The Shrinking Puddle
A class sets two identical shallow trays of water on a windowsill. Tray A is left open. Tray B is covered with a clear lid that has a small gap at one edge. Both start with 200 mL of water at 22 °C. After two days, Tray A holds 96 mL and Tray B holds 168 mL. The room temperature stays at 22 °C and the trays sit in the same sunlight. A student says, "Tray A lost more water, so the water in Tray A must have been hotter." Another student says the lid simply kept the water from escaping as vapour. The teacher asks the class to decide which explanation the evidence supports and to predict what would happen if both trays were moved to a dark cupboard at the same temperature.
1. Which explanation does the evidence support? Use the numbers to justify your choice.
Read the worked answer
The lid explanation is supported. Both trays started at the same temperature and volume, so the difference must come from the lid. Tray A lost 200 − 96 = 104 mL. Tray B lost 200 − 168 = 32 mL. The lid did not heat the water; it slowed evaporation by trapping vapour above the surface, so fewer water molecules escaped into the room. The claim that Tray A was hotter is not supported, because nothing in the setup made it hotter - the only difference was the lid. Evaporation rate depends on temperature, surface area, and airflow, and here only the lid differed.
2. Predict the result if both trays are moved to a dark cupboard at 22 °C for two more days. Explain your reasoning.
Read the worked answer
Both trays would keep losing water, but more slowly than in sunlight, because light adds heat and warmth speeds evaporation. Tray A would still lose more than Tray B, since the lid still slows escape of vapour. The gap in the lid means Tray B is not sealed, so it will not stop losing water entirely - it will just lose it more slowly. A good prediction states the direction (A loses more than B) and the reason (the lid reduces vapour escape; darkness reduces heat input). Saying 'nothing happens in the dark' would be wrong, because evaporation continues at room temperature without light.