What are isotonic and hypotonic solutions?

The two solutions which are having equivalent osmotic pressure are called isotonic solutions. The isotonic solutions at the same temperature also have same molar concentration. If we have solutions having different osmotic pressures then the solution having different osmotic pressures is said to be hypertonic solution and the solution having lower osmotic pressure is called hypotonic solution.

0.9% (mass/volume) aqueous solution of sodium chloride is called normal saline solution and is isotonic with the fluid inside the human red blood cells (RBC). Therefore, during intravenous injections, the medicines are mixed with saline water before the injection. This avoids the blood cells from shrinking or bursting. However, solutions having concentration grater or less than 0.9% (mass/volume) are not safe. For example, if the NaCl solution is less concentrated i.e. hypotonic w.r.t. human blood, its osmotic pressure will be low and finally burst. On the other hand, if the NaCl solution is more concentrated, i.e. hypertonic, its osmotic pressure is high and the water will move out of the cell causing the cell to shrink due to plasmolysis.

Some common phenomena and osmosis

Osmosis provides logical explanation to some common processes that we come across.

(i) Shrieving of raw mangoes to pickle.

Raw mangoes when place in concentrated solution of common salt lose water through osmosis and ultimately shrivel into pickle.

(ii) Revival of wilted flowers and limped carrots: flowers revive and regain their freshness when place in fresh water because of osmosis. Carrots get limed due to loss of water to atmosphere. However, when limped carrots are placed in water, they become firm due to inflow of water because of osmosis.

(iii) Swelling of tissues in people consuming more salt: people consuming more salt and excessively salty food suffer from edema which is swelling and puffiness produced because of retention of water in the tissue cells and intercellular space.

(iv) Preservation of meat: the preservation of meat against bacterial action is done by salting it. Similarly, fruits are preserved against bacteria by adding sugar. The bacteria on salted meat or on candid fruits loses water through osmosis shrivels and ultimately dies.

(v) Absorption of water by plants: plants absorb water from the soil through their roots due to osmosis because the cell sap in the root hair cells has higher osmotic pressure than that of soil water.

Osmotic pressure is a colligative property. For a given solvent the osmotic pressure relies only upon the molar concentration of solute but does not depend on its nature. Osmotic pressure is linked with the number of moles of the solute by the subsequent relation:

πV = nRT

Or, π = n/V (RT)

π = CRT

C = conc. In mol L-1

R = gas constant (bar LK-1 mol-1)

T = temperature (K); n = mol of solute

V = volume of soln. (L);

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