Osmosis is the movement of water molecules from an area of high concentration to an area of low concentration across a semi-permeable membrane. It goes on until both solutions have the same water concentration.
That's sure a mouthful and does not really tells us much about osmosis, so let's take our definition apart to understand it better:
-Osmosis is the movement of water. When other substances move because of a difference in concentrations the process is called diffusion so osmosis always means WATER is moving around.
-Osmosis happens from an area of high concentration to an area of low concentration of water. In all cases where osmosis is concerned, water acts as a solvent to other molecules. This means that the other molecules are dissolved in water. In any given volume of a solution then, there is LESS WATER because some water molecules get kicked out by the dissolved molecules. So, the fewer dissolved molecules that are present the more water there is.
-Keep in mind that the overall amount of water does not matter. What matters is the concentration difference or the amount of water in any given volume of solution. So, for example, if you have a human cell with 2X amount of water and you drop it in a bucket of a solution with X amount of water, the cell will have a higher water concentration, even though the bucket will have far more water than a single cell.
- If that was not complicated enough, often you will see solutions expressed in amount of SOLUTE per volume of water (for example, 10 mg of NaCl (table salt) in 1 L of water). You should always keep in mind that the higher the amount of solute in a solution, the lower amount of water.
- Osmosis occurs across a semi-permeable membrane. A semi-permeable membrane is a membrane that lets some things through (such as water) but stops the movement of others. This is important because without something to separate the two solutions they would mix and become one solution. However, if the membrane is impermeable (it lets nothing through) water will not be able to move across it so there will be no osmosis taking place.
So, let's wrap up what we have learned so far. Osmosis is the movement of water across a membrane that will let water through but will not let solutes through, from an area of high water (low solute) concentration to an area of low water (high solute) concentration.
When does osmosis stop? Osmosis will carry on until one of two possible outcomes happens. Ideally, osmosis will go on until both solutions have equal water concentrations at which point water will still diffuse across the membrane but it will go in both directions at the same rate, so there will be no changes in concentration of the solutions. However, that is not always possible, because water takes up space and in many real-life examples space can be limited. So it may happen that water will rush in until the water pressure breaks the membrane that separates the two solutions, which will cause the solutions to mix and become one with a constant water concentration.
Last, but certainly not least, there is "tonicity". Tonicity is a comparative statement of the water concentration between two solutions separated by a semi-permeable membrane. One cannot say "Solution A is hypertonic", but rather should say "Solution A is hypertonic to solution B". However, what does hypertonic, isotonic and hypotonic mean?
ISOTONIC solutions are the easiest to define. When solutions are isotonic they contain the same amount of water concentration and therefore there is no net movement of water from one to the other. Isotonicity is the end-product of osmosis!
A HYPERTONIC solution contains more water (less solutes) than another. So if solution A is hypertonic to solution B that means that solution A will have a higher water concentration. As we have seen before, that means that water will move from solution A to solution B.
A HYPOTONIC solution is the opposite of a hypertonic solution. It contains less water (more solutes) than another solution. So if solution C is hypotonic to solution D, water will move from solution D to solution C.
Finally, we can say this:
"Osmosis is the movement of water molecules from a hypertonic solution to a hypotonic solution across a semi-permeable membrane. It goes on until both solutions become isotonic."