PURIFYING YOUR WATER
A reverse osmosis membrane.
Say you pour a bit of gasoline onto a plate in a room.
Initially, you can only smell the gasoline right next to the
plate, but not on the other side of the room. Eventually,
the gasoline evaporates and diffuses throughout the room
until the molecules are equally spaced from each other
and are as far apart from each other as they can get. This
process is driven by entropy, a basic operating principle of
the universe. The probability that the gasoline molecules
will spontaneously all find themselves back together as a
liquid on the plate at some point in the future is so small
(although it isn’t zero!) that we confidently assume it to be
an impossible event.
The same principle applies to solids such as salts when
they are dissolved in water. To the extent that a substance
is soluble in a solvent such as water, a solute will distribute
itself randomly in the solvent until equilibrium is reached
and the concentration of solute is the same everywhere in
the solution. The universe likes things this way.
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Cell membranes are selectively
permeable. They allow water
and many other things to pass,
but there are even more things,
such as sugars and large protein
molecules, that cannot pass.
Maximum Yield USA | June 2016
Let’s move onto osmosis. Here’s a simple situation to help
explain it: For osmosis to occur, a barrier must exist between
two solutions that have different solute concentrations. This
barrier must allow water to pass through it while blocking
the passage of whatever solute molecules exist in solution
(remember, this is a highly simplified example). The water
will move across the selectively permeable membrane in the
direction that decreases the concentration of solute molecules. This makes the solute particles farther apart from
each other and less organized (entropy in action). Osmosis
continues until a new equilibrium state is reached. Water
molecules continue crossing the membrane, but the rate of
crossing in one direction is equal to the rate of crossing in
the opposite direction.
The reason people have an easier time understanding
“regular” diffusion is because it’s more intuitive to imagine
the solute molecules moving until they are as randomized as
possible. It is harder to picture the solvent molecules diffusing to randomize the solute particles. But that’s the way it is.
Here is one final example to help solidify your understanding of osmosis: Say you place some red blood cells in a solution of pure water—a classic experiment. Cell membranes
are selectively permeable. They allow water and many other
things to pass, but there are even more things, such as sugars
an d large protein molecules, that cannot pass. Since the
solute concentration inside the cells is much greater than
the solute concentration outside the cells, which is essentially
zero, water flows into the cells by osmosis to try and make
the two concentrations equal. In fact, the cells will eventually