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Basic Processes |  |
| Issue | Naive Idea* | Scientific Idea |
| Concentration | Students often base their predictions regarding the overall direction of osmosis on the concentration of the solute (e.g., 70% sugar, 20% sugar). | The concentration of water is the factor that determines the overall direction of osmosis, and H2O concentrations are approximately inversely proportional to solute concentrations (e.g., 30% H2O, 80% H2O). |
| Movement | Common solute molecules such as salt and sugar will diffuse through a semipermeable membrane. | Only water and other small, neutral molecules (such as oxygen and carbon dioxide)Ęcan diffuse across a semipermeable membrane; sugar molecules and salt ions are too large and carry too much charge. |
| Constraints | Osmosis will continue until equilibrium is reached, with equal concentrations of solute on both sides of the semi-permeable membrane. | Osmosis often fails to reach equilibrium levels because of other factors, such as pressure inside a plant cell or atmospheric pressure on a column of water. |
| Rate | Students often miss questions about rate of reaction, presumably because this is rarely emphasized in texts and their mental model of molecular movement is not very robust. | The higher the temperature, the faster the rate of diffusion, all other things being equal. |
* A misconception or alternative idea has three primary features: it is a cognitive idea that differs in a significant way from the scientific idea, it is held by a sizable proportion of the population, and it is notably resistant to being taught away; it is often described as a conceptual primitive (Clement, 1982). Some investigators believe there are clear misconceptions or alternative ideas associated with osmosis, but we feel less certain about this.
There are many other types of errors in understanding besides misconceptions (Fisher & Lipson, 1986). The errors common to understanding osmosis seem to be largely a function of the dynamic and abstract nature of the phenomenon. Generally speaking, they are more readily taught away than is typical of alternative conceptions, although acquiring deep understanding seems to require significant opportunities for practice with problems of gradually increasing difficulty.
Clement, J. (1982). Students' preconceptions in introductory mechanics.
American Journal of Physics, 50 (1), 66 - 71.
Fisher, K. M. & Lipson, J. I. (1986). Twenty questions about student errors.
Journal of Research in Science Teaching, 23 (9), 783-803.
