In chemistry, the osmole (Osm or osmol) is a non-SI unit of measurement that defines the number of moles of solute that contribute to the osmotic pressure of a solution. The term comes from the phenomenon of osmosis, and is typically used for osmotically active solutions. For example, a solution of 1 mol/L NaCl corresponds to an osmolarity of 2 osmol/L. The NaCl salt particle dissociates fully in water to become two separate particles: an Na⁺ ion and a Cl^- ion. Therefore, each mole of NaCl becomes two osmoles in solution, one mole of Na⁺ and one mole of Cl^-. Similarly, a solution of 1 mol/L CaCl₂, gives a solution of 3 osmol/L (Ca²⁺ and 2 Cl^-).