What’s Osmosis, Anyway?
Osmosis is the passage or diffusion of water or other solvents through a semipermeable membrane that blocks the passage of dissolved solutes [source: Encyclopedia Britannica].
What, you don’t get it? No fear. Most of us don’t, which is why there are countless explanations and analogies to clarify osmosis. We’ll explore a few of those, but first let’s break osmosis down to its parts to get a grasp on it.
First, we’ll make our solution. We start with a boring old cup of water. To spice things up, we’ll call water the “solvent” — which is convenient, because that’s what it is. To make our solvent a little tastier, we’ll dissolve in some delicious sugar. The sugar is the solute. Just to keep track, we now have water (solvent) that we’ve dissolved sugar (solute) in, to make sugar water (our solution).
Now that we have our solution of sugar water, we’ll grab a U-tube. This is not an internet video of kittens and monkeys hugging; a U-tube is a beaker, shaped in a u-shape. Right in the middle of the tube, imagine a bit of Gore-tex that cuts the U in half. Gore-tex is our “semipermeable membrane.” Gore-tex is a thin plastic, dotted with a billion tiny little holes that allow water vapor to pass through, but liquid to stay out. (Saran wrap wouldn’t let anything through, and a piece of cotton fabric would let just about anything.)
In one arm of the U-tube, we pour our sugar water mixture. On another we pour our plain old water. That’s when the magic of osmosis begins, if you find the movement of water magical. The level of liquid in the sugar water arm will slowly rise, as the solvent (water) moves through the Gore-tex, to make both sides of the arm more equal in a sugar-to-water ratio.
But why does that happen? Simply put, because water wants to find equilibrium. And because the one side of the arm is crowded with sugar, pure water from the other side decides to move on over to make the concentration more equal or until the osmotic pressure (the pressure that happens as the molecules move) is reached.
This is what a typical “under the sink” RO system looks like.