The Melting Ice Puddle Mystery
Two students place identical ice cubes on two identical plates. Plate 1 sits in the sun on a black metal tray; Plate 2 sits in the shade on a white plastic tray. After 10 minutes, the ice on Plate 1 has completely melted and the ice on Plate 2 is still mostly solid. One student concludes, "The black tray made the ice melt faster." The other says, "No - the sunlight made it melt faster. The tray color has nothing to do with it." They cannot agree, so they design a new test.
1. Explain why the original setup cannot tell us whether the tray color or the sunlight caused the difference. Then describe a fair test the students could run.
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Why the original setup is inconclusive: Two variables changed at once - the amount of sunlight (sun vs. shade) and the tray color (black metal vs. white plastic). When more than one variable changes, you cannot tell which one caused the result. The metal vs. plastic material also differs, adding a third variable.
A fair test: Keep everything the same except the variable being tested. For example, put two identical ice cubes on two identical black trays, place one in the sun and one in the shade, and compare melting times. Then run a second test with two identical trays of different colors (one black, one white) both placed in the same sunlight, and compare. Only one thing changes between the two setups in each test, so any difference in melting can be attributed to that variable.
2. The students run the fair test with black and white trays both in full sun. The ice on the black tray melts in 8 minutes; the ice on the white tray melts in 14 minutes. Write a conclusion the students could defend, and explain what evidence supports it.
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Defensible conclusion: "When two trays are placed in the same sunlight, ice on a black tray melts faster than ice on a white tray. This suggests that tray color affects how quickly ice melts, with darker surfaces causing faster melting."
Evidence: Both trays were in the same sunlight, the ice cubes were identical, and the only difference was tray color. The measured melting times (8 min vs. 14 min) show a clear difference of 6 minutes.
Caveat to include: This is one trial with one pair of trays. To be more confident, the students should repeat the test several times and average the results, and they should be careful not to claim why it happens (for example, that dark surfaces absorb more light energy) unless they have additional evidence - the melting-time data alone shows the pattern but does not by itself prove the mechanism.