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What exactly causes osmotic pressure?

  1. Jan 29, 2014 #1
    Hello everyone,

    If there is a semi permeable membrane that allows only water to pass through. Also this membrane separates the sides into A and B.

    There are big molecules like colloids in side A along with water. Side B has only water . What mechanism causes side A to have more osmotic pressure, and thus allow water to flow from side B to A.

    1) Is it because the particles are so big , that they physically reduce the interactions of water molecules in side A with the semi permeable membrane.

    2) Is it because the colloid create a chemical interaction with water molecules and prevent them from moving away.

    I just want to know if this osmotic pressure created by a pure physical block (like big guy covering small guy) or an actual chemical interaction (Eg big guy holding the small guy)

    3) Is osmotic and oncotic pressure mean same thing.

    Thanks a lot :smile:
  2. jcsd
  3. Jan 30, 2014 #2
    First of all, osmotic pressure is not the pressure applied by the liquid on one side on the semipermeable membrane. Rather, it is the pressure required to stop the process of osmosis applied against the side having more dilute conditions.

    So there is nothing like one of the sides is having higher osmotic pressure or something.

    Now for clearer understanding of osmotic pressure, visit this link: http://h2g2.com/edited_entry/A686766
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