Osmotic Pressure

Osmotic Pressure is a (not-so-great) name for the process that shrivels slugs when they encounter salt and prevents honey from rotting.

To understand this idea, we need to start with osmosis. In biology class, osmosis is defined this way:
Osmosis is the movement of water through a semi-permeable membrane from an area of high concentration to low concentration.
Let's pull that apart.

Permeable means that things can go through. So, a permeable membrane is one that allows things to move through it. Essentially, a permeable membrane is a membrane with holes.

SEMI-permeable would mean a membrane with holes that are large enough for some things to go through, but small enough to block other things.

Specifically, the holes are large enough for water to go through.

Remember that when something (like table salt) dissolves, the pieces of the solute are surrounded by water molecules.

Because those solute particles and water molecules move together, these "clumps" area too large to move through the membrane.

It is important to realize that there is no "need" for water particles to go through. The molecules are moving randomly. Some of those particles will go through the membrane simply because they can.

So, if the only thing present on one side of the membrane is water, then (effectively) 100% of the particles "can" go through the membrane.

At the same time, if the other side of the membrane has some water and some solute, less than 100% of the particles "can" go through that membrane.

The result is that water will go faster in one direction than it will in the other.

We can see this happen, if we set up the equipment below.

In this diagram, we have taken special tube, called a thistle tube (sort of like a funnel with a REALLY long tube) and sealed the top with a semi-permeable membrane.

That tube was then flipped upside down and submerged into a beaker of water.

The tube was then filled with a sugar solution so that the level of the solution matches the level of water in the beaker.

In this set-up, 100% of the water in the beaker "can" move up through the membrane. At the same time, only some of the water in the tube can move out.

Then, because the water can move INTO the tube faster than the water can move OUT of the tube, the level of liquid in the tube will rise.

The final result of this process is a tube sticking into a beaker of water with liquids at different heights. The lat time we looked at this, we were discussing gas pressure. (Think about straws and barometers.) Since this set-up allows osmosis and results in something that looks like pressure, we call this process osmotic pressure.

Slugs

This same process is the reason that salt kills slugs. The skin of a slug is a semi-permeable membrane. Inside that membrane is some water (and other stuff -- but that's bio!). Outside the membrane is the mucus coating (which is water and other stuff -- also bio). As a result some water can move in and some water can move out. Slugs "work" because the rate at which water moves in matches the rate at which water moves out.


If however, you put salt on the slug, the water on the outside will stick to the salt and that water will NOT be able to move in through the membrane. Suddenly, water will be going out (as it always did...not because it is being pulled) but it is not being replaced.

Honey

Osmotic pressure is also the reason that honey (and other thick things like syrup and ketchup) doesn't rot.

Remember (from biology) that rotting is just what happens when bacteria grows on (and eats) something.

Honey is a solution made with a HUGE amount of sugar and a very a small amount of water.

Bacteria can be thought of as little bags of water with a semi-permeable membrane as "the bag." Bacteria thrive when they find themselves in a warm wet environment. Warmth helps them function, but the "wet" part is really important part. For a bacteria to live, it needs to be in a place where water can move in and out  at equal rates.

When a bacteria lands on honey, the water inside the bacteria can move out, but there is so much sugar and so little water in the honey that essentially "no" water moves back in. As a result, the bacteria lose all of their water, dehydrate and die.

The math:

Osmotic pressure can be measured just like gas pressure, in terms of the height of the liquid. Just like the other colligative properties, osmotic pressure depends on the concentration of the solution and can be calculated using the formula:

Where Π is the osmotic pressure, M is the molarity, R is the universal gas constant, and T is the temperature.