Lot's of things seem to defy common sense. Take osmosis, for example. Technically, osmosis is the diffusion of water through a differentially permeable membrane. We all recognize diffusion when we see it. Like when we deposit a drop of food coloring into water, and it spreads out evenly throughout the water. Just the tendency of molecules in a liquid or a gas to move out from where they're concentrated until they're evenly distributed.
A differentially permeable membrane is one where some things, like
water, can pass through easily, but bigger molecules, like dissolved sugar, can't.
Take a glass tube, fix such a membrane over one end, and pour in a bit of syrup. Then
put the membrane end of the tube into pure water so the top of the water and the top of
the syrup are even. The syrup slowly rises up the tube. Magic? Nope. The dissolved
sugar molecules have diluted the water in the syrup; so, naturally, the concentrated
pure water diffuses into the syrup through the membrane, expanding its volume. A really
sweet experiment!
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.