Four Cups of Bean Seeds
In the science club, four groups each fill a paper cup with 10 cm3 of damp cotton wool and press five bean seeds against the inside of the cup. The cups are treated as follows for ten days.
Cup P: on the window sill, 20 mL of water added each morning. Cup Q: inside a closed cupboard, 20 mL of water added each morning. Cup R: on the window sill, 5 mL of water added each morning. Cup S: inside the closed cupboard, 5 mL of water added each morning.
After ten days the group measures the tallest shoot in each cup: P 12.0 cm, Q 11.8 cm, R 6.0 cm, S 5.5 cm. They also notice that the shoots in Q and S are pale yellow, while those in P and R are green.
The club writes in its report: 'Light does not affect plant growth. Water is the only thing that matters, because the cups given 20 mL grew twice as tall.'
1. Which two cups must be compared to test the effect of light? Name the variable being changed, two variables kept the same, and one thing that was not controlled.
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Compare P with Q (or R with S): the pairs differ only in light, since both are given 20 mL (or both 5 mL).
- Variable changed: light (window sill vs closed cupboard).
- Kept the same: volume of water (20 mL), number of seeds (5), type and size of cup and cotton, length of the test (10 days), how height is measured.
- Not controlled: temperature. A closed cupboard and a window sill are not at the same temperature, and the cupboard also has still, humid air while the sill has moving air. Any difference between P and Q could be caused by warmth rather than light.
A second weakness in control: each condition was run once only, from a single cup with five seeds, so a single bad seed or a single warm corner can move the result.
2. Test the club's two claims against the numbers and the colour observation. Which parts stand and which parts must be withdrawn?
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Claim 1, 'Light does not affect plant growth': half right, too broad. For height, P 12.0 cm vs Q 11.8 cm is a 0.2 cm difference - about the width of a pencil line, well within measuring error - so light made little difference to how tall the shoots grew in ten days. But the colour result contradicts the general claim: Q and S shoots are pale yellow, meaning little or no chlorophyll was made, so light clearly affects how the plant develops. A fair statement is: 'In ten days, light made little difference to shoot height, but shoots grown in the dark were pale and unhealthy.'
Claim 2, 'Water is the only thing that matters': not supported. Water plainly matters - P (20 mL) 12.0 cm against R (5 mL) 6.0 cm, and Q 11.8 cm against S 5.5 cm, both roughly double. But 'only' cannot be concluded: there was no cup with zero water to show water is essential, and the colour difference shows light matters too. Also, the 5 mL plants are short but still green, so they are not simply dying.
The report should also say that 'tallest shoot' is a weak measure: a tall pale shoot is not a healthy plant.
3. Suggest one change to the design and one change to the recording that would make the club's conclusion trustworthy.
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Design change: add a control cup kept on the window sill with no water at all after the cotton is damped at the start, and run at least three cups for every condition. Three cups per condition show whether a result is repeatable; the zero-water cup shows what 'no water' actually looks like and proves the 5 mL plants are not simply dying of drought. Keeping all cups at the same temperature (e.g. two identical boxes, one open to the lamp, one covered) removes the temperature problem as well.
Recording change: stop measuring only the tallest shoot. Measure all five shoots per cup and report the mean, and record the colour and number of leaves on a simple scale (green / pale green / yellow), plus the height of each plant. Health, not just height, is the outcome the club cares about - as the pale, tall cupboard plants show, the two can point in opposite directions.