2.Osmosis and osmotic pressure:-

Osmosis may be defined as “The process of passage of solvent from solution of lower concentration into the solution of higher concentration through semi permeable membrane is called osmosis”.

The process of osmosis is different from conversional process of Diffusion in the following ways:-

Osmosis	Diffusion
In osmosis the movement of molecules takes place through semi permeable membrane.	In diffusion there is no role of semi permeable membrane.
It involvers only movement of the solvent particles from one side to another.	It involves the passage of solvent as well as solute molecules from one region to another
Osmosis is limited to solutions only	Diffusion can take place in liquids, gases and solutions.
Osmosis can be stopped or reversed by applying additional pressure on the solution side.	Diffusion can neither be stopped nor reversed.

The flows of solvent particles from lower concentration (or pure solvent) into the solution through semi permeable membrane can be prevented by applying an external pressure over the solution side as shown in figure1. “This minimum external pressure which must be applied  to solution side in order to just prevent osmosis is called OSMOTIC PRESSURE.”

However if more external pressure (greater than osmotic pressure) is applied on the solution side, the solvent will flow from the solution (high conc.) into solvent (low conc.)through SPM.

This process is called reverse osmosis. It is widely used in de-salination of water and makes water useful for drinking . (R.O. Technique)

Derivation of expression for osmotic pressure:-

Van’t Hoff on the basis of his studies concluded that Os

π α C

                α T

                ∴ π α C X;;; T

                Or           π = R X C X T

Where R is a constant called “Solution constant” and its value is found to be same as “Gas Constant” (8.314 JK^-1 mole ^-1 or 0.0821 L atm K^-1 mole^-1 or 0.0831 L bar K^-1 mole^-1)

                Or           π = CRT

But   C = n/V       where n = No. of moles &            V= volume of solution

               

or            π V = nRT

This equation is called Van’t Hoff equation.

From above at a particular temperature the quantity RT is a constant, hence π α C. Since osmotic pressure depends upon the molar conc. (no of moles of solute) of solution therefore it is a Colligative property.

Determination of molar mass from osmotic pressure:-

According to Van’t Hoff equation:

Isotonic, hypertonic and hypotonic solution:-

The solutions of different compounds which have same osmotic pressure at a temperature are called isotonic solutions. The solutions having equimolar concentrations are Isotonic solutions. If a solution has more osmotic pressure than some other solution, it is called hypertonic solution. On the other hand, a solution having less osmotic pressure than the other is called hypotonic solution. The hypertonic solution will be more concentrated than other solution and hypotonic solution will be less conc. than other solution.

Experimental demonstration of Osmosis:-

The two eggs are deshelled by placing them in dilute hydrochloric acid. One of these is put in water and another in a conc. NaCl solution (brine). The egg placed in water will swell because, due to osmosis water flows from low conc. (water) to high conc. (egg) through SPM (cell walls of egg). This process, in which membrane permits the solvent particles to enter inside, is called Endosmosis. The egg placed in NaCl solution will shrink because , due to osmosis water flows from low conc. (egg) to high conc.(NaCl solution) through SPM (cell wall of egg). This process, in which membrane permits the solvent particles to come out, is called Exosmosis.

Biological importance of Osmosis (Applications of osmosis):-

Osmosis is (biologically) important in following ways:

1. Plants absorb water from the soil through their roots due to osmosis.
2. In animals water moves into different parts of the body under the influence of osmosis.
3. The stretching of leaves, flowers etc. is controlled by osmosis.
4. Osmosis helps in rapid growth of the plants and germination of seeds.
5. The solutions to be injected in veins are isotonic with blood (blood is isotonic with 0.91% NaCl solution). If solution if high conc. is used the blood cells will shrink and ultimately collapse. It is called plasmolysis. However if solution of low conc. is used the blood cells will swell and ultimately burst. It is called hemolysis.