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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?

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    $\begingroup$ Consider why the salts lower the vapor pressure. Now consider how the opposite effect could happen. $\endgroup$
    – Jon Custer
    Commented Aug 2, 2021 at 21:57
  • $\begingroup$ @Mithoron Isn't that the melting point, not boiling point? $\endgroup$ Commented Aug 3, 2021 at 18:39

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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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You should probably look at understanding colligative properties.

Effectively non volatile salts lower the vapour pressure by taking the place of water. Colligative means only depending on the amount not the nature. Just the fact they are there, rather than interacting is what counts. Calcium chloride has more of an effect as there are more ions, 2 vs 3 for sodium chloride.

If you need to know more try here. https://chem.libretexts.org/Courses/University_of_Arkansas_Little_Rock/Chem_1403%3A_General_Chemistry_2/Text/13%3A_Solutions/13.04%3A__Colligative_Properties

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That is an interesting question. The vapor pressure of water at a given T is defined by its chemical potential which is effectively the amount of water present. Dissolving anything in water, or anything for that matter, lowers its concentration and its chemical potential, thereby decreasing its vapor pressure. At first blush it seems that the vapor pressure must be lowered. However, there exist low boiling azeotropes where the VP of at least one of the components is raised so there is a mechanism by which VP can be higher than normal. This can be accomplished by physical means, the best example being reverse osmosis. Applying pressure to a water solution raises its chemical potential compared to pure water. This phenomenon must exist in membranes especially in living cells. What is needed is a solute that increases the pressure on the water molecules, a hydrophobic hydrogel, an oxymoron, but who knows?

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