What is osmosis? Can you give an example?  

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ncchemist eNotes educator| Certified Educator

Osmosis is defined as the movement of solvent molecules through a semi-permeable membrane down a concentration gradient. The membrane is semi-permeable because it allows solvent molecules to pass through it but solute molecules (solids dissolved in the solvent) cannot pass through it. If you have two solutions of a solute in solvent (like salt or sugar dissolved in water) of differing concentrations on either side of the membrane, the water will naturally flow from the side where it is present in higher concentration to the side where it is present in lower concentration. In other words, osmosis will have the water flow in the direction through the membrane that will equalize the concentrations of the two different solutions over time. The natural osmotic flow of solvent from high to low concentration is called an osmotic gradient.

An example of osmosis is taking a tomato slice (or any vegetable) and immersing it in a saltwater solution. The tomato skin and pulp is the semi-permeable membrane that will allow water to pass through it. The saltwater solution surrounding the tomato will have a lower concentration of water than the relatively fresh water inside the tomato. As a result, the water will flow over time from the inside of the tomato to the saltwater solution to try to equalize the two. The removal of water from the tomato via the osmotic gradient will cause the tomato to lose mass and visibly shrivel up.

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