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There exist eutectic mixtures, in which the freezing point of the mixture is below those of either component of the mixture, i.e., the freezing point of the mixture is not intermediate between that of each component, but is instead lower than both.

Can the same occur for density? Are there mixtures of substances $\ce{A}$ and $\ce{B}$ such that either $\rho_\mathrm{mix}$ is greater than both $\rho_\ce{A}$ and $\rho_\ce{B},$ or $\rho_\mathrm{mix}$ is less than than both $\rho_\ce{A}$ and $\rho_\ce{B}?$

Or is the density of a two-component mixture always bracketed by (i.e., some intermediate value between) the densities of its individual components?

andselisk
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TheEnvironmentalist
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  • May I ask why this question is being downvoted? It seems a relevant question to those exploring chemistry, though perhaps technically more in the realm of physics? – TheEnvironmentalist Jul 05 '20 at 04:03
  • No, the density of a mixture of A and B is not necessarily the weighted average of the densities of A and B. This is equivalent to saying that, when mixing two liquids, the total volume is not necessarily the sum of the two starting volumes; look Partial molar volume up. A common example is water-ethanol mixtures. – Thomas Jungers Jul 05 '20 at 06:39
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    The problem is how it's written. It wasn't sufficiently precise, plus you're not actually asking if density is additive. Densities are never simply additive, since density is an intensive property (like temperature -- you can see that temperature is not additive). You're asking whether the density of a 2-component mixture is always bracketed by (i.e., some intermediate value between) the densities of its individual components. I've edited your question to make it clearer. – theorist Jul 05 '20 at 08:03
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    Well, optical density can often be treated as additive for the non-reacting mix, but I see no reason why mass density must ever be additive. Maybe have a look at the Wikipedia page on intensive and extensive properties – andselisk Jul 05 '20 at 08:58
  • Even using the logic of density compared to the nature of compounds tells you that variations will occur. Mass will obviously stay the same but volume can be drastically different compared to the pure elements. – Eric Hedengren Jul 05 '20 at 15:40
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    https://chemistry.stackexchange.com/questions/23534/how-does-volume-contraction-in-solvent-mixing-work https://chemistry.stackexchange.com/questions/23534/is-it-possible-to-have-volumesolution-volumesolvent-volumesolute https://chemistry.stackexchange.com/questions/83765/is-it-possible-to-have-volumesolution-volumesolvent-before-adding-solute – Mithoron Jul 05 '20 at 17:00
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    @Mithoron Those answers all give examples of volume contraction, which is different from what's being asked. Volume contraction is a necessary but not sufficient criterion for the mixture to have a density that exceeds that of its individual components. Correspondingly, none of the volume contraction examples in those answers meet the OP's criterion: Ethanol-water solutions never have a density greater than that of pure water (https://www.engineeringtoolbox.com/ethanol-water-mixture-density-d_2162.html) and NaCl(aq) solutions never have a density greater than that of NaCl(s) (2.16 g/cm^3). – theorist Jul 05 '20 at 22:47
  • ....continued: http://butane.chem.uiuc.edu/pshapley/genchem1/l21/1.html [Assuming, of course, the pure components and the mixture are measured under the same conditions, particularly T and p.] – theorist Jul 05 '20 at 22:50

1 Answers1

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No, there is no law that requires the density of a mixture to fall between the densities of pure components. It does so most of the time, but the exceptions are not unheard of. Here's one.

Water: density $1.00\ \rm{g/cm^3}$
Hydrazine: $1.02\ \rm{g/cm^3}$
Hydrazine hydrate: $1.03\ \rm{g/cm^3}$

So it goes.

Ivan Neretin
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  • Any examples of the opposite, where the density of the mixture is lower than that of both the pure components? – theorist Jul 05 '20 at 22:56
  • Can't think of any, but there is no fundamental law forbidding that, either. – Ivan Neretin Jul 06 '20 at 00:39
  • Ethanol/benzene and ethanol/toluene: https://www.researchgate.net/figure/Excess-volume-of-ethanol-benzene-top-and-ethanol-toluene-bottom-present_fig2_282982793. Benzene/m-xylene: http://www.ddbst.com/en/EED/VE/VE0%20Benzene%3Bm-Xylene.php. – peruca3d Jul 06 '20 at 02:30
  • Positive excess volume per se is not enough to ensure the condition in question. – Ivan Neretin Jul 08 '20 at 17:07