Illustration from the Physical chemistry textbook (click to enlarge)
Colligative properties have particularly interesting…well, properties! The most important of them is, as far as I understand from my students, their independence on the chemical nature of the solute.
How can a property of a solute-solvent system NOT depend on the nature of the former? Well, to explain this behavior we can follow two roads; the first one, surely the most significant one from a mathematic point of view, is in the Physical chemistry textbook (section 5.8 pages 5-63 to 5-78).
It clearly explains how to calculate these properties. I won’t add anything to this Chapter (basically since I wouldn’t be able to do better 😀 ), but maybe ask for some animations about this topic to be added in a future release of the Book.
What I’ll try now is to explain colligative properties by means of simple and qualitative concepts. The most important key: if a substance (A) is diluted in a solvent (B), this means that interactions between solute and solvent are energetically favorable.
What does this mean? Simple enough: molecules A like molecules B, and vice versa.
How can this explain colligative properties? Let’s start from the first one: freezing point depression. When molecules A interact with molecules B, the latter won’t like to form a solid phase, because this will mean to abandon the interaction to form a perfect crystal structure, where molecules A have no room to interact.
For this reason, you have to force molecules B to accept this separation. How can you do this? By leaving them less energy than they actually have; or, in other words, by decreasing temperature to a lower value than the one needed by the solvent alone. Poor molecules B will then hate their “depleted kinetic energy wallet”, and will gather themselves in solid phase to receive some reticular energy, even if this means losing their interactions with molecules A.
Almost the same can be said about boiling point elevation; it may seem contradictory that previously the temperature had to DEcrease and now it has to INcrease, but it is easy to understand!
Molecules B reach a temperature which would allow them to pass in gaseous phase, but if they do, they lose their interaction with molecules A. How can you ask them to forget bonding with beautiful molecules A? Well, you force them by “paying” more kinetic energy (that is, temperature). In this way, the need for higher boiling temperatures can be explained.
And osmotic pressure is no exception: a semipermeable membrane is something that lets B pass, and stops A. Now, if molecules B do like molecules A, they would like to go wherever molecules A can be found… and for this reason, they will prefer to “overcrowd” in the solution with the highest concentration of A.
But, in doing so, they will leave the most diluted solution, creating a level inequality. A higher level means more pressure. To equilibrate this higher pressure, you have to push harder, forcing some molecules B to go back into the first solution. And this is exactly what osmotic pressure means!