Where Did the Puddle Go? Separating Evaporation from Infiltration
At 9:00 a.m. a fourth-grade class measures a 3 cm-deep puddle on the blacktop after overnight rain; the air is 22°C. By 1:00 p.m. the puddle is gone and the air is 29°C with a steady breeze. Beside the blacktop, on a grass slope, students find small mud channels and a fan of sand washed onto the sidewalk at the bottom - but the storm drain at the edge of the blacktop is dry. Asked where the puddle went, one student says, 'It soaked in,' and another says, 'It dried up.' The teacher has playground soil, a heat lamp, two clear containers, a balance, and 30 minutes. The class must decide which explanation the evidence supports before moving into the water-cycle unit.
1. Which explanation - soaking in or drying up - does the existing evidence better support for the blacktop puddle, and what single test would best settle it?
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Reasoning from the evidence available
- Soaking in requires a permeable surface beneath the water. Blacktop is designed to shed water, so this claim needs direct evidence, not just plausibility.
- The dry storm drain argues against the puddle having drained away as runoff, which is a third possibility students often skip.
- The mud channels and sand fan prove that runoff and erosion happened on the grass slope, not on the blacktop. Students must not transfer evidence from one surface to the other.
- The 7°C temperature rise, the breeze, and the seven hours of daylight are conditions that favor evaporation.
Best test: place equal masses of water in two identical clear containers, one open and one sealed, under the heat lamp. Weigh both after 30 minutes. If the open container loses mass and the sealed one does not (with droplets forming on the lid), water is leaving as vapor - evaporation - not draining downward. To test permeability directly, pour 100 mL onto a tilted tray of soil and onto a tilted tray of blacktop-like material, and compare how much water passes through.
Judgment point: the class should conclude 'most likely evaporated' rather than 'proven,' and should state what the sealed-container result adds.
2. A student builds a 'water cycle in a bag' - wet paper towel sealed inside a clear bag set in the sun - and says it proves the puddle evaporated. Evaluate what this model does and does not demonstrate, and how you would frame it for students.
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What the model does show
- Liquid water disappears from the towel while vapor appears as droplets on the inside of the bag: evaporation and condensation are real and observable, and they occur without boiling.
- It makes the invisible visible, which is the main reason to use it.
What it does not show - and why this matters
- The bag is sealed, so nothing is lost to the room. In the real puddle, the vapor is carried away by the breeze and never returns; the model can mislead students into thinking rain is guaranteed to come back from that same puddle.
- The bag has no soil, no slope, no drain, and no impermeable surface, so it cannot represent infiltration or runoff - exactly the processes the class is trying to separate.
- The sun warms the bag itself, so the heating is not equivalent to sunlight on open ground.
Instructional framing: present the bag as a model of two processes, then ask, 'Which parts of the playground does this bag leave out?' Students should name runoff, infiltration, and the open system. A good extension is an unsealed cup beside the bag so students can see the difference between a closed and an open system.