Why Does Adding Salt Mix Increase Volume

[Randy Holmes-Farley Verified Community Expert]

Anything in solid or liquid state added to the water would add volume. Basic physics.

It's not so simple, and it is definitely not basic physics. It is a very complicated analysis of how water molecules interact with dissolved ions.

 

[Randy Holmes-Farley Verified Community Expert]

I believe this is only happening on a very small scale where only small amount of salt is added. You're dumping kgs of salt into your mixing bin.

The stated rationale in the link is correct in general for all salt levels.

 

[Bret]

The really interesting question for a chemist is why the volume of two things mixed together can be smaller than the sum of the two before mixing. But I'll leave that for another day.

I think this is the question we are getting to.

Does the volume of pure fresh water actually decrease when adding NaCl?
What effect do the other components in the salt mix have on the volume of the solution, and by how much?

 

[BZOFIQ]

It's not so simple, and it is definitely not basic physics. It is a very complicated analysis of how water molecules interact with dissolved ions.

I was going to tag you. Who else to explain it better.

I said its simple because you can't add a kg of salt into 1L of water and end up with 1L of mixture.

I can see what you're saying about not so simple.

 

[Randy Holmes-Farley Verified Community Expert]

I think this is the question we are getting to.

Does the volume of pure fresh water actually decrease when adding NaCl?
What effect do the other components in the salt mix have on the volume of the solution, and by how much?

I gave the exact data for seawater. It rises. That's the summation of many effects, but mostly Na+, Cl-, Mg++, and SO4--.

But, there are chemicals that actually cause a decrease, emphasizing that it can decrease and is not "simple physics".

Dissolving a small amount of magnesium sulfate in fresh water results in a smaller final volume than the starting volume of fresh water. That is because magnesium and sulfate strongly bind water ions, pulling them closer and increasing the density.

See this article:

9.2 Partial Molar Quantities

chem.libretexts.org

.

from it:

" It is possible for a partial molar volume to be negative. Magnesium sulfate, in aqueous solutions of molality less than 0.07mol kg−10.07mol kg−1, has a negative partial molar volume. Physically, this means that when a small amount of crystalline MgSO44 dissolves at constant temperature in water, the liquid phase contracts. This unusual behavior is due to strong attractive water–ion interactions. "


This article has highly detailed data for true enthusiasts:

Densities and apparent molar volumes of aqueous magnesium sulfate and sodium sulfate to 473 K and 100 bar

https://pubs.acs.org/doi/10.1021/je00045a018

 

[Randy Holmes-Farley Verified Community Expert]

I was going to tag you. Who else to explain it better.

I said its simple because you can't add a kg of salt into 1L of water and end up with 1L of mixture.

I can see what you're saying about not so simple.

It can reduce in volume with certain salts. See above post.

 

[BZOFIQ]

It can reduce in volume with certain salts. See above post.

Wow, even in these quantities? That's just magic!

 

[mitten_reef]

As the remaining salt dissolves, and finds its way into the water molecules, the volume will then decrease.

The volume will decrease from solid to solution state, yes. That, I'd agree on. as far as explanation goes, I don't recall freshman year chemistry. But an ionic solution of salt, or any other soluble compounds, has the ionic states of Na and Cl "floating" between water molecule, nothing is "absorbed" by water. thus, there should still be displacement, you do not magically lose volume of water.

One possible exception, when your dissolving reaction generates so much heat some water escape in the form of vapor.

 

[Randy Holmes-Farley Verified Community Expert]

The volume will decrease from solid to solution state, yes. That, I'd agree on. as far as explanation goes, I don't recall freshman year chemistry. But an ionic solution of salt, or any other soluble compounds, has the ionic states of Na and Cl "floating" between water molecule, nothing is "absorbed" by water. thus, there should still be displacement, you do not magically lose volume of water.

One possible exception, when your dissolving reaction generates so much heat some water escape in the form of vapor.

Yes, you do magically lose volume. I love chemistry magic!!!

The simplistic way to think of it is that magnesium and sulfate pull water into a tighter and denser configuration around them than they form without the salts present.

 

[BZOFIQ]

.... thus, there should still be displacement, you do not magically lose volume of water.

....but the posts above by @Randy Holmes-Farley would suggest that with certain salts it does.

 

[BZOFIQ]

Yes, you do magically lose volume. I love chemistry magic!!!

The simplistic way to think of it is that magnesium and sulfate pull water into a tighter and denser configuration around them than they form without the salts present.

Makes sense - still gets filed under magic.

 

[Bret]

I gave the exact data for seawater. It rises. That's the summation of many effects, but mostly Na+, Cl-, Mg++, and SO4--.

let me ask the question in another way. Can an aqueous solution of NaCl have a negative partial molar volume, as suggested by the article I included in my post?
If so, can it still have a negative partial molar volume at the temperatures we run our tanks, and at less than a fully saturated solution?

 

[BZOFIQ]

It can reduce in volume with certain salts. See above post.

Certain salts, just not THE salt.

So with table salt or salt mix just not the case?

 

[Randy Holmes-Farley Verified Community Expert]

let me ask the question in another way. Can an aqueous solution of NaCl have a negative partial molar volume, as suggested by the article I included in my post?
If so, can it still have a negative partial molar volume at the temperatures we run our tanks, and at less than a fully saturated solution?

I quickly looked through the literature and only found positive partial molar volumes for NaCl, which is consistent with salt mix having a positive partial molar volume, despite the presence of magnesium and sulfate.

 

[Randy Holmes-Farley Verified Community Expert]

Certain salts, just not THE salt.

So with table salt or salt mix just not the case?

Magnesium and sulfate (the 3rd and 4th most abundant ions in seawater,). yes.
Salt mix, obviously no.
Sodium chloride, my literature looks says no.

 

[Alenya]

Specific gravity is a measure of density not volume alone.

If you are talking volume then water has some natural ‘gaps’ due to the shape of H2O. Small ions like Cl- can slot into those spaces. Other ions and cations can shuffle all the molecules into tighter alignments. But at some point you’ve added some much additional material the overall volume will increase.

It’s like having a jar of sand (water) then pouring water (salts) into the jar - the spaces get taken up but eventually the jar will overflow.

 

[Bret]

Specific gravity is a measure of density not volume alone.

If you are talking volume then water has some natural ‘gaps’ due to the shape of H2O. Small ions like Cl- can slot into those spaces. Other ions and cations can shuffle all the molecules into tighter alignments. But at some point you’ve added some much additional material the overall volume will increase.

It’s like having a jar of sand (water) then pouring water (salts) into the jar - the spaces get taken up but eventually the jar will overflow.

This makes sense.
I am a geologist, so the porosity example is relatable.
I'm going to see if i can set up a simple experiment to see how much my salt increases the volume of a know volume of water at reef tank salinity.

 

[JimWelsh]

I'm going to see if i can set up a simple experiment

Your results will vary depending upon, among other things, how much water of hydration is contained in the salts your mix of choice is composed of.

 

[Bret]

Your results will vary depending upon, among other things, how much water of hydration is contained in the salts your mix of choice is composed of.

Yes, that's why I said MY salt mix

 

[Alenya]

This makes sense.
I am a geologist, so the porosity example is relatable.
I'm going to see if i can set up a simple experiment to see how much my salt increases the volume of a know volume of water at reef tank salinity.

Geologists unite! (I only studied though)