Osmosis is a process by which a solvent (the liquid that dissolves another substance) in solution passes through a barrier. The solvent may pass through the barrier, but the solute (the substance dissolved in the solvent) either does not go through it, or passes through much more slowly than the solvent. The solvent will pass through the barrier until the concentration of solvent is the same on both sides of the barrier. The barrier is a membrane that is either permeable, allowing solvent and solute molecules to pass through, or semipermeable, allowing only solvent molecules to pass through. The pressure of the water passing through the membrane is called osmotic pressure.

The process was first investigated by a French physicist, Abbé Jean Antoine Nollette (1700-1770), in 1748. Nollette covered a glass tube containing sugar water with a piece of paper. He placed the tube, paper end down, into...