If the cell is set up so that the liquid level is initially the same in both compartments, you will soon notice that the liquid rises in the left compartment and falls in the right side, indicating that water molecules from the right compartment are migrating through the semipermeable membrane and into the left compartment. This migration of the solvent is known as osmotic flow, or simply osmosis.
The escaping tendency of a substance from a phase increases with its concentration in the phase. What is the force that drives the molecules through the membrane? Osmosis is a consequence of simple statistics: the randomly directed motions of a collection of molecules will cause more to leave a region of high concentration than return to it; the escaping tendency of a substance from a phase increases with its concentration in the phase.
Suppose you drop a lump of sugar into a cup of tea, without stirring. Initially there will be a very high concentration of dissolved sugar at the bottom of the cup, and a very low concentration near the top.
Since the molecules are in random motion, there will be more sugar molecules moving from the high concentration region to the low concentration region than in the opposite direction. The motion of a substance from a region of high concentration to one of low concentration is known as diffusion. Diffusion is a consequence of a concentration gradient which is a measure of the difference in escaping tendency of the substance in different regions of the solution.
There is really no special force on the individual molecules; diffusion is purely a consequence of statistics. Osmotic flow is simply diffusion of a solvent through a membrane impermeable to solute molecules. Being semipermeable, the membrane is essentially invisible to the solvent molecules, so they diffuse from the high concentration region to the low concentration region just as before. This flow of solvent constitutes osmotic flow , or osmosis.
Osmosis will be to the right, since water is less concentrated there. This results in a net osmotic flow of water from the right side which continues until the increased hydrostatic pressure on the left side raises the escaping tendency of the diluted water to that of the pure water at 1 atm, at which point osmotic equilibrium is achieved.
In the absence of the semipermeable membrane, diffusion would continue until the concentrations of all substances are uniform throughout the liquid phase. With the semipermeable membrane in place, and if one compartment contains the pure solvent, this can never happen; no matter how much liquid flows through the membrane, the solvent in the right side will always be more concentrated than that in the left side.
Osmosis will continue indefinitely until we run out of solvent, or something else stops it. The reader is therefore invited to create their own, and it will be as valid as the next. In case one needs a plausible-sounding third option, here is the definition from Ferner in Quantitative Human Physiology 2nd ed. The key points here are that, unlike diffusion considered more broadly, osmosis narrowly refers to the movement of the solvent itself, rather than particles or molecules of the solute.
Most of the definitions also seem to incorporate a semipermeable membrane of one sort or another, and so it seems essential; various unofficial sources claim that without a semipermeable membrane in the way it's not osmosis, it's just diffusion. That one is from the IUPAC Gold Book , and therefore definitive as those people are literally responsible for defining the terms used in chemistry and chemical engineering.
The original definition, from Wilhelm Pfeffer , went:. Or something probably with more Druckkraft in it. Anway, the point is that osmotic pressure is literally a hydraulic term. Specifically, the definition is a fluid pressure, which can be expressed in normal pressure units be it kPa or mmHg , and which describes the pressure necessary to oppose the movement of solvent.
Observe, a simple experiment where, to two chambers filled with some solvent and separated by a semipermeable membrane, some sort of solute gloop is added. The gloop disperses into one chamber only, the membrane being gloop-impermeable.
Owing to this, solvent moves into the gloop chamber, and the fluid level rises. So far, so osmosis. Now, for the most important part: the osmotic pressure is the pressure that would need to be applied to the gloop chamber in order to keep this fluid movement from happening:.
For any solution, osmotic pressure is directly proportional to its absolute temperature, and at a constant temperature, it is directly proportional to the solute concentration.
If the reader is ready to remark that this resembles the ideal gas equation , they would be correct. In fact, in most editions of Ganong, the reader is offered this rearranged version:. This view of osmotic pressure originates with Jacobus Henricus van 't Hoff. The van 't Hoff model imagines that a substance dissolved in a fluid medium behaves as if it were a gas in a vacuum.
What is the freezing point of a 1. We use the equation to calculate the change in the freezing point and then subtract this number from the normal freezing point of C 6 H 6 to get the freezing point of the solution:. Now we subtract this number from the normal freezing point of C 6 H 6 , which is 5. What is the freezing point of a 3.
Freezing point depression is one colligative property we use in everyday life. Many antifreezes used in automobile radiators use solutions that have a lower freezing point than normal so that automobile engines can operate at subfreezing temperatures.
We also take advantage of freezing point depression when we sprinkle various compounds on ice to thaw it in the winter for safety Figure The compounds make solutions that have a lower freezing point, so rather than forming slippery ice, any ice is liquefied and runs off, leaving a safer pavement behind. Before we introduce the final colligative property, we need to present a new concept. A semipermeable membrane is a thin membrane that will pass certain small molecules but not others.
A thin sheet of cellophane, for example, acts as a semipermeable membrane. Consider the system in Figure A semipermeable membrane separates two solutions having the different concentrations marked. Curiously, this situation is not stable; there is a tendency for water molecules to move from the dilute side on the left to the concentrated side on the right until the concentrations are equalized, as in Figure This tendency is called osmosis.
In osmosis, the solute remains in its original side of the system; only solvent molecules move through the semipermeable membrane. In the end, the two sides of the system will have different volumes. This pressure difference is called the osmotic pressure , which is a colligative property. The pressure exerted by the different height of the solution on the right is called the osmotic pressure.
The osmotic pressure of a solution is easy to calculate:. What is the osmotic pressure of a 0. Now we can substitute into the equation for osmotic pressure, recalling the value for R :. The units may not make sense until we realize that molarity is defined as moles per liter:. Now we see that the moles, liters, and kelvins cancel, leaving atmospheres, which is a unit of pressure.
This is a substantial pressure! It is the equivalent of a column of water 84 m tall. Osmotic pressure is important in biological systems because cell walls are semipermeable membranes. In particular, when a person is receiving intravenous IV fluids, the osmotic pressure of the fluid needs to be approximately the same as blood serum; otherwise bad things can happen. Figure Only when the solutions inside and outside the cell are the same isotonic will the red blood cell be able to do its job.
Neither of these last two cases is desirable, so IV solutions must be isotonic with blood serum to not cause deleterious effects.
You can drink the water, but ingesting it will pull water out of your cells as osmosis works to dilute the seawater. Ironically, your cells will die of thirst, and you will also die.
It is OK to drink the water if you are stranded on a body of freshwater, at least from an osmotic pressure perspective. Osmotic pressure is also thought to be important—in addition to capillary action—in getting water to the tops of tall trees.
What are the three colligative properties that involve phase changes? Which colligative property does not involve a phase change? Give an example of its importance.
If If g of N 2 are mixed with g of O 2 , what is the mole fraction of each component? An alloy of stainless steel is prepared from
0コメント