Osmotic Pressure
Osmosis and Diffusion
Diffusion is the spontaneous process of movement of both solute and solvent from their higher concentration to lower concentration.
Osmosis is the spontaneous process of movement of solvent only, from its higher concentration to lower concentration through semi-permeable membrane.
Demonstration of Osmosis
When grapes placed in concentrated solution of NaCl, the grapes start shrink due to exosmosis. This is because the concentration of H2O (solvent) is high in grapes sap than in NaCl solution. Hence, movement of water takes place from grapes sap to NaCl solution and grapes start shrink.
Similarly, when kismis placed in pure water, the kismish start swell up due to endosmosis. This is because the concentration of H2O (solvent) is low in kismish sap. Hence, movement of water takes place from pure water to kismish sap and kismish start swell up.
Semi-permeable Membrane
It can be defined as the membrane which is selectively permeable to substances and allow solvent molecules to pass through it but do not allow solute molecule. It is of two types :
Natural semi-permeable membranes: They are vegetable membranes or animal membranes which are found in outer skin of plants and animals.
Artificial semi-permeable membranes: They are synthesized artificially. Common examples are: parchment membrane, cellophane membrane, copper ferrocyanide etc.
Osmotic Pressure
It can be defined as the additional pressure applied on the solution to prevent the entry of the solvent into the solution through semi-permeable membrane. It is represented by ‘π’.
Reverse Osmosis
If a pressure higher than the osmotic pressure is applied on the solution, the solvent will flow from the solution into the pure solvent through the semi-permeable membrane. The process is called reverse osmosis (RO).
Expression for Osmotic Pressure
Osmotic pressure (π) of a solution is found to be directly proportional to molar concentration (C) of the solution and its temperature (T).
π ∝ C —(i) π ∝ T —(ii)
Compare the equation (i) and (ii)
π ∝ CT or π = RCT
Where ‘R’ is a constant and its value is found to be same as that of the ‘Gas constant’. ‘C’ is molar concentration (n/V). ‘n’ is number of moles of solute and ‘V’ is the volume of solution in litre. The above equation is usually written as :
\( \pi = \frac{n}{V}.RT~~~or~~~\pi V=nRT \)
Determination of Molecular mass from Osmotic Pressure :
\( \left [ n=\frac{W_{B}}{M _{B}} \right ] ~~~~~~~ \pi V = \frac{W_{B}}{M _{B}}.RT \)
or
\( M _{B}=\frac{W_{B}\times RT}{\pi~ \times V} \)
Hypertonic, Hypotonic and Isotonic solutions
- The solution which has high osmotic pressure with respect to another solution is called hypertonic solution. It contains relatively large quantity of solute.
- The solution which has low osmotic pressure with respect to another solution is called hypotonic solution. It contains relatively small quantity of solute.
- The solution which has same osmotic pressure with respect to another solution is called isotonic solution. It has equimolar concentration of solute.
By study of osmosis, it helps us to conclude the following :
In case of RBCs
- When RBCs are placed in hypotonic solution, having salt concentration less than 0.9 %, they will start swell up due to endosmosis.
- When RBCs are placed in hypertonic solution, having salt concentration more than 0.9 %, they will start shrink due to exosmosis.
- When RBCs are placed in isotonic solution, having salt concentration 0.9 %, the cell never swell and nor shrink because no osmosis takes place.
In case of Plant Cell
- When water enters into the cell, it starts swell up due to endosmosis, the cell wall of the cell then exert pressure to oppose the flow of water into the cell, this exerted pressure is called turgor pressure and the cell is celled turgid cell.
- When water going out the cell, it starts shrink due to exosmosis, the protoplasm of the cell attains small spherical shape, this phenomenon is called plasmolysis or crenation and the cell is called plasmolysed cell.
Biological Importance of Osmosis
- Plants absorb water from the soil through their roots due to osmosis.
- In animals, water moves into different parts of the body due to osmosis.
- Bursting of RBCs when placed in water is also due to osmosis.
- Opening and closing of stomata of plant leaves is also due to osmosis.
- Raw mangoes shrink to pickle when placed in common salt is due to osmosis.
- Wilted flowers revive when placed in fresh water is due to osmosis.
Explanation of Some Phenomenon on the Basis of Osmosis :
- People taking a lot of salt in food develop swelling of their tissues, a disease is called edema. This is due to retention of water in the tissue cells because of osmosis.
- Raw mangoes shrink into pickle when placed in common salt solution. This is due to outflow of water from the cells of mangoes because of osmosis.
- Wilted flower revives again when placed in fresh water. This is due to flow of water into the flower cells because of osmosis.
Remember that osmosis takes place in the direction of :
- High solvent concentration to Low solvent concentration
- Low solute concentration to high solute concentration
- Less concentrated solution to More concentrated solution
- Low osmotic pressure to high osmotic pressure
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