Editing Colligative properties (section)

{{Short description|Properties of solutions that depend only on the number of solute particles}}
{{refimprove|date=May 2010}}
[[File:Freezing point depression and boiling point elevation.png|thumb|Freezing point depression and boiling point elevation]]

In [[chemistry]], '''colligative properties''' are those properties of [[Solution (chemistry)|solution]]s that depend on the ratio of the number of solute particles to the number of [[solvent]] [[particle]]s in a solution, and not on the nature of the chemical species present.<ref>McQuarrie, Donald, et al. ''Colligative properties of Solutions" General Chemistry Mill Valley: Library of Congress, 2011. {{ISBN|978-1-89138-960-3}}.</ref> The number ratio can be related to the various units for [[concentration]] of a solution such as [[molarity]], [[molality]], [[normality (chemistry)]], etc. The assumption that solution properties are independent of nature of solute particles is exact only for [[ideal solution]]s, which are solutions that exhibit [[Thermodynamics|thermodynamic]] properties analogous to those of an [[ideal gas]], and is approximate for dilute real solutions. In other words, colligative properties are a set of solution properties that can be reasonably approximated by the assumption that the solution is ideal. 

Only properties which result from the dissolution of a nonvolatile solute in a volatile liquid solvent are considered.<ref name="ReferenceA">KL Kapoor ''Applications of Thermodynamics'' Volume 3</ref> They are essentially solvent properties which are changed by the presence of the solute. The solute particles displace some solvent molecules in the liquid phase and thereby reduce the concentration of solvent and increase its entropy, so that the colligative properties are independent of the nature of the solute. The word colligative is derived from the Latin ''colligatus'' meaning ''bound together''.<ref>[[Keith Laidler|K.J. Laidler]] and J.L. Meiser, ''Physical Chemistry'' (Benjamin/Cummings 1982), p.196</ref> This indicates that all colligative properties have a common feature, namely that they are related only to the number of solute molecules relative to the number of solvent molecules and not to the nature of the solute.<ref>{{cite book |last1=Castellan |first1=Gilbert W. |title=Physical Chemistry |date=1983 |publisher=Addison-Wesley |isbn=978-0201103861 |page=281 |edition=3rd |url=https://books.google.com/books?id=YI9aDwAAQBAJ&q=colligative+%22bound+together%22&pg=PA281 |access-date=20 July 2019}}</ref>

Colligative properties include: 
* [[Relative lowering of vapor pressure]] ([[Raoult's law]])
* [[Boiling-point elevation|Elevation of boiling point]]
* [[Freezing-point depression|Depression of freezing point]]
* [[Osmotic pressure]]
For a given solute-solvent mass ratio, all colligative properties are inversely proportional to solute molar mass.

Measurement of colligative properties for a dilute solution of a non-ionized solute such as [[urea]] or [[glucose]] in water or another solvent can lead to determinations of relative [[molar mass]]es, both for small molecules and for [[polymers]] which cannot be studied by other means. Alternatively, measurements for ionized solutes can lead to an estimation of the percentage of [[Dissociation (chemistry)|dissociation]] taking place.

Colligative properties are studied mostly for dilute solutions, whose behavior may be approximated as that of an ideal solution. In fact, all of the properties listed above are colligative only in the dilute limit: at higher concentrations, the freezing point depression, boiling point elevation, vapor pressure elevation or depression, and osmotic pressure are all dependent on the chemical nature of the solvent and the solute.