate depends on how much surface is exposed, how dry the air above it is, and how fast that air is replaced. A cover reduces the exposed area and holds a layer of humid air in place, so fewer molecules escape per minute, and the loss that remains can happen at the edges or through the cover itself. That mechanism makes testable predictions: a perforated cover should fall between the two, a draught should widen the gap, and a wider container should lose more. The limits are worth stating plainly. This is one comparison over one day in one setting, so it does not establish how the effect would scale with time, temperature or wind. The record states that starting water, container and temperature were comparable, but not that the exposed surface areas were equal, and a difference there would be a rival explanation for part of the 7 g.
Scientific references for this unit
Parent / teacher teaching notes
Ask for a because-statement and a labeled relationship, not just a memorized word. Work through one calculation step at a time, permit oral or diagram responses, and use flexible sessions for difficult reading. Compare the learner’s actual evidence with the supplied model; do not reward copying or agreement with a personal preference. No private family, genetic, or medical records are required.
The key idea
A scientific explanation connects a claim with evidence and reasoning. The claim answers the question; the evidence consists of relevant observations or measurements; reasoning explains why a scientific idea connects that evidence to the claim. Restating a claim in different words is not enough. An explanation should also identify what the evidence does not settle.
Worked examples
Example 1 · Rebuild a weak explanation
- State the claim precisely, with the comparison and the conditions: seedlings 20 cm from a window gained more height over fourteen days than matched seedlings 2 m away.
- Attach the evidence with numbers: the near group averaged 9.2 cm (range 2 cm, ten plants), the far group 3.1 cm (range 3 cm, ten plants).
- Supply the mechanism rather than restating: chlorophyll in the chloroplasts absorbs light energy, which drives photosynthesis and builds the glucose used to make new cells, so more light available means more material for growth up to a limit.
"Because it is good for them" has a because in it and no mechanism. Mechanism is the test.
Example 2 · Find the rival explanation
- Ask what else could produce less measured loss besides reduced evaporation.
- Rival one: the pots did not start with the same amount of water, so the covered one had less to lose. Check by weighing both at the start and recording the figures.
- Rival two: the pots were not in matched positions, so the covered one was cooler or more sheltered. Check by recording temperature and air movement at both positions, or by swapping them and repeating.
Write the rival down before you write the conclusion, not after someone objects.
Avoid these mistakes
- Mistake: reasoning means saying because. "The cover reduced loss because less was lost" restates the evidence; reasoning names the mechanism — reduced exposed surface and trapped humid air.
- Mistake: a claim can be stronger than the data. The covered soil still lost 5 g, so the investigator's own record refutes "prevents all evaporation"; reduced by 7 g is what is supported.
- Mistake: a model is wrong if it is incomplete. Every model leaves things out on purpose; judge it by whether it is adequate for the question, and name its main limitation.
- Mistake: an observation and an inference are the same kind of statement. "Lost 12 g" can be rechecked by repeating the weighing; "covering reduces evaporation" is an argument about what the measurement means.
Check your understanding
Which part of “the soil is wet because it rained” is an inference?
Discuss the answer
The proposed rainfall cause is an inference; wet soil is the stated observation. Watering or another source could also explain it.
How is reasoning different from listing numbers?
Discuss the answer
Reasoning explains a scientific connection between the observations and the claim. Numbers become relevant evidence through that connection.
Words to know
- claim
- The statement being asserted — what you think is true — stated precisely enough that evidence could contradict it.
- evidence
- The specific observations and measurements supporting a claim, given with numbers and units.
- reasoning
- The connection between evidence and claim, usually a mechanism explaining how one would produce the other; restating the evidence is not reasoning.
- model
- A deliberate simplification used to think with, judged by whether it is adequate for the question rather than whether it is complete.
- limitation
- A respect in which a model or an investigation does not match reality; a good statement names its own.
Extend your scientific explanation
Compare a second supplied example or the linked educational reference. Identify a connection, a model limit, and an additional observation that would test the explanation.
In the real world
Use a clear question, reliable observations, and a biological mechanism to evaluate a claim. State what remains uncertain before applying a classroom model to a different setting.
Exit check
Does naming a limitation make an explanation worthless?
Check your explanation after answering
No. It makes the conclusion more accurate and identifies what further evidence would strengthen or change it.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.