[galpinj]

Hello guys,

Apparently, homogeneous solutions can only be separated by chromatography or distillation and not by filtration; however, I find this a little difficult to believe. If I had a semi-permeable membrane that was only permeable to Chlorine atoms, for example, wouldn't I be able to filter the Cl- ions out of an aqueous (and homogeneous) NaCl solution? Why is this not the case?

Also, I'm having a lot of difficulty with grasping the concept of "solid solutions". I've read that alloys can be thought of as solid solutions, which include at least two different metals that are mixed together. However, if they are not fully mixed, the two metals can form a heterogeneous mixture (insoluble metals).

I have a couple questions about these facts: What causes this difference between solid solutions and heterogeneous mixtures for metals? Is a solid solution the same thing as a substitutional alloy? How about an interstitial alloy? What is it that causes certain metals to be "insoluble" as seen in heterogeneous mixtures?

Thank you for your time and assistance

[Corribus]

Regarding alloys, I think this is really too general a question to answer on a forum like this. You'd do well to find yourself a good materials science textbook that covers the chemistry and physics of solids, in particular metals. I recommend Callister and Rethwisch, Materials Science and Engineering.

[galpinj]

Regarding alloys, I think this is really too general a question to answer on a forum like this. You'd do well to find yourself a good materials science textbook that covers the chemistry and physics of solids, in particular metals. I recommend Callister and Rethwisch, Materials Science and Engineering.

Thank you so much; I found the book and, as you said, it provides succinct and explicit answers to all my questions regarding solid solutions. For anyone that may be interested, both interstitial and substitutional metal alloys can form solid solutions. Differences in atomic size, EN, cell unit structure, etc. will all influence whether a homogeneous or heterogeneous solution will form.

I have yet to figure out the answer to my first question, however, and appreciate any input from the community

[Corribus]

Let's consider two scenarios based on a container that is separated into two chambers by a membrane. In the first case, the membrane is only permeable to glucose. In the second, it's only permeable to chloride ions. Now in the first case: on one side of the chamber you have distilled water and the other chamber you have a concentrated glucose solution. In the second, you have water on one side and a concentrated solution of sodium chloride on the other. At t ∞, how do you imagine the equilibrium points of each system will appear?

[galpinj]

Certainly I would expect a homogeneous mixture for glucose. In the second, I would expect chloride ions to move into the other chamber (along with more water, if possible) while the sodium remains in the chamber to the right. In the second example, the solutions would have different compositions, so I'm tempted to say that the two are no longer homogeneous; however, homogeneous solutions can't be filtered, so....

[Corribus]

Like most everything, diffusion (along with related phenomena like mixing) is governed by thermodynamics. In an ideal solution, the responsible party is basically entropy. Indeed, in the first scenario the glucose will spontaneously diffuse through the membrane into the other chamber until the concentrations on both sides are equal - the rate of forward diffusion is balanced by the rate of backward diffusion. This would happen in the second scenario as well except there are other forces at work. As chloride diffused, this would result in a gradually increasing positive charge in the original chamber, and a gradually increasing negative charge in the other one. Separating charges requires energy, quite a bit, in fact, and therefore this diffusion would not be a spontaneous process. We could envision making it happen by applying an electric bias, but even then you'd quickly reach a point where the energy cost is too great for further charges to move effectively. (This is, by the way, essentially why a salt bridge is required in an electrochemical cell.)

In a filtration, gravity is a force of bias (slowing down the rate of backward reaction), to drive the equilibrium point forward. But gravity would likely not be enough of a forward bias to overcome the negative electrostatic force that would accompany the separation of charges occurring if chloride only passed through your filter.

This all presumes the hypothetical chloride-only filter could be created in the first place.