Osmosis and Osmotic Pressure

You have studied the principle of osmosis. You have possibly done some experiments with salt water as well. Haven’t you seen pickles being made at home? That is all the magic of osmosis. But, how does osmosis work? Do you know that osmosis is one major reason for the survival of many species? In this chapter, we will study all about osmosis and osmotic pressure.

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What is Osmotic Pressure?

Osmotic pressure is the pressure that we need to apply to stop the flow of solvent molecules from a dilute solution to a concentrated solution through a semi-permeable membrane. Now, the above statement has many terms that need a thorough explanation. We will look at them one by one.

Let us consider the semi-permeable membrane. It is a membrane that selectively allows the movement of particles through it. At some point or the other, we all have seen shrivelling of raw mango when it is pickled in salt water. Blood cells collapse when exposed to saline water. What causes these phenomena? Let us learn about it.

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Osmotic Pressure

What is Osmosis?

We have observed that when we separate a dilute solution from a concentrated solution of the same type by a membrane, the solvent molecules flow from the dilute solution to the concentrated solutions. This movement continues until it reaches an equilibrium. This process of flow of solvent molecules due to the difference in concentration is osmosis.

We can stop the flow of solvent molecules through the membrane by applying external pressure. As we discussed at the beginning, the pressure that just stops the flow of solvent particles is the osmotic pressure.


It is true that osmotic pressure is a colligative property as it only depends on the concentration of the solution. In other terms, osmotic pressure is the extra pressure that we need apply in order to prevent osmosis.  Experimental data shows that this pressure is directly proportional to the molarity (number of moles of solute per litre of solution) of the solution at a certain given temperature. Two solutions which have the same osmotic pressures at a given temperature are Isotonic solutions. Learn more about Different Types of Solutions here.


Mathematical Proof

π = CRT


  • π=osmotic pressure
  • C=molarity of the solution
  • R=Universal Gas constant:
  • T=Temperature

Molarity is the number of moles of solute per litre of the solution. Suppose w2 is the mass of solute having molar mass M2 in a solution of volume V, then we can write,


We use the above equation extensively in the determination of the molar mass of complex molecules of proteins, polymers and other macromolecules. Scientists use this preferably for bio-molecules as it shows a significant value even at room temperature.

Learn how to express the Concentration of Solution here.

A Solved Example for You

Q: Is osmosis a reason for cholera?

Ans: Osmosis allows for terrible things to happen, as well. Cholera would not be possible without osmosis. The choleric bacteria populate in our intestines and begin to reverse the intestinal cells’ ionic orientation. In other words, it changes the way ions and, subsequently, water transport in our intestines. It means that cholera performs a perfect coup.

First of all, when our ions’ orientations are switched, the intestinal cells are no longer able to absorb water into the body. Now osmosis happens in the other direction and water moves from our intestinal cells into our intestines. This is what causes cholera’s infamously deadly watery diarrhoea. Second, this compounds the rate at which you get dehydrated. Not only can you not absorb water, you are literally being drained dry. This is why cholera can kill you so quickly because it does not rely on how much water you consume.

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