Osmosis will occur if a vessel is separated into two compartments by a semipermeable membrane, both compartments are filled to the same level with a solvent, and solute is added to one side. The level of the liquid on the side containing the solute will rise as the solvent flows from the side of its higher concentration to the side of lower concentration. If an external pressure is exerted on the side containing the solute, the transfer of solvent can be stopped and even reversed (reverse osmosis). Two solutions separated by a semipermeable membrane are said to be isotonic if no osmosis occurs. If osmosis occurs, transfer of solvent is from the hypotonic solution to the hypertonic solution, which has the higher osmotic pressure.
The minimum pressure necessary to stop solvent transfer is called the osmotic pressure. Since the osmotic pressure is related to the concentration of solute particles, there is a mathematical relationship between osmotic pressure, freezing-point depression, and boiling-point elevation. Properties such as osmotic pressure, freezing point, and boiling point, which depend on the number of particles present rather than on their size or chemical nature, are called colligative properties. For dilute solutions the mathematical relationship between the osmotic pressure, temperature, and concentration of solute is much like the relation between pressure, temperature, and volume in an ideal gas (see gas laws). A number of theories explaining osmotic pressure by analogy to gases have been devised, but most have been discarded in favor of thermodynamic interpretations using such concepts as the entropy of dilution.