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Osmosis

• Osmosis is the movement of a solvent of a solution, from a low concentration area of the solute (i.e. location of high solvent concentration) to a loacation of high solute concentration (low solvent concentration), through a membrane pervious and selective to the solvent involved, until the both concentrations become equal.
  
  
• This effect can be demonstrated in a 'U' tube with two volumes separated by a membrane pervious only to the solvent involved, resulting in an increase in the liquid level in the arm of the initial lower solvent concentration (see diagram).
  
  
• To many people this seems a mysterious and magical phenomenon.
  
  
• I like to use the analogy of a room full of blind folded people that are allowed to roam at random.
  
  If without their knowledge a sliding partition is opened to provide excess to an adjoining room, with time the odds of people moving in to and occupying the adjoining room would be high, until equal numbers occupy each room and the crossover into rooms would be equal.
  
  That is, the more concentrated room, full of people, will see a larger number leaving that room for the room of lower concentration. The room with the lower concentration will see a less people moving into the more concentrated room, This will continue, with changing dynamics, until the same number will occupy each room and will see the same number leaving and entering each room.
  
  That is until a dynamic equilibrium is reached.
  
  It is just a matter of chance, odds and sheer weight of numbers.
  
  
• If the movement of people from one room to the other, in either direction, were to be selective by some form of criteria, such as size for example, you would have an analogy for the membrane used in osmosis.
  
  
• Molecular size can in fact be a criteria for osmosis, as can electrical charge by selecting against charged molecules in favour of neutrally charged molecules.
  
  
• Note: the above analogy can also apply to dynamic situations such as -
  
  
  • solutes or dye dispersion in solutions with time
    
    
  • water movement in plants; where a high concentration of water molecules within a leaf, relative to the surrounding air, sees water molecules move out through the leafs' pores. This creates a water concentration gradient between the soil and plant, with a similar dynamics causing water uptake by the plant's roots.