People can find understanding osmosis a little tricky – but this is because all the textbooks do a horrible job of explaining it! Let’s see if we can do a better job….
Osmosis is the diffusion of water through a semi-permeable membrane, like Osy’s cell membrane.
When you add a solute, (e.g., table salt) to water, some of the water molecules attach to the solute, so you end up with two kinds of water – “solute-bound water” and “free water”.
Only the free water can diffuse through the membrane.
Because diffusion is the movement of molecules from an area of high concentration to an area of low concentration, the concentration of free water determines which direction osmosis occurs. For example, when you add bits (solutes) to Osy, some of the free water molecules attach to the bits, and this reduces the concentration of free water inside Osy – so, free water diffuses into Osy and she swells up!
When you remove the bits from Osy, the water molecules are released from the bits, so the concentration of free water inside Osy increases – so free water diffuses out of Osy and she shrinks.
OK, that’s the science-y bit, now go and have some fun with Osy!
We are developing materials to help teachers understand how Osy works without needing to play through the game. If you have some ideas on what you would like to see, please let us know in the comments, on facebook, or on twitter.