When chemicals are dissolved in water they interact with the water molecules and change the physical properties of the resulting solution. Dissolving polyethylene glycol (antifreeze for your car) in water decreases the solution's freezing temperature, raises its boiling temperature, changes the weight of a liter of the solution and increases the osmotic pressure of the solution. All of these things happen because the solute molecules (polyethylene glycol in this case) interfere with the way the water molecules interact with each other and lower the water concentration (in milliliters per liter) of the resulting solution. The last statement is easily proved by dissolving some sugar in exactly one liter of water. The volume of the final solution will be more than one liter and, thus, the number of water molecules in a liter of the solution will be lower than that in a liter of pure water. (NOTE: solutes really change the "activity" or "water potential" of the water to which they are added, but it's a bit easier to think of them as lowering the water concentration).

Molecule for molecule, chemicals which do not dissociate into ions (sugars, for example) change the solution's physical properties less than those which ionize (e.g.,table salt, NaCl, which dissolves into Na+ and Cl- ions). All molecules have a molecular weight which is determined by their composite atoms. Amedeo Avogadro (1776-1856) determined that the molecular weight of a given gas expressed in grams, that is, its gram molecular weight, contains 6.022 x 10^23 molecules, no matter which gas is considered; this relationship was later found to hold true for all molecules. Thus, if one gram molecular weight (abbreviated GMW) of any nondissociating solute, 180 gm of glucose, for example, is dissolved in a liter of water, the effects on the solution's physical properties will be the same, regardless of which nondissociating molecule is used. One GMW of such a solute dissolved in one liter of pure water is called a one molal solution and it has an osmotic pressure of one osmole or 1000 milliosmoles. If less than one GMW of a solute is dissolved in the liter of water, the osmotic pressure of the resulting solution will be lower than 1000 milliosmoles (abbreviated mOsm) and if more solute is used, the osmotic pressure will be greater than 1000 mOsm.

Human blood plasma has an osmotic pressure of about 290 mOsm and water from the open ocean has an osmotic pressure of about 1010 mOsm. This indicates that there is much more salt in sea water than in human blood plasma and also that there is less pure water in a liter of sea water than in a liter of plasma. Just as ions diffuse from areas of high concentration in a solution to areas of low concentration, so do water molecules diffuse from areas of high water concentration (= low osmotic pressure) to areas of low water concentration (= high osmotic pressure). In considering this it is well to remember that salt added to water lowers the water concentration of the resulting solution. When water diffuses through membranes such as those around cells (called semipermeable membranes because they are nearly impermeable to ions but very permeable to water), this diffusion of water is called osmosis. Osmosis is the reason that the skin on our hands wrinkles into "prune fingers" when we stay too long in the bath or shower. Water diffuses from the higher water concentration (but low salt concentration) of the fresh water in the bath into the lower water concentration (but higher salt concentration) of our skin, causing it to swell and be thrown into wrinkles. The exact opposite happens when we immerse ourselves in sea water at the beach and our skin, particularly on our hands and faces, shrinks by osmosis and feels tight.

In summary, an organism with an internal osmotic pressure of 500 mOsm will swell by osmosis when put in fresh water (about 20 mOsm). This is why live starfish brought home from the ocean will always die when put in fresh water in the bathtub at home. A creature with an internal osmotic pressure of 500 mOsm will also shrink by osmosis when put in sea water (1000 mOsm). Estuaries are constantly being flushed with water of high osmotic pressure by the incoming tide and, on the outgoing tide, they are flushed with water of low osmotic pressure from the rivers which flow into them. The constantly changing osmotic pressure of estuarine waters makes them difficult places to live because resident plants and animals are constantly swelling and shrinking. As in other ecosystems, this stress means that fewer species of organisms can thrive in estuaries. However, those few species which can make a living there are very abundant and the estuary as a whole, although less diverse in species, is a very productive place.