Puddle Detectives: Where Did the Water Go?
After a heavy rain, students map puddles on the playground at 9:00 AM. They find a large puddle on black asphalt (2 cm deep), one on a concrete sidewalk (1.5 cm), one on grass (1 cm), and one in a sandbox (1 cm). The day is sunny, 26°C, with a steady breeze. By 1:00 PM the asphalt and sidewalk puddles are gone, the grass puddle is half its size, and the sandbox puddle is still there but smaller. A student says, "The water soaked into the ground - that's why the asphalt puddle disappeared." Another says, "The sun dried them all up equally."
1. Which student's explanation fits the evidence better? Use the fate of each puddle to support your answer.
Read the worked answer
Neither is fully right, but the first student's idea of soaking in is closer for some surfaces while the second's 'equally' is clearly wrong.
Evidence against 'dried equally': the puddles disappeared at very different rates. If the sun acted equally, all four would shrink at the same pace.
Evidence about soaking in: asphalt and concrete are not porous in the same way as grass and sand, yet the asphalt and sidewalk puddles vanished first. That points to evaporation from a hot, dark, exposed surface rather than soaking in. The black asphalt heats up strongly in the sun, speeding evaporation.
The sandbox puddle lasted longest, consistent with water soaking into sand and being shaded by the sand's texture. A complete answer separates two processes - evaporation (water into the air) and soaking in (water into the ground) - and matches each puddle to the surface.
2. The sandbox puddle lasted longest. Give two different reasons this could be true and describe an observation that would help decide between them.
Read the worked answer
Reason A - soaking in: water drains down into loose sand, so less stays on the surface to be seen, but the surface puddle shrinks slowly because sand holds water within it.
Reason B - slower evaporation: the sand surface may be lighter or rougher, or partly shaded, so it heats less and evaporates more slowly than dark asphalt.
Deciding observation: Check whether the sand below the puddle is wet (dig a small hole and feel it). If the sand is wet well below the surface, soaking in is happening. Also compare surface temperatures of the sand and asphalt with a thermometer: if the sand is much cooler, slower evaporation is likely.
A strong answer notes that both processes can happen at once, so the test should measure each one rather than assume only one is at work.
3. The class predicts the puddles would behave differently on a cold, cloudy, windy day. Make a prediction and justify it using what they learned.
Read the worked answer
Prediction: the puddles should last longer on a cold, cloudy day, because evaporation is slower without strong sunlight and warmth. Wind still speeds evaporation, so the breeze would work against this, but the loss of sun and heat should dominate.
Justification from the earlier data: on the sunny 26°C day, the exposed dark asphalt puddle vanished within four hours, showing that sun and warmth drive evaporation quickly. Removing the sun and lowering the temperature should slow that process.
A strong answer notes that soaking in would continue at a similar rate regardless of sun, so the sandbox puddle's behavior would change least - a useful way to separate the two processes.