Colligative Properties
Up until now, we have discussed the various properties of solutions by referring to the properties of the molecules present. For example, a glass of water with NaCl dissolved in it conducts electricity due to the dissolution of NaCl into its component ions. If we dissolved glucose into that same water it wouldn’t conduct electricity as glucose doesn’t break apart into ions in water readily. Colligative properties on the other hand only focus on how much of a solute is present rather than what that solute is. What matters is not what has been dissolved as seen above but how much.
Osmotic Pressure
The most straightforward and well known colligative property is osmotic pressure. While not the technical definition, osmotic pressure is the desire of a solvent, water or otherwise, to move across a semipermeable membrane. In osmosis, the solvent will always move towards the higher concentration of dissolved solute. As they say, water wants to go to the ion party and thus it flows towards higher concentrations of ions.
This is why diabetics with markedly elevated blood sugar levels often appear incredibly dehydrated. The sugar dissolved in their blood increases its osmotic pressure causing water from the cells to be “sucked” into the blood by its high osmotic pressure.
In contrast, people with advanced liver disease are unable to produce albumin, the major dissolved protein in our blood, thus dropping their blood’s osmotic pressure. As a result, water flows out of the blood and into the cells and cavities of the body causing conditions such as ascites, which is a large collection of fluid within the abdomen.
As we can see it didn’t matter if the solute was glucose or albumin only that the concentration of both changed, which is the hallmark of a colligative property. The greater the solute concentration the greater the osmotic pressure so in some sense what you dissolve into a solution does matter.
For example, for every albumin molecule you dissolve in water you get one dissolved albumin molecule. Whereas for every NaCl molecule you dissolve you get two new dissolved molecules one sodium ion and one chloride ion. This means that every NaCl molecule essentially counts twice.
So if we dissolved 1L of 1M Albumin in 1L of water and 1L of 1M NaCl in 1L of water the NaCl solution would have double the osmotic pressure since it has double the solute concentration.
Of course we can quantify the osmotic pressure of a solution if we wish using the following equation.
[latexpage]
\[\Pi=iMRT
\]
Here 𝚷 stands for the osmotic pressure, i for the number of solute molecules a compound produces, R the ideal gas constant, and T is the temperature.
Vapor Pressure
Vapor pressure, another colligative property, refers to the ability of solvent molecules to turn into their gaseous form. The higher a solvent’s vapor pressure the more easily it evaporates and vice versa. When comparing the vapor pressure of a pure solvent to a solvent with dissolved solutes the vapor pressure of the solute containing dissolved solutes will always be lower. This occurs because solute molecules “block” the exit of escaping solvent molecules.
We have already seen how this works by considering an analogy so let’s look at what is going on from a different perspective, electrostatics. Let’s imagine that are planning on boiling water to make pasta and per the instructions on the package, we salt our pasta water appropriately.
When we add the NaCl into the pasta water it will quickly dissolve into its component ions. Since water is a polar molecule it will be attracted to these ions and form ion-dipole interactions with the salt molecules. While the water molecules can evaporate the sodium and chloride ions can’t and are stuck in solution keeping their attracted water molecules stuck in solution with them.
In this way water molecules have little salt anchors tied to them, meaning they will have a harder time transitioning into the gas phase. This occurs because they not only have to overcome their intermolecular forces with other water molecules but also with the solute molecules too which can’t evaporate.
Boiling Point
Ultimately lowering the vapor pressure of our solution increases the solution’s boiling point because as vapor pressure drops the ability of the solvent molecules to transition from liquid to gas also drops. Since our new salty solution has a harder time turning into a gas it has a harder time boiling and as a result, the temperature needed for it to boil increases. Thus dissolved solutes increase the boiling point of a solution.
Freezing Point
Lastly, solutes lower the freezing points of solutions making it harder for solutions to freeze. This occurs because dissolved salts disrupt the tightly packed molecular structure of solids. I like to think of the salt molecules like little fires with a bunch of solvent molecules huddled around them. It reminds me that salts lower the freezing point of solutions by disrupting solvent molecules’ ability to form the tightly packed molecular structure of a solid.
Again some salts are better than others at lowering the freezing point mainly those that dissociate into a greater number of ions. That is why MgCl2 is used to salt the roads in the winter to prevent them from forming dangerous black ice. Since MgCl2 dissociates into three ions we can three ions for the price of one salt molecule.
Colligative Questions
While the MCAT doesn’t tend to ask a ton of colligative based questions when they do they tend to all have the same feature in common. That is they all require you to compare the concentration of solutes among different types of solutions. The key to getting these questions correct is making sure that you are comparing apples to apples. Which is to say solute concentration to solute concentration.
For example, a question might ask which of the following solutions has the highest osmotic pressure.
- 0.4M Glucose
- 0.2M NaCl
- 0.35 MgBr2
- 1M Albumin
Looking at this list it might be tempting to choose albumin, the solution with the highest overall concentration, however that would end up being incorrect. While glucose and albumin don’t break apart into additional solute molecules, NaCl and MgBr2 do. Therefore we must first determine the total solute concentration of both prior to making comparisons between the four options.
To do this we first determine the total number of solute molecules each compound will produce by counting the number of ionizable components, which is a fancy way of saying the number of parts that break off and dissolve. For ionic salts that is simply the number of atoms they contain. In this case, MgBr2 has three components while NaCl has two components.
Now we multiply the total number of solute molecules present in each compound by its overall concentration. This gives us the total solute concentration of each solution which can be compared to one another.
- 0.4M Glucose = 0.4M Solute
- 0.25 Nacl = 0.25M x 2 Solutes = 0.5M Solute
- 0.35 MgBr2 = 0.35M x 3 Solutes = 1.05M Solute
- 1M Albumin = 1M Solute
Here we can see that the MgBr2 solution actually has the highest solute concentration and will therefore have the highest osmotic pressure as well. These same exact steps are always used for colligative questions. From there you have to match up the solute concentration trend with the colligative property discussed, which follows the trend summary below. Additionally, these questions are almost always most/least questions so make sure to eliminate any middle answers.
Colligative Trend Summary
