The Shrinking Puddle
A student places two identical clear plastic cups on a windowsill. Cup A contains 50 mL of water and is left open. Cup B contains 50 mL of water and is sealed with a tight lid. Both cups start at the same temperature. After 3 days, the water level in Cup A has dropped noticeably, while the water level in Cup B looks unchanged. The student concludes, "Water in Cup A disappeared because the cup is open to the air, but water in Cup B cannot evaporate because the lid traps it." A classmate says, "The water in Cup B is still evaporating - it just can't escape." The teacher asks the class to decide which explanation better fits what is happening at the particle level.
1. Evaluate both explanations. Which one better matches what happens to water particles in each cup?
Read the worked answer
The classmate's explanation is better. In both cups, some liquid water particles gain enough energy to leave the liquid and become water vapor (evaporation). In Cup A, the vapor escapes into the room, so the liquid level drops. In Cup B, vapor forms but is trapped by the lid. The particles cannot leave the cup, so the vapor builds up above the liquid. The liquid level looks unchanged because water is leaving the liquid and returning to it at about the same rate once the air above becomes humid. The student's claim that evaporation "cannot happen" in a sealed cup is wrong - evaporation still occurs; it is just balanced by condensation.
2. Predict what would happen to the water level in Cup B if the lid were removed after 3 days and the cup left for another 3 days. Explain using particles.
Read the worked answer
The water level in Cup B would begin to drop, just as in Cup A. Once the lid is removed, the trapped water vapor can escape into the room. With vapor leaving, fewer particles return to the liquid, so the escape of particles from the liquid outweighs the return. Over the next 3 days the liquid level would decrease. The key idea is that evaporation depends on whether vapor can leave the area, not on whether the cup is "open" or "closed" in some magical sense.