Osmosis

Osmosis is a natural process in which solvent molecules (typically water) pass through a semipermeable membrane (which allows some substances to cross but blocks others), moving toward the side where the dissolved substance is more concentrated. The result is a gradual equalization of concentration on both sides of the membrane.

Osmosis is a type of diffusion, but it involves solvent molecules only. It plays an important role in the life of cells: biological cells have a cell membrane through which water passes, so the amount of water inside the cell depends on the concentration of its surroundings.

Osmotic Pressure

Osmotic pressure is the pressure required to stop the flow of solvent across a membrane. It increases as the number of dissolved particles in the solution rises, and does not depend on the type of chemical substance itself, but only on the number of particles. It is therefore classified as one of the colligative properties of solutions.

Solution Comparison

When comparing two solutions: a solution with higher concentration is called hypertonic, one with lower concentration is hypotonic, and solutions with equal concentration are called isotonic. A cell placed in a hypotonic solution absorbs water and swells; in a hypertonic solution, water leaves the cell and it shrinks. In medical practice, fluids injected into the bloodstream are usually prepared to be isotonic with blood.

Technological Application

Reverse osmosis is a technology that uses external pressure greater than osmotic pressure, allowing water to pass through the membrane from the side of higher concentration and escape as pure water, leaving behind salt and contaminants. It is used for water purification and desalination of seawater.