Unit – V A

Osmosis and Osmotic Pressure:

Semipermeable Membrane:

  • Semipermeable membrane is a membrane which allows the solvent molecules, but not the solute molecules through it.
    Examples: Cellulosde, cellulose nitrate, animal bladder.

Osmosis:

  • The spontaneous and unidirectional flow of solvent molecules through a semipermeable membrane, into a solution OR flow of solvent from a solution of low concentration to a solution of higher concentration through semipermeable membrane is called osmosis.

Everyday Examples of Osmosis:

  • Raw mangoes when placed in concentrated solution of common salt lose water through osmosis and ultimately shrivel into pickle.
  • Flowers revive and regain their freshness when placed in a fresh water because of osmosis.
  • Carrots get limed due to loss of water to atmosphere. But, when limped carrots are placed in water, they become firm due to inflow of water because of osmosis.
  • People consuming more salt and excessive salty food suffer from edema which is swelling and puffiness produced in the body due to retention of water in tissue cells and intracellular spaces.
  • The preservation of meat and fishes against bacteria is done by salting it. The bacteria on meat or fishes loses water through osmosis and ultimately die.
  • The preservation of fruits against bacteria is done by adding sugar to it. The bacteria on meat or fishes loses water through osmosis and ultimately die.
  • Plants absorb water from soil through roots due to osmosis because the root hair cells have higher osmotic pressure than that of soil water.
  • Red blood cells burst when kept in water due to endoosmosis..
  • Blooming, opening and closing of flowers is governed by osmosis.
  • Dead bodies swell under water due to endoosmosis.

Experiment Exhibiting Phenomenon of Osmosis OR Abbe Nollet Experiment:

Osmosis 01

  • A wide mouthed thistle funnel with a narrow long stem was taken. Then pig’s bladder (semipermeable membrane) is tied tightly around the wide mouth of the funnel with the help of a thread or rubber band. Now dilute sugar solution is carefully poured into the stem of the funnel to a certain level.
  • The wide mouth of the funnel containing sugar solution is now kept in a beaker with the help of an Iron stand. Now three fourths of the beaker is filled with pure water. The apparatus are left undisturbed for some time.
  • After few hours we find the level of sugar solution increases from its initial level. This indicates that there is net flow of solvent molecules into the solution through the semipermeable membrane.
  • We have to apply excess sufficient pressure from stem side on solution to stop this migration of solvent molecule and this excess pressure is called osmotic pressure.

Osmotic Pressure:

  • The excess of pressure on the side of solution that stops the net flow of solvent into solution through semipermeable membrane is called osmotic pressure.
  • The equilibrium is reached when hydrostatic pressure of the column is equal to that of osmotic pressure.
  • Osmotic pressure is not created by the solution but it comes into existence when the solution is separated from solvent by a semipermeable membrane.
  • If the pressure applied on the solution is greater than the osmotic pressure of the solution then solvent starts passing from solution to solvent. This phenomenon is called reverse osmosis. This process is used to for purification of sea water and hard water.

Osmosis 02

Types of Solutions on the Basis of Osmosis:

Isotonic Solutions:

  • Two or more solutions having same osmotic pressure at a given temperature are called isotonic solutions.
  • When such solutions are separated by semipermeable membrane no osmosis occurs between them.
  • For example, the osmotic pressure associated with the fluid inside the blood cell is equivalent to that of 0.9% (mass/ volume) sodium chloride solution, called normal saline solution and it is safe to inject intravenously.

Hypertonic Solution:

  • A solution having osmatic pressure higher than that of another solution is called as hypertonic solution.
  • For example, the osmotic pressure associated with the fluid inside the blood cell is less than sodium chloride solution having concentration more than 0.9% (mass/volume). Thus solution of sodium chloride is hypertonic. In this case water will flow out of the cells and cells would shrink.

Hypotonic Solution:

  • A solution having osmatic pressure lower than that of another solution is called as hypotonic solution.
  • For example, the osmotic pressure associated with the fluid inside the blood cell is more than sodium chloride solution having concentration less than 0.9% (mass/volume). Thus solution of sodium chloride is hypotonic. In this case water will flow into the cells and cells would swell.

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