Watering Trial with Two Basil Plants
A student grows two basil plants from the same seed packet in identical 2 L pots filled with the same soil. Both sit on the same windowsill. Plant A gets 200 mL of tap water every morning. Plant B gets 200 mL of water with 10 g of sugar dissolved in it every morning. After 14 days, Plant A has 12 green leaves and is 18 cm tall. Plant B has 7 leaves, 4 of which are yellow at the edges, and is 13 cm tall. The student concludes that sugar water harms basil. A classmate says the result may be caused by something else because the pots were not rotated and Plant B stood closer to a radiator that came on for 3 hours each afternoon.
1. Identify one variable the student controlled and one variable that was not controlled. Explain how the uncontrolled variable weakens the conclusion that sugar water harms basil.
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Controlled: both plants received the same volume of liquid (200 mL) each morning and were in identical pots with the same soil. Not controlled: distance to the radiator and heat exposure. Plant B was closer to a radiator that ran 3 hours each afternoon. Heat can cause leaf yellowing and reduce growth, so Plant B differed from Plant A in two ways at once: sugar and heat. The comparison therefore cannot isolate sugar as the cause. The conclusion is a reasonable hypothesis, not a demonstrated result.
2. The class repeats the trial with five plants per group and no radiator. Name one further change that would improve reliability and one additional measurement that would make the comparison stronger. Justify each.
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Reliability change: rotate the pots a quarter turn daily or randomize their positions on the windowsill, so no plant gets a permanent light or heat advantage. With five plants per group, this reduces the chance that one unusual plant drives the result. Additional measurement: count yellow leaves and record a simple leaf-color score (green/yellow) on days 0, 7, and 14, plus stem height. This quantifies the harm instead of relying on a single final description and lets the class compare the size of the effect.
3. The student now wants to test whether the amount of sugar matters. Describe two treatment groups and one control group, including amounts, and state what result would support a dose effect.
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Use three groups of five plants, all with 200 mL per day: control 0 g sugar; low 25 g/L (5 g in 200 mL); high 50 g/L (10 g in 200 mL), the same concentration as the first trial. Keep soil, pot size, light, and heat the same. If yellowing and growth reduction increase from control to low to high, that supports a dose effect. If only the high group is affected, the effect may depend on concentration but is not graded.