Explaining how osmosis occurs

Osmosis is the movement of water molecules from a region of higher concentration to lower concentration… (not forgetting the partially permeable membrane) But what on earth does it means? 🙂

Isn’t water just another compound? Why is it so special that there is a term called osmosis to describe how it moves?

It is actually quite simple to understand and you will find out that water is not really that special after all!

1st thing to note:

Large quantities of water molecules constantly move across membranes by random movement!

For example, it has been estimated that an amount of water equivalent to roughly 250 times the volume of the cell diffuses across the red blood cell membrane every second; the cell doesn’t lose or gain water because equal amounts go in and out.

2nd thing to take note of

Water molecules can dissolve other substances.

dissolve_saltYou put a cube of salt into a cup of water and the cube of salt rapidly disappears!





The reason is because the water molecules attract the salt particles out of the cube…







and distribute the salt particles all around the liquid

That is why light can pass through the gaps and we cannot see our salt particles anymore!



3rd thing to take note of

Water molecules are locked with their solute particles because of the attraction towards the solute particles.


Take note of the cluster of water molecules that are binded to the solute! The other water molecules are free!




Take note of the water molecules that are also binded to the sodium ions!  Take note of there is a lot more FREE water molecules on the left than on the right





4th thing to take note:

You must have a partially permeable membrane to block solute particles from going over to the other side


Now finally,

we can go into the idea that it is the free water molecules moving randomly that causes this phenomenon called osmosis


Notice that the volume on both sides of the membrane is the same

There is a lot more FREE water molecules on the right

So there is a higher concentration of FREE water molecules on the right compared to the left

Therefore by random movement alone, more water molecules will find themselves on the left side!


Same thing for this picture!








Last but not least, see if you understand this picture below


You will definitely encounter pictures of some visking tubing etc having higher water levels on one side than the other…

the reason is because osmosis balances out the number of FREE water molecules on both sides. But one side has more solute particles and binded water molecules to them so the water level is higher!

Hope you have gained a better understanding of osmosis! 🙂




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