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Osmosis is the diffusion of water through a semi-permeable membrane, moving from a dilute solution to a more concentrated solution.

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Background Notes Osmosis is the diffusion of water through a semi-permeable membrane, moving from a dilute solution to a more concentrated solution. A semi-permeable membrane can also be known as being selectively permeable, which means it only allows some particles to pass through it. The membrane contains microscopic pores and when placed in a solution it acts as a gateway for the particles. The pores (dependant on their size) allow the smaller individual molecules in the solution to flow through, whilst preventing the larger particles. Water molecules are small molecules, and their movement is due to the process osmosis. This is when water molecules move from a region where they are at a higher concentration to a region where they are at a lower concentration. Osmosis only regards water molecules and their movement. The diagram above shows the movement of the water molecules from the dilute solution, across the membrane into the more concentrated solution. We can see the osmotic action has taken place from the change in the water level, the dilute solution level falls, whilst the more concentrated solution level rises, due to the extra water molecules increasing the volume. However when a substance is dissolved in water to create a solution, for example a sugar solution, the water molecules are attracted to the sugar particles. Therefore they are no longer free to move and this also reduces the water concentration of the solution. Water molecules Sugar molecule The more concentrated a solution is (e.g. the more sugar particles it contains), the fewer water molecules there will be. This means the water concentration will be low, which makes the water molecules diffuse more rapidly from the dilute solution through the semi-permeable membrane. Sugar molecules Water molecules DILUTE SUGAR SOLUTION CONCENTRATED SUGAR SOLUTION Fewer water molecules flow in this direction More water molecules flow in this direction Semi-permeable membrane This diagram represents a dilute sugar solution and a concentrated sugar solution, both separated by a semi-permeable membrane. ...read more.


* Using a cork borer cut out some long thin strips of potato, carefully pushing them out of the borer using the mounted needle making sure you are leaning on the white tile. * When an adequate amount of pieces have been cut, measure them all equally using a ruler to 4cm. Cut seven pieces to the correct size using the scalpel, making sure no potato skin is left on the pieces themselves. * When all of the pieces are close to identical, they must be weighed and the results recorded. Place a weighing boat onto the top-pan balance and then zero the scales before placing each separate piece into the weighing boat and reading its weight in grams. * Before starting the experiment, the temperature of each solution can also be measured to check that they are all at more or less equal temperatures. * After unscrewing the lids of each tube, using the forceps each potato piece must be put into a solution (making sure you have noted which piece is in which concentration of solution for the final results table). * As quickly as possible the stopwatch then needs to be started, once all of the potato pieces are in a solution. Remember to re-screw on all of the lids, and then leave the tubes undisturbed for 15 minutes. * When the time is up, you must remove the pieces as best as you can straight away, using the forceps, being as efficient as possible. * They can then be placed on the white tile once again, after shaking of any excess solution. * Once again, each piece needs to be individually weighed using the top-pan balance. The new weights must then be recorded of each piece alongside the original weight ensuring you have them against their previous reading for comparing them correctly. * After all the results are recorded, you need to clear away any apparatus and equipment used and tidy up the area generally. ...read more.


After completing my pilot experiment, I have been able to choose the size of my potato pieces. They need to be a good size for the efficiency of osmosis taking place in the experiment, not too large or too small. In my pilot experiment I used 4cm long pieces of potato and came to realise they were too long, as the solution did not fully cover the pieces as well as I would have liked. Therefore I will alter their length slightly to 3cm in length when I come to do the real thing. The last main decision I need to make is how long I am going to leave the potato pieces in solution for. In my pilot experiment I left the pieces in solution for only 10 minutes, which at the time I thought was not long enough. My results in my pilot experiment were good, but to ensure osmosis has entirely taken place I would have liked to leave them a bit longer. So in my final experiment I intend to leave them in for a total of 20 minutes. Outline Plan The experiment that I intend to carry out will be investigating osmosis in potatoes. I plan to use a variety of different concentrates of a particular solution, as osmosis is the diffusion of water particles from a high to a low concentration, so a solution is necessary to the investigation. I will then place equal pieces of potato into each of the different concentrations (one of them being pure water), which will have been poured into specimen tubes. They will then remain in the solutions for a decided amount of time and I will observe their changes in weight after the experiment, comparing them to the weights of the potato pieces before I started the experiment. This quite simplistic experiment should prove decent results, which I can hopefully obtain a satisfactory conclusion from, confirming that the process osmosis took place. Although there are areas in the experiment that need to be looked over and decisions must be finalised to ensure its reliability. ...read more.

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