Water moves toward the solute
- The idea
- Predict
- See it
- Why
- Check
The idea
“Water flows to the side with less solute, where there's already more water.”
Solutes spread toward the side with less solute, so it's easy to picture water going that way too. Water actually moves toward the solute.
The short version
The mistake: Water flows to the side with less solute, where there's already more water.
What's actually true: Water moves from higher water potential to lower. With equal pressure, that's toward the side with more solute.
- Dissolving a solute lowers water potential, and more solute lowers it further. So 0.5 M sucrose has lower water potential than 0.1 M.
- Osmosis is water moving across a selectively permeable membrane from higher water potential to lower. With equal pressure, that's toward more solute.
- Water crosses both ways all the time. Osmosis is the net flow, the difference between the two.
- A cell gains water in a hypotonic solution and loses it in a hypertonic one. Solute added outside can draw water out, but it never pushes water in.
On the exam: Water moves toward lower water potential. With equal pressure, that's the side with more solute.