# Colligative properties and sizes of particles

Why do colligative properties only depend on the number of particles and not the size?

*My argument being that since the root cause of these properties is the occurrence of solute particles on the surface, reducing the vapour pressure of the solvent, shouldn't bigger molecules, such as urea, give a much more appreciable deviation in properties, as compared to smaller ones like NaF?

• Colligative properties depend upon the "relative number" of solute particles and not on the number of solute particles. Mar 24, 2017 at 15:39
• relative? i beg your pardon, am i missing something? @Cotton Mar 24, 2017 at 16:01
• Consider two solutions-A and B. Say the number of solute molecules in A is 5 and number of solvent molecules is 10000. In B, the number of solute molecules is 5 and number of solvent molecules is 10. In which solution do you think relative lowering of vapour pressure will be more? Mar 24, 2017 at 16:42
• You are confused probably because the formule given in your textbook are valid for dilute solutions. But they are just special cases. Colligative properties are applicable for both dilute and concentrated solutions. Mar 24, 2017 at 17:09
• Please. PLEASE pay attention to the tags you're creating. You've been creating a lot of typo-y and wrong tags lately, and they've been a pain to clean. Don't edit, and take a second look at the tags on your questions. Mar 27, 2017 at 13:17

Colligative properties From Wikipedia

In chemistry, colligative properties are properties of solutions that depend on the ratio of the number of solute particles to the number of solvent molecules in a solution, and not on the nature of the chemical species present. The number ratio can be related to the various units for concentration of solutions. The assumption that solution properties are independent of the nature of solute particles is only exact for ideal solutions, and is approximate for dilute real solutions. In other words, colligative properties are a set of solution properties that can be reasonably approximated by assuming that the solution is ideal.

Emphasis in above text is mine.

So it the assumption that colligative properties are independent of particle size, not the reality. The assumption is very useful, but not perfect.

Vapor pressure reduction is not truly explained as a mass transport phenomena where solute particles at the surface of the solvent prevent solvent molecules from entering the vapor phase. Since vapor pressure depression is dependent on solute concentration, which is uniform throughout the solution, the proper explanation is the increased entropy of the solution as compared to the pure solvent. The entropy increase is not dependent on the size of the solute, but the relative quantity of solute to solvent molecules (expressed as the concentration of the solute).

http://www1.lsbu.ac.uk/water/colligative_properties.html