You must be familiar with the term diffusion. It refers to the process in which molecules intermingle due to their kinetic energy of random motion. For example; two containers of gas A and B are separated by a partition.
Assume that the molecules of both gases are in constant motion and continuously colloid with the partition. If we remove the partition, the gases will mix due to their random velocities and form a uniform mixture of A and B molecules in the container.
The tendency of molecule to be diffused depends upon the thermal energy of molecules.Let’s discuss about another term related to diffusion that is known as Osmosis. Osmosis definition states that it is a movement of particles from high concentration to low concentration through a semi-permeable membrane.If we separate two solutions of different concentration by a semi-permeable membrane then the solvent molecules will tend to diffuse across the membrane from the less concentrated to the more concentrated solution.
Remember the semi-permeable membrane is permeable for the smaller solvent molecules but cannot allow larger solute molecules.This process is much more common in biological processes where water acts as solvent. As the transport of water and other molecules through the biological membranes is essential to many processes in living organisms.
This selective diffusion process is driven by the internal energy of the solvent molecules which can express in terms of osmotic pressure. It can consider as the amount of hydrostatic pressure which is necessary to prevent fluid transfer by osmosis. Those solutions which have same osmotic pressure are known as isotonic solution and show no transport of solvent molecules. On the other hand hypotonic and hypertonic solution shows diffusion of solvent molecules from higher concentration to low concentration. A hypotonic solution has low concentration of solute compare to surrounding while a hypertonic solution has high concentration of solute than surrounding.
The semi-permeable membrane can allow the passage of water, but it is impermeable for ions like Na+, Ca2+, Cl- or larger molecules such as glucose, urea, and bacteria.It’s a thermodynamically favourable process and will continue until the equilibrium state. But we can reverse this process by applying sufficient pressure to the membrane from the 'concentrated' side of the membrane. This process is called as Reverse Osmosis.
In a reverse osmosis System, water is moved across the membrane from lower concentration to higher concentration. For example; if we take a semi-permeable membrane with fresh water on one side and a concentrated aqueous solution on the other side. According to normal process; the fresh water will move from dilute to the concentrated solution. While in reverse process, pressure is exerted on the side with the concentrated solution and water molecules move toward the fresh water side