With regard to osmosis, distilled water will always be hypotonic compared to an aqueous solution containing any amount of a solute. Because distilled water is pure and contains no dissolved substances, an aqueous solution with any concentration of solute will be hypertonic when compared to distilled water. Osmosis is a process based on the concentration of solute contained in two aqueous solutions on either side of a semipermeable membrane, and is not dependent on the dissolved substance.
When aqueous solutions are separated by a semipermeable membrane, the solution containing the lesser concentration of solute, the hypotonic solution, will pass over to the side containing the greater concentration of solute, which is the hypertonic solution. Only the water will pass through the membrane, leaving the solute behind.
Osmotic pressure refers to the diffusion of water across a semipermeable membrane, and is the result of two aqueous solutions striving toward a state of equilibrium. When this state is reached, the osmotic pressure is the same on both sides. This equalized degree of osmotic pressure is called the hydrostatic, or "water-stopping," pressure.
Because plant cells contain enzymes, salts and proteins dissolved in an aqueous solution, a simple school lab experiment can be used to demonstrate osmotic pressure. Placing a stalk of celery in a beaker of distilled water will cause the hypotonic water in the beaker to flow across the celery stalk's cell membranes to equalize the pressure difference. The celery stalk will become rigid as its cells fill with distilled water. Placing another celery stalk in a beaker containing a salt solution will cause the reverse effect. The water leaving the hypotonic solution within the plant cells to cross over to the hypertonic salt solution in the beaker will cause the celery stalk to shrivel and become limp.