Mung Bean Light Color Experiment
A Grade 10 student grows mung beans in four identical boxes. Each box has 20 seeds in the same soil and receives 200 mL of water every two days. Box A sits under white light, Box B under red light, Box C under blue light, and Box D under green light. All boxes are kept at 28°C. After 14 days, the student measures average seedling height: A = 12.4 cm, B = 14.1 cm, C = 10.8 cm, D = 6.2 cm. Box D also has 7 seeds that did not sprout; the other boxes have 2 or fewer. The student concludes that green light is worst for mung bean growth and that red light is best. A classmate says the light color cannot be the cause because the boxes were not rotated, and the lamp above Box D was 5 cm closer to the soil.
1. Identify the independent variable, the dependent variable, and two controlled variables in this experiment.
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Independent variable: light color (white, red, blue, green). Dependent variable: average seedling height after 14 days; germination count is a second dependent measure. Controlled variables include the number of seeds per box (20), soil type, water volume and frequency (200 mL every two days), and temperature (28°C). The lamp distance is supposed to be controlled but is not: Box D's lamp was 5 cm closer. That is a confounding variable because closer lamps usually deliver more light intensity.
2. Evaluate the student's conclusion. Which part is supported and which is not? Use the data and the classmate's point.
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The data support only a descriptive statement: in this setup, the red group had the tallest average seedlings (14.1 cm), and the green group had the shortest (6.2 cm) and the most unsprouted seeds (7). The causal conclusion that red is 'best' and green is 'worst' is not supported. Box D's lamp was 5 cm closer, so the green result may reflect higher light intensity, heat, or uneven placement, not green light itself. The boxes were also not rotated, so position near a window or lamp could affect growth. With one box per color, there is no replication to separate treatment from chance. The classmate is right to question the causal claim.
3. Describe one modification to improve the experiment and explain what it would control.
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Set all lamps at the same height and measure light intensity with a lux meter at soil level; rotate the boxes every two days. Use at least three boxes per color, not one. These changes control light intensity and position, so any remaining height difference is more plausibly due to color. Also record germination daily and use the same seed batch.