External pressure applied to a solution can counteract and even reverse osmosis; the pressure required to completely stop osmosis is known as the osmotic pressure of the solution.
Here's a more detailed breakdown:
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Osmosis Basics: Osmosis is the movement of solvent molecules (typically water) from an area of high solvent concentration (low solute concentration) to an area of low solvent concentration (high solute concentration) through a semi-permeable membrane.
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Pressure's Influence: Pressure can influence this movement in two main ways:
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Counteracting Osmosis: When external pressure is applied to the solution with the higher solute concentration, it opposes the osmotic flow of solvent into that solution. Think of it like pushing back against the water trying to enter.
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Osmotic Pressure: The specific amount of pressure needed to completely stop the net movement of solvent across the membrane is termed the osmotic pressure. It's a fundamental property of the solution and is directly related to the solute concentration. A higher solute concentration leads to a higher osmotic pressure.
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Reversing Osmosis: If the applied pressure exceeds the osmotic pressure, the solvent flow reverses. This is known as reverse osmosis, and it's used in various applications like water purification. Solvent is forced from the high concentration solution to the low concentration solution.
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Mathematical Relationship: Osmotic pressure (π) can be described by the van't Hoff equation:
π = iMRT
Where:
- π = Osmotic pressure
- i = van't Hoff factor (number of particles the solute dissociates into)
- M = Molar concentration of the solute
- R = Ideal gas constant
- T = Absolute temperature (in Kelvin)
This equation demonstrates that osmotic pressure is directly proportional to the solute concentration.
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Examples:
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Plant Cells: Plant cells rely on osmosis to maintain turgor pressure, which keeps them rigid. If a plant is placed in a hypertonic solution (high solute concentration), water will flow out of the cells, causing them to shrink (plasmolysis) and the plant to wilt. Applying pressure could theoretically counteract this.
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Reverse Osmosis Water Filters: These filters use high pressure to force water through a membrane, leaving behind dissolved salts and other impurities. The applied pressure overcomes the osmotic pressure, effectively separating pure water.
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In summary, pressure plays a crucial role in osmosis. It can be used to counteract, stop, or even reverse the flow of solvent across a semi-permeable membrane. The osmotic pressure is the exact pressure needed to halt the process.
