Chapter 13 · Solutions
13.10Osmosis: Why Drinking Salt Water Causes Dehydration
10 min · two checks
Predict
What crosses a semipermeable membrane during osmosis, and in which direction?
The idea
- Describe osmosis and osmotic pressure.
- Explain why seawater worsens dehydration.
A semipermeable membrane lets solvent through and blocks solute. Osmosis is the net flow of solvent toward the more concentrated solution. The driving idea is the same as other colligative effects: the side with more solute has a lower tendency for solvent to escape, so solvent arrives faster than it leaves. Osmotic pressure π = iMRT, with R in a pressure unit, is the external pressure that would stop the flow. More particles, higher osmotic pressure.
Cells are bags of solution behind membranes. If the outside fluid is too concentrated (hypertonic), water leaves the cell and the cell shrinks. If the outside is too dilute (hypotonic), water enters and the cell can burst. Seawater is saltier than your body fluids. Drinking it pulls water into the intestine from the blood rather than the other way around, which is the opposite of what a thirsty person needs. Isotonic fluids match the body’s particle concentration.
Keep these
- Osmosis: solvent flows toward higher solute concentration.
- π = iMRT.
- Hypertonic surroundings shrink cells. Seawater is hypertonic to body fluids.