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Author: Subject: Are solutions always even?
KidCurry
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[*] posted on 7-7-2005 at 11:27
Are solutions always even?


Theoretically a solution (with the solute completely dissolved) always is even. For example, if you dissolve 100mg of a compound in 100ml of water and you take 1ml it will always contain 1mg. To what degree is this true practically? Can you always be assured that the solution is even as long as everything is dissolved?
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12AX7
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[*] posted on 7-7-2005 at 12:18


Even in what way? Ratio of weights? Not counting the quantum possibility of all molecules shifting in unison to one side in one moment, it'll be valid for some pretty small "solutions", I'd say on the order of thousands of molecules. I don't know if water is still water at that droplet size...

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unionised
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[*] posted on 7-7-2005 at 12:53


Even ignoring the quantum effects there is some "unevenness" but, except for very small amounts or very dilute solutions, it's not significant.

The other thing is that to a totally unimportant (and probably immeasaurable) extent, the denser molecules tend to be more concentrated at the bottom of the solution.

On a practical note there can be a much bigger effect if the solute is absorbed by the walls of the container.
If I want to make a 1 ng/ml solution of something and I do this by taking a gram of it (enough to weigh easily), disolving that in a litre, taking 1 ml of this solution and diluting that to a litre and repeating the dilution, I should get the solution I want. But, if I make that solution which only has a microgram of stuff in it, some of that might stick to the bottle and the solution won't be what I expect.

[Edited on 7-7-2005 by unionised]
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Quantum
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[*] posted on 7-7-2005 at 14:46


I would say it is accurate for all pratical use as this is how they lay sheets full of our well known lysergic based compound. Of course even in that case it could be +-25µg and not matter.



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Simon
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[*] posted on 7-7-2005 at 16:41


Using a slow siphon through paper towel I once managed to put a layer of water on top of a permanganate solution. Despite the denser liquid being happily at the bottom, the two mixed themselves together over the next few hours.
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[*] posted on 7-7-2005 at 20:28


Hey Simon that sounds neat. You kow if you add concentrated HCl or any strong highly soluble solution to pure water in sufficient light you can see "invisible" eddy current movements of the liquids if they are not stirred. Sometimes I like to be as gentle as possible with this dilution so I can see the dynamic fluidic events unfold. Given sufficient surface interface and time though solutions find there stable equilibrium. There are entire volumes of text devoted to this topic(YAWN!) but I am content in just watching the swirly wirlies:P;)



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12AX7
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[*] posted on 8-7-2005 at 00:27


Sulfuric acid does that quite well, being well...dramatically more dense than water.

You can also chuck rock salt in the bottom of a jar and pour water on top. Everything settles before the salt dissolves appreciably, then time diffuses Na+Cl- through the bottom mostly. Shift it to the side and there goes the twirlies. ;)

Quantum: exactly. Individual molecules are so small, you've got billions of them on a square milimeter of that sheet.

Tim




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[*] posted on 8-7-2005 at 04:01


The classic "crow foot" voltaic cell might be relevant here. IIRC, they poured copper(II) sulphate at the bottom of the container, and then carefully added another layer of zinc sulphate at the top. The diffusion of ions through the ions were a important part of the cell.



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