Osmosis is a process which occurs when there is a difference in solute concentration between two solutions separated by a semipermeable membrane. A semipermeable membrane allows passage of water but not of solutes. The passage of water occurs because of a pressure difference of water between the two solutions. The pressure needed to to stop the flow of water from a solution with low osmolarity to a solution with higher osmolarity is in fact called osmotic pressure, and is calculated with the van’t Hoff equation. The concentration of osmotically active particles in a solution is called osmolarity. When two solutions have the same osmolarity they are called isosmotic, otherwise one is defined hyposmotic and the other hyperosmotic. When two solutions have the same osmotic pressure they are called isotonic. If one solution has higher osmotic pressure than the other, the first is called hypertonic and the second hypotonic.