After dissolving salt in water there is a reduction in volume? 2

[davidd314152wind4]

Someone pointed out to me that when salt is disolved in water that the volume of the water would not change.

While trying to learn how such a calculation would be made in chemistry I came across a web page that stated that there would actually be a 2.5% reduction in volume:

http://www.practicalphysics.org/go/Experiment_174.html

Why does this happen? Is there a straight-forward way to calculate the volume of a mixture? I remember working calculations with mols in chemistry but do not recall if we had ever calculated volumes of mixtures.

Thanks,

windo4life

 

[EngAbdelkader]

windo4life,
Tricky post you've found there! this issue have been studied first in 19th century!!
the oldest scientific study and publication that I've found myself was carried out around 1816 in the French Chemistry and Physics annals (which was the equivalent of our actual scientific journals), in that study they relate the change of volume to the shrinkage of the salt (the salt contracts as they say) keep in mind that it was in 1816's so don't expect to read something about lattice or atoms!

This was for the history, now let's speak technical!
the phenomenon is divided in two distinct phases:

1- before that the salt dissolves in the water, we will have an increase of volume that will be the addition of the two volumes separately (Vwater + Vsalt) as if you put a stone in a glass of water, you'll notice that the water will over flood! (if the glass was full of water obviously) remember Archimedes!

2- as dissolution occurs the total volume decreases for 2 reasons:

2-a. crystal breakage
the dissolution of salt that means the breakage of the salt lattice (as it is a crystal) into "free" ions Cl- and Na+ that will take less place in the common language (so less volume technically speaking)you'll notice that this joins the theory of the French scientist we've spoken about before.

2-b. water bonding
the water molecules bond to the free ions (attraction):
-the negative oxygen ends of water molecules will surround the positive sodium ions (Na+);
-the positive hydrogen ends will surround the negative chlorine ions.
The ions are "FILMED" by water
This will cause a shrinkage of the total volume
You can say in this case that as the salt ions are smaller than the water molecules they enter the voids between them (which is another way to say filmed)

Now the question is "is it applicable to sugar"?
the answer is straight in the last lines of my post! the sugar molecules are greater than the water ones and the sugar DOESN'T FORM IONS!

P.S: here is the original title of the French publication
"Mémoire sur les dissolutions salines" by
Alphonse MICHEL and L. KRAFFT
in "Annales de chimie et de physique" Série 3, Tome 41
pages 471 to 484

 

[Joesteam]

Very interesting. One question, does the volume get smaller based on the initial volume of just the water or on the increased volume of the salt & water before the salt dissolves?

Joe Lambert

http://www.control-associates.com/

 

[25362]

I may be wrong, but my reading of tabulated densities of water and NaCl solutions at 20 deg C vs water at the same temperature shows no volume contraction. See, please


____________solution__________ Just water
Water, g NaCl, g Density, g/mL Vol, mL

99.9 0.1 0.9989 100.11 100.08
99.0 1.0 1.0053 99.473 99.178
90.0 10.0 1.0707 93.397 90.162
85.0 15.0 1.1085 90.212 85.153

Are you implying that there is a cooling effect that should be taken into account ? Kindly comment.

 

[EngAbdelkader]

Hi guys I see that now you are all looking to this unusual quiz or let us call it puzzle!

First, Joesteam by volume decrease I meant the total volume taken apart (before dissolution) is less than the volume observed after dissolution.

Secondly, 25362 2 observations for your interesting post and computation!

1- the solution I was speaking about is saturated. in this issue I am not so clever but I think that the solubility of the NaCl in water at room temperature is about 35.9 g/ 100 ml that means that your calculations where far from the point!
that's for the computation

2- and YES there is a slight cooling effect (3 : 5 °C) as the dissolution of NaCl salt in water occurs.
The reaction NaCl(s) --> Na+(aq) + Cl-(aq) is endothermic!

Finally I would say that we are in need of a chemical engineer in here to correct us if we are wrong! hahaha
As you've noticed I am flagged "petroleum" in here but actually I am mechanical engineer evolving in the oil industry labyrinth!

Kader.

 

[Zoobie]

There is no disputing 25362's data. I think this thread (and the linked experiment) illustrates simply that the volume of water plus undissolved salt is greater than the combined solution. If you use a density for salt of 2.16 g/ml (density of crystal not bulk density) with the masses that 25362 gave you get that the solution volume is slightly less than the sum of the two volumes for the constituent materials. This approaches the 2.5% number as quoted in the experiment at higher concentrations.

 

[EngAbdelkader]

Thanks Zoobie so you saved us!
I told you we were in need of a "chemical" in this side

 

[25362]

I understood the question was whether the solution has a smaller volume than water by itself before adding salt.
Anyhow, it was an interesting subject...

 

[25362]

BTW, not all salts actually cool the solution upon dissolving in water, NaCl (endothermic heat of solution: +3.9 kJ/mol), NH₄Cl and NH₄NO₃ (cold packs) do; but, for example, anhydrous CaCl₂ and MgSO₄ heat up and are used in hot packs.