Will dissolving stuff always increase density of solution?

[chiddler]

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Prompted by the following question:

An organic compound with a density that is less than 1 g/mL is added to an organic liquid also with a density that is less than 1 g/mL. What can be said about concentration of the solution?

The density of the solution is greater than that of pure organic liquid.
Or
**The relative densities of the solution and organic liquid cannot be determined without more info.

When it comes to dissolving a solid and liquid, wouldn't the density always increase?

 

[MT Headed]

It depends on the nature of the solvation. If the solvation shell that the liquid forms around the solid puts the liquid molecules super close together, the density could be higher. If the liquid molecules are spaced further apart when making the shell, the density would go down.

An organic solvation would be more like a physical, not chemical, process.

 

[MedPR]

Prompted by the following question:

An organic compound with a density that is less than 1 g/mL is added to an organic liquid also with a density that is less than 1 g/mL. What can be said about concentration of the solution?

The density of the solution is greater than that of pure organic liquid.
Or
**The relative densities of the solution and organic liquid cannot be determined without more info.

When it comes to dissolving a solid and liquid, wouldn't the density always increase?

At first I agreed with you, since if you add one to another, you've added more mass. But you also increase the volume, so depending on the actual densities of both organic compounds, you could have decreased, same, or increased overall density.

 

[dsoz]

dissolving alcohol (ethanol) into water DECREASES the density of the mixture (compared to pure water), but INCREASES the density compared to the alcohol.

Since most solids have a density greater than that of water, dissolving a solid in water will increase the density. However, in your example, you have two organic substances with unknown densities. You don't have enough information to make a guess. If the solute has a density < solvent density then the overall density will go down (most of the time... depending on colligative properties...)

dsoz

 

[chiddler]

dissolving alcohol (ethanol) into water DECREASES the density of the mixture (compared to pure water), but INCREASES the density compared to the alcohol.

Since most solids have a density greater than that of water, dissolving a solid in water will increase the density. However, in your example, you have two organic substances with unknown densities. You don't have enough information to make a guess. If the solute has a density < solvent density then the overall density will go down (most of the time... depending on colligative properties...)

dsoz

If they're both liquid, is it "dissolving" or mixing? Can you think of an example with a solid?

Second, suppose you have something that is very not dense but can dissolve in water very well. Its density in its solid state does not determine how much the density will increase when dissolved.

But I think I understand. I guess we must know a lot more information about the nature of the solvation as MT headed mentioned.

 

[MedPR]

If they're both liquid, is it "dissolving" or mixing? Can you think of an example with a solid?

Second, suppose you have something that is very not dense but can dissolve in water very well. Its density in its solid state does not determine how much the density will increase when dissolved.

But I think I understand. I guess we must know a lot more information about the nature of the solvation as MT headed mentioned.

Depends really. Some solids, like NaCl will dissociate in water. Other solids, like a sugar cube, will dissolve. TBR has some definitions in Chem 3. I assume liquids will dissolve into other liquids, such as alcohols in water, since the alcohol (solute) is still an alcohol, and not the constituent parts of an alcohol.

I can't think of any example where dissolving (or dissociating) a solid would not result in the solution being more dense than the solvent. <-- I think I have too many negatives in there, it's kind of confusing. What I meant was, all solid solutes result in a more dense solution.

 

[chiddler]

Depends really. Some solids, like NaCl will dissociate in water. Other solids, like a sugar cube, will dissolve. TBR has some definitions in Chem 3. I assume liquids will dissolve into other liquids, such as alcohols in water, since the alcohol (solute) is still an alcohol, and not the constituent parts of an alcohol.

I can't think of any example where dissolving (or dissociating) a solid would not result in the solution being more dense than the solvent. <-- I think I have too many negatives in there, it's kind of confusing. What I meant was, all solid solutes result in a more dense solution.

Ah this makes a world of difference! I didn't think of dissociating vs dissolving! I was thinking strictly dissociating.

Thanks a lot.

Hmm on second thought, it doesn't make a big difference in regards to my initial question because dissolving might be compared to dissociating a single ion salt. I understand the answer though so no worries.