Osmosis

Water, water, everywhere: ... not a drop to drink. You do know that it's dangerous to drink salt water, yes? Good. But do you know why?

Come along and you'll see! Two glasses. Water in both of them. We dissolve salt in one of the glasses. Now, we place a carrot in each glass, and wait a couple of days.

What do you think will happen? The carrot in the salt water has shrunk! Some of the water that was inside the carrot's cells moved out. It's as if the carrot tried to dilute the saltwater. At first, the water inside the carrot had a lower concentration of salt; and the water outside the carrot had a higher concentration of salt.

And as the carrot released its water, the concentration balanced out. The salt stayed where it was. And it was the water that moved, to even out the concentration. Hmmm. It seems water always tries to get to the more concentrated solution to dilute it?

This phenomenon is called osmosis, and is necessary for all - life - on earth. It's osmosis that enables plant roots to absorb water from the soil. There's osmosis going on in your body right now too: in your cells. Osmosis moves water in and out of the cells. It's driven by the difference in concentration of salts and minerals inside the cell, compared to outside.

To understand how osmosis really works, we need to look closely: Here's a beaker, divided in two sections, with a thin wall in between. This wall represents the outer wall of a cell -- the cell membrane. On one side we have water with a low concentration of salt. And high on the other side. We wait a bit, and notice that: the water level raises on the saltier side!

This has to do with that cell membrane. It's not just a wall, sitting there. That cell membrane has tiny holes in it like a super fine filter. The water molecules are small enough to get through. And they do.

Water molecules move around randomly and some of them get through, from one side to the other, in both directions. But look what happens with the salt! The salt splits up into sodium ions and choride ions. Since ions have electrical charge, they attract the water molecules, and collect a kind of shell around them. Now they are so bulky, they can't get through the membrane!

These large clumps move randomly, just like the water molecules. Sometimes the clumps block the way for water molecules which otherwise would have made it this way through the membrane. The fascinating thing is that from this side the water gets through despite there being a clump in the way. It's more likely that a water molecule gets from the low concentration side to the high concentration side, than the other way around. Osmosis occurs in a solution, which has different concentrations of a solute, on either side of a membrane, ...

which only lets the solvent through, but not the solute. Such a membrane, that only lets some things through, is called a semi-permeable membrane, "half-passable". The osmosis continues until the two solutions on either side of a semi-permeable membrane have the same concentration. That's why it's dangerous to drink sea water. When your cells contact water with an overly high concentration of salt, they let their own water out, until there is the same concentration of salt inside and outside!

So, if you drink seawater, your body will eventually dry out!