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Osmosis-Diffusion of Solvent or Caused by Diffusion of Solutes?

Physics and Chemistry of Water

  1. Karl Erik Zachariassen

Published Online: 15 JUL 2005

DOI: 10.1002/047147844X.pc1503

Water Encyclopedia

Water Encyclopedia

How to Cite

Zachariassen, K. E. 2005. Osmosis-Diffusion of Solvent or Caused by Diffusion of Solutes?. Water Encyclopedia. 4:520–521.

Author Information

  1. Norwegian University of Science and Technology, Trondheim, Norway

Publication History

  1. Published Online: 15 JUL 2005


The prevailing theory describes osmosis as a diffusion of water down a water concentration gradient created by different concentrations of solutes. However, this description is at odds with several features of osmotic phenomena, and it does not explain why osmotic pressures are related to solute concentrations as described by the universal law of gases. An alternative theory, which appears to explain the observed features of osmosis, was published by George Hulett in 1903. This theory ascribes osmosis to a negative pressure created by the diffusive movement of the solute molecules. Like molecules in a gas, the solute molecules create an expansive pressure when they collide with the boundary wall. However, water cannot expand, but the expansive pressure of the solutes creates a negative solvent pressure, which sucks solvent water into the solution from the outside if the boundary wall has sufficient water permeability, i.e., is like a semipermeable membrane.

The osmotic pressure varies precisely proportionally with melting point, vapor pressure, boiling point, and osmolality, and these features are, therefore, referred to as the colligative properties of a solution.


  • colligative properties;
  • Hulett;
  • osmolality;
  • osmolarity;
  • osmosis;
  • osmotic pressure