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Generally water soluble salts tend to 'fit between' the molecules of water such that the volume of the resulting solution does not increase much relative to the volume and added mass of the salt. So the result is that the solution generally has a higher density than pure water.

But are there any special cases; any salts or compounds that result in an expansion of the resulting solution volume such that its density actually decreases?

Sodium polyacrylate comes to mind, but if I'm not mistaken, the expansion doesn't take place until a significant amount of the compound is added, and then only after the liquid is changed to rather a gel or solid. My question relates to the resulting solution actually being a liquid with density < 1.0 g/cm³.

So then if not for water, are there solutions of other solvents that might behave this way?

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  • $\begingroup$ You're asking about reversion of contraction of volume? $\endgroup$
    – Mithoron
    Commented Aug 10, 2015 at 20:27
  • $\begingroup$ I think so. Volume is the only thing that can change to reduce density since mass is being added. $\endgroup$
    – docscience
    Commented Aug 10, 2015 at 20:53
  • $\begingroup$ You mention "compounds" after salts. Does this refer solely to solids as otherwise methanol, as an example, would meet your criteria. $\endgroup$
    – Beerhunter
    Commented Aug 12, 2015 at 17:22
  • $\begingroup$ @Beerhunter I'm considering any compound, including a liquid. So if you could explain details as an answer, please do so. Thanks $\endgroup$
    – docscience
    Commented Aug 12, 2015 at 17:26

2 Answers 2

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Beerhunter is correct in his comment. Water and methanol are mutually soluble in all proportions, and methanol has a lower density than water, so, if you add lots of methanol to the water, the density of the solution will be less than that of water.

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No, you can't dissolve a salt to make a lower density solution.

Instead of thinking about salt ions just fitting into water molecules, think about it as if the salt ions are letting everything fit together better. The water molecules are achieving their current density because of the presence of hydrogen bonding. When you add the ions you introduce better bonds between the charged ions and the polar water molecules. These bonds allow everything to fit together tighter leading to a rise in density.

This decrease in overall bond length is a key to the spontaneity of the solvation reaction. If you introduce an ion that increases the bond length you would see no solubility because they system would be more energetically stable as a bulk solid and water with hydrogen bonding.

The polyelectrolyte you bring up raises a good point, but it is not an exception to this rule. Instead, the polyelectrolyte is so large that it "solvates" into water to form a gel. This new material is not really a solution so much as just a new material with water in it. Hydrated salts would be another example.

This should be the case for all solute-solvent systems. However, this is not to say what you want can't be done. You may have to looks at some reversible reaction or cis/trans type switching material. A simple solute-solvent system won't get it done.

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  • $\begingroup$ Following up with other materials, certainly mixing a less dense fluid like methanol or ammonia would decrease the density. As a first order answer you can take the average density of both fluids based on the molar ratio. This will get you the ideal mixture density. For materials like methanol that are highly interactive with water there is some ideal nature to the mixture density, but it is usually small and can be calculated using excess volume data. A good resource for this info is DDBST: ddbst.com/ddb-ve.html $\endgroup$
    – Murenrb
    Commented Aug 25, 2015 at 17:03

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