Tonicity

The terms hypotonic, isotonic, and hypertonic are used when comparing the osmotic pressure between different solutions separated by a semipermeable membrane. Often this comparison is between a solution, such as saline (salt dissolved in water) and a cell or bodily fluid.

Hypotonic

A hypotonic solution is a solution that has a comparatively lower concentration of dissolved molecules. It is therefore said to have low osmotic pressure and high water potential. Water will move from the hypotonic solution into the solution with more dissolved molecules.

Isotonic

An isotonic solution is a solution that has exactly the same concentration of dissolved molecules as another solution. It is therefore already in equilibrium and no water will move from one solution into the other across a semipermeable membrane.

Hypertonic

A hypertonic solution is a solution that has a comparatively higher concentration of dissolved molecules. It is therefore said to have high osmotic pressure and low water potential. Water will move into the hypertonic solution from an area with fewer dissolved molecules.