What is Osmotic Pressure?
It is the pressure that must be applied on the solution side to stop the movement of the fluid when a semipermeable membrane separates the two mixtures.

The phenomenon of osmosis is a colligative property of water. This means that osmosis depends on the concentration of molecules or ions in the solute, but does not depend on the nature of the solute. For example, other colligative properties of water would be the boiling point or the freezing point.

The equation to identify the osmotic pressure of a solution goes as follows:

  • M: amount of moles of solute per unit of solution or the molarity
  • R : the ideal gas constancy
  • T : the absolute temperature
  • Π : osmotic pressure





More explicitly, let us picture a container separated into two parts by a semipermeable membrane.


On the left (blue) is the concentrated solution, on the right (orange) the diluted solution and in the middle (yellow) the semipermeable membrane. In a natural way, due to the phenomenon of osmosis, the diluted solution will move towards the concentrated solution.


Now, by adding a piston in the top of the concentrated solution and applying a pressure forcing the dilute solution to stop crossing and thus restore the balance between the two solutions, this is called osmotic pressure.


When the pressure on the side of the concentrated solution is higher, the flow will be reversed, and the concentrated solution will be diluted to be on the side of the diluted solution. This is what is known as the principle of reverse osmosis.


So, this explains in a very simple way how reverse osmosis system’s work. Obviously, the actual operation of reverse osmosis is more complicated than that. If you want to learn more about this technology, here are a few articles on the subject of reverse osmosis.

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Precipitation Reactions
Precipitation is a chemical reaction that results in the formation of an insoluble product, the precipitate.