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When solute concentrations differ (left), water flows through a membrane toward the solution with the higher concentration of solute. The resulting osmotic pressure causes the difference in water levels shown at right.
Video: Hypotonic vs. hypertonic:
A hypertonic solution (pronounced "HĪ-per-TAWN-ik") is a solution with a higher concentration of solute (dissolved substance) than some other, specified solution (and which therefore has a higher osmotic pressure than the other solution). The solution with the lower concentration is then termed hypotonic. A solution with the same concentration as some other solution is called isotonic (iso- means equal). In general, pure water will be hypotonic in comparison with any solution, because it contains no solute whatsoever.
In a medical setting, these comparisons are usually made with blood. Typically, the cells of the body have about the same osmotic pressure as blood. A hypotonic solution, which has a lower concentration of salt than blood, causes cells in contact with it to swell, or even burst. A solution with a higher salt concentration than blood will, when mixed with blood, cause blood cells to shrink. Isotonic solutions cause neither shrinkage nor swelling since they exert the same osmotic pressure as blood (7.2 atm, the same as a 0.9% sodium chloride solution). The figure at right (above) shows how water flows from a hypertonic solution toward a hypotonic one. Water always moves in the direction that will cause dilution of the solution that contains more solute (the hypertonic one)
Try this experiment: (1) Remove the hard outer shell of two raw chicken eggs by placing them in vinegar for three days (this process leaves the egg enclosed in only the flexible semi-permeable membrane ordinarily concealed within the shell); (2) place one egg in pure water (hypotonic solution), place the other in heavy syrup (hypertonic); wait a few hours and see what happens. Which egg gets smaller? Which gets bigger?