The Radish Mass Mystery: Matter, Energy, and a Confident Wrong Answer
In a ninth-grade integrated science class of 27, students grow radish seedlings in 500 g of dry potting soil under a grow light. After four weeks, the seedlings' combined dry mass has increased by 3.4 g, while the soil's dry mass has decreased by only 0.2 g. During a consensus discussion, 21 students conclude that the plant's mass 'came from the soil, because that is the only material we gave it.' Three students cite your earlier statement that 'plants make their own food from sunlight' as proof that soil is unnecessary. A parent email praises the investigation but asks why the class is 'teaching that plants eat sunlight instead of nutrients.' You have one 50-minute period before the unit test, which includes conservation of matter, and a colleague expects all classes to use the same assessment. Your data table and plant samples are available.
1. Diagnose the conceptual root of the students' conclusion. What does it reveal about your earlier explanation, and why did the numerical data fail to disconfirm the soil idea?
Read the worked answer
Students are using a substance-transfer schema: mass must come from a visible material that was added. They are not tracking gases or atoms, so a 0.2 g soil loss and a 3.4 g plant gain do not feel contradictory; soil is still the only 'input' they can see. The deeper issue is a matter-energy conflation. 'Food' in everyday language means fuel, while in biology it means organic carbon compounds used both as building material and energy source. My earlier line-'plants make their own food from sunlight'-was incomplete: sunlight supplies energy, but carbon and oxygen come from CO2 and hydrogen and oxygen from water. The phrase can reinforce the idea that plants do not need matter at all, or that soil nutrients are 'food.' The data alone cannot disconfirm the schema because students can assimilate it: soil provided nutrients, and nutrients are 'food.' I need to make the system boundary explicit (pot, soil, air, light, water), ask where the carbon atoms in glucose come from, and have students represent gases. I should also separate 'nutrient' (mineral ions) from 'food' (organic molecules) and avoid saying plants 'eat sunlight.'
2. Design a 50-minute instructional sequence that uses the class data to support conceptual change. Specify the sequence, the representations, and the talk moves you will use.
Read the worked answer
0–5 min: Return to the data without evaluation. Ask students to draw the pot-soil-plant system and label every input and output, including invisible ones. Collect two competing models on the board: soil-to-plant versus air-to-plant.
5–15 min: Predict-observe-explain with a sealed clear chamber containing a radish seedling and a CO2 indicator. Students predict CO2 change in light and dark. This creates a discrepant event: CO2 falls in light while plant mass rises.
15–30 min: Particle-level modeling. Give groups glucose, CO2, and water cards; have them assemble the atoms and account for O2. Use a mass-balance table: carbon in CO2 versus carbon in dry plant mass. Ask: 'If 3.4 g of dry mass is about 45% carbon, where did roughly 1.5 g of carbon come from?'
30–40 min: Structured talk-think-pair-share with sentence frames ('The data challenge my model because...'; 'The mass could come from...'). Deliberately ask a student holding the soil model to respond to the gas evidence.
40–50 min: Exit ticket: revise the whole-class model and write one sentence distinguishing matter and energy. I will not simply tell the answer; the sequence makes gas flux visible and forces atom accounting. I will correct my 'food' language explicitly and preview the assessment item format.
3. Write a summative item that separates equation recall from conservation-of-matter reasoning, and provide a 3-level rubric. Explain how you would use the results to inform the next unit.
Read the worked answer
Item: 'A radish seedling is sealed in a clear 2 L chamber with moist soil and air. The chamber is exposed to light for 10 days. The plant's dry mass increases by 1.2 g, and the chamber's CO2 concentration falls by 0.8%. The soil's dry mass is unchanged. (a) Draw a diagram tracing the path of carbon atoms from the air to the plant. (b) Explain why the plant's mass increase does not violate conservation of matter. (c) A student says, "The plant gained mass because it absorbed sunlight." Identify the error in that statement and correct it using matter and energy.'
Rubric: 3-traces carbon from CO2 through photosynthesis into glucose and plant biomass; accounts for O2 release; explicitly states sunlight is energy, not matter; uses conservation of atoms. 2-correct carbon source but vague about energy and matter or omits a gas. 1-says soil or sunlight supplies mass, or recites the equation without applying it. 0-no relevant reasoning. Score the diagram and explanation separately to see whether a student can recall the equation but not trace atoms. If many students score 1 on carbon source, reteach with gas measurements before the ecology unit. Give a word bank and allow labeled diagrams so language demand does not mask the science construct.