Plain old NaCl salt can slightly lower the vapour pressure of water for a given temperature. Other salts such as CaCl₂ lower the vapour pressure to a larger degree.

Do any salts, or compounds of any type, do the reverse, and raise the vapour pressure of water? Is this provably impossible/possible?

  • 2
    $\begingroup$ Consider why the salts lower the vapor pressure. Now consider how the opposite effect could happen. $\endgroup$
    – Jon Custer
    Aug 2 at 21:57
  • $\begingroup$ @Mithoron Isn't that the melting point, not boiling point? $\endgroup$ Aug 3 at 18:39

Chemistry is very complicated and I'm probably missing certain edge cases, but I think the answer is almost certainly 'no'.

Salts lower the vapour pressure of water because intermolecular forces attract the salt ions to the water, raising the energy needed for water molecules to escape.

In order for water's vapour pressure to rise, the salt would need to (on average) repel the water ions, the typical term for this is an insoluble salt.

Alternatively, something in the gaseous mixture could attract the water molecules, but this is going to be difficult, since the defining feature of gases is their minimised inter molecular forces.

I'd note this is not a rigorous proof, but I cannot see a way around it. Perhaps the minimal amounts of insoluble salts that dissolve in water due to the statistical nature of chemistry raise the vapour pressure?


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