The Child's Ideas for 2. Water to Vapor
The Challenges of Learning about Evaporation and Condensation
The idea that water vapor exists in the air as an invisible odorless gas (distinct from steam) strains their imaginations.
Children have many experiences with evaporation and condensation in their everyday lives. The puddles that dot the street after a morning rainstorm may be gone by late afternoon. What happened to all that water? Dew appears on the grass each morning; little “beads of sweat” form on the outside of cans of soda on hot humid days; and the bathroom mirror fogs up after taking a shower. Where do the dew, sweat, and fog come from?
Explaining all these mysterious things, in ways that rigorously account for all the available evidence, is particularly challenging for children because they must use the existence of invisible entities—gases—which they do not clearly understand and may not even believe exist—to explain visible events. Further, because children know so little about gases and how they behave, it is hard for them to generate plausible mechanistic explanations.
Instead, children initially provide much shallower explanations, ones that posit a closer match between how things appear and how they are. So if the puddle disappears, then the water must be gone. Some may say that the water has “gone into the air” or even use the word “evaporated.” What they mean by that, however, isn't that the water has transformed into a water vapor gas, which is in the air, but somehow has become ethereal like the air. They no longer think it retains its identity as water.
Further evidence that they don't believe air contains a distinct water vapor gas is the much greater difficulty they have explaining condensation than evaporation. They go to great lengths to find visible sources for the dew, sweat, and fog—proposing that it rained last night or that water seeped through (or over) the glass. The idea that water vapor exists in the air as an invisible odorless gas (distinct from steam) strains their imaginations.
Clearly, explaining evaporation and condensation go hand-in-hand with each other as well as with developing an understanding of the material nature of gases and the particulate nature of matter. It is important that these ideas are all developed together, rather than treated in isolation, because they are mutually reinforcing. Without considering condensation as well as evaporation, students are not challenged to think of water vapor as an invisible gas, pre-existing in the air. Without considering the particulate model, including the idea of particles in motion, children have no mechanistic way to explain what is happening and no way to envision the invisible. Models help make the invisible more visible and real to students, facilitating using invisible entities in meaningful real-world explanations.
—Carol L. Smith
