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The phenomenon of deliquescence wherein a water soluble substance absorbs water from atmosphere and dissolve in it forming a saturated solution, has thermodynamic roots. I have read the fact that the vapour pressure of the saturated solution should be lower than the the vapour pressure of water at that temperature. Why is this so?

Moreover, since deliquescence is a spontaneous process and therefore thermodynamically favourable, there must be a negative $\Delta G$ for the absorption of water vapour and conversion of solid substance into its saturated solution? How can we derive, understand or quantify various thermodynamic parameters ($\Delta G, \Delta S, \Delta H$ etc) for the process of deliquescence?
I am comfortable with basic thermodynamic manipulations, binary phase diagrams, multiphase systems and equilibria.

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The vapour pressure lowering is a consequence of Raoult's law with a non-volatile solute, i.e. a solvent that has zero vapour pressure (or at least, lower vapour pressure than the solvent).

Regarding the second part (“How can we derive, understand or quantify various thermodynamic parameters (ΔG,ΔS,ΔH etc) for the process of deliquescence?”), I am not sure exactly what you are looking for. This is a complex phenomenon, and there is a lot of research literature of the topic. To give only one example, see

Water-Solids Interactions: Deliquescence”, Lisa J. Mauer and Lynne S. Taylor, Ann. Rev. of Food Sci. Technol., 2010, 1, 41-63.

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  • $\begingroup$ As for the first part of the question, how is the lowering of vapour pressure essential for deliquescence? And the paper you cited is behind a paywall. Can you provide a link to some other literary source? $\endgroup$ – Satwik Pasani Aug 25 '13 at 15:04
  • $\begingroup$ @SatwikPasani I'm sorry, but I didn't find a decent open access source on that (although I may not have done an exhaustive search, of course)… your university library probably has access $\endgroup$ – F'x Aug 25 '13 at 19:04
  • $\begingroup$ “how is the lowering of vapour pressure essential for deliquescence?” — I am not sure, you quoted that as an established fact… do you have a source on this? $\endgroup$ – F'x Aug 25 '13 at 19:05
  • $\begingroup$ I did read it in a physical chemistry book long ago but can't seem to remember the name of the book. It was also asked in a chemistry olympiad mock paper. $\endgroup$ – Satwik Pasani Aug 26 '13 at 12:32

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