I've been doing various simple experiments with table salt/NaCl. I just recently did an experiment where I tried to find the density of some
re-crystalized NaCl solids by the water displacement method in a graduated cylinder. The results I came up with (around 2.5g/mL) were obviously
wrong(should be 2.16). Upon doing some research I read that the water displacement method cannot be used for finding the density of water soluble
substances.
My question for you all is: why is that so? Shouldn't the atoms from the crystal still take up the same amount of space in the water when dissolved?
I even observed that since the volume of the water didn't change when I dissolved all of the solid that was already in the water.
[Edited on 21-8-2008 by HydroCarbon]Fleaker - 21-8-2008 at 19:30
Without going into great detail let's think of it this way: the salt you attempted to take a density on was in a crystalline state meaning its atoms
were arranged in a specific order (face centered cubic in this case). Now think, take this salt and break it out of its neat, ordered, favored crystal
lattice and let it loose into your randomized solution.
The other option is that your salt had water in it still.Ozone - 21-8-2008 at 19:41
Perhaps displacing acetone and correcting for density would yield the result that you are looking for.
Cheers,
O3Klute - 22-8-2008 at 02:54
Becasue of the interactions between the ions and water, the sum of volume of the solid and the volume of the liquid will not be equal to the volume of
the solution.
This is also the case with organic substance which can make hydrogen bonds: mix 50mL of water and 50mL of EtOh, and you will not have 100mL solution.
But, if you mix 50gr of water and 50g of EtOH, you will obtain 100g of solution (matter conservation).
The ions being solvated, the molecules of water will "crowd" around each ion via hydrogen bonding, and so diminish the the space between the
molecules.woelen - 22-8-2008 at 05:11
This effect of decreasing volume is very common. For almost all single phase systems with compounds A and B, and mixing of their volumes vol(A) and
vol(B), it is true that vol(A) + vol(B) > vol(mix of A and B).
Are there any compounds A, B which expand on mixing: vol(A) + vol(B) < vol(mix of A and B)?chemkid - 22-8-2008 at 05:53
Yes i believe a mix of sugar and water does this. I can find out....12AX7 - 22-8-2008 at 06:46
If the mixture expands, that would also imply lower solubility at higher pressures. But how much higher? Are we talking pipe-bursting pressures
here?
TimHydroCarbon - 27-8-2008 at 20:59
Thanks for the responses.
I salted out the water from some isopropanol and tried it in that I got much better results: 2.26g/mL. Only .1 off which is most likely just accuracy
error from my graduated cylinder.Nicodem - 28-8-2008 at 07:22
HydroCarbon, try using some hydrocarbon for your density by displacement experiment. For example, by using toluene.
NaCl is not particularly soluble in isopropanol, but if it contains some moisture (apparently you didn't use dry isopropanol) the crystals will
absorb some and this will influence your measurement.bfesser - 28-8-2008 at 07:46
You could also try using a calibrated specific gravity bottle or pycnometer to get a more precise volume measurement. Of course, they're often
expensive or hard to obtain...unionised - 28-8-2008 at 10:39
Salt is not soluble in a saturated solution of salt.
Salt water is easier to get than toluene.chemkid - 28-8-2008 at 19:18
Thats a fantastic method unionised!
Is this sort of post discouraged? I think i read something somewhere....
[Edited on 28-8-2008 by chemkid]bfesser - 28-8-2008 at 19:32
Quote:
Originally posted by unionised
Salt is not soluble in a saturated solution of salt.
Salt water is easier to get than toluene.
But won't the saturation concentration vary with temperature anyway?12AX7 - 29-8-2008 at 03:17
Actually, NaCl has the advantage of very little change in solubility vs. temperature.
