Question QUESTION: Hi, thanks for suggestions you gave me a few days ago although
you probably won't remember. I am trying to make a 3 layer liquid that can last for more than a week. My idea was to have a fully saturated solution of salt water then a layer of fully saturated vinegar (acidic acid) with water and top it off with oil. The theory I had behind this is that bottom layer's water is so saturated with salt that it won't mix with the fully saturated vinegar on the second layer and oil just have a tendency not to mix. Can you tell me if my method is correct or not and problems that might possibly occur? Also I am not sure how I can tell if vinegar is fully saturated with water.
ANSWER: hi, and thank you for your question.
I'm afraid your method at present won't work; I did try it myself to be sure, and what you end up with is a two layer liquid.
The problem is the aqueous layer; the salt and the vinegar, particularly the vinegar. Specifically, the problem is that you can't saturate an aqueous solution with water; if I put a cup of vinegar in a large beaker, I have a solution of ethanoic acid in water (vinegar is about 4% ethanoic acid). I can add as much water as I want, and all I'll end up with is more and more dilute vinegar. If I start with 100ml of vinegar and add 100ml water, i end up with 100ml vinegar in a total of 200ml, 100/200 = a 50% solution. If I'd added 1,000,000 ml water, the calculation's the same; I have 100ml vinegar in 1,000,100 total; this is a 9x10-5 (0.00009%). A very dilute solution, but still a solution.
So all that happens is some of the salt-water (which in any case has a tenancy to dilute by diffusion) mixes with the vinegar to get a 3-component solution (water, vinegar and salt) in one layer, with the oil floating on top.
As for possible workable systems, I don't know if you have access to a lab; if you do there are a few good ideas you could use. If not, on a household level, your best bet is to use a viscose dense liquid as your base layer, e.g. "golden syrup", maple syrup or similar. These are concentrated sugar solutions, and will eventually dissolve in water, but because of their thickness, it takes ages for this to happen, certainly longer than a week..
My suggestion would therefore be; a layer of syrup as the base layer, a saturated solution of sugar in water as the next layer, and vegetable oil as the third. The reason for using sugar solution rather than plain water is that it lowers the rate at which sugar escapes from the syrup layer, and so slows down the dissolving process.
You could also add a couple of drops of food coloring to the aqueous layer, and try using a mineral oil as the oil layer; the science doesn't change, just the appearance of your density column.
Hope this helps, again let me know any other points you want to discuss. Thanks.
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QUESTION: Yes, I do have access to a lab. I thought the definition of the word "saturated" means that it is at its maximum dissolving point that it can no longer dissolve anything anymore, what I thought is that in order for water to dissolve vinegar water need to form salt crystals first then be able to dissolve vinegar. I am not sure how the theory work in this case compare to the definition. I don't think I can use syrup because I am trying to do a biphospholipid layer model of a cell membrane that when you bump it on the side everything in there can move freely like a cell membrane.
ANSWER: Thanks for your response.
Firstly, it's not necessary for water to form salt crystals in order to dissolve. This will be explored in the next section, but you're confusing your large scale system (where dilution and transfer occurs by diffusion), and cell-membrane dilutions and movements, where one of the suggested methods (though it is not a completely accurate model) is encapsulation by fatty acids.
1) Why won't the large-scale work?
The essence of the reason your method doesn't work (again, this is experimentally verified, if only in my kitchen!), that when you mix the two layers, you are essentially dissolving salt water in a vinegar solution, forming a complex but, to all intents and purposes, complete solution.
And the point I was making in the first response is that, in no part of your suggested system are you going to be saturating a solution (except for the salt solution being saturated before you start). Remember, water is the solvent for both salt and vinegar (vinegar being about 96% water anyway). If you take a concentrated glass of orange squash (which is a solution of juice in water) and a more dilute solution of strawberry squash, and mix them, you wouldn't expect them to separate in to two distinct layers, because the water is the same solvent for both the orange and the strawberry, so you're just going to make a mixed solution with twice the volume of the original solution. The same logic applies here.
When things dissolve, they form intermolecular bonds with the solvent; water is a great solvent because of its hydrogen atoms which can easily form hydrogen bonds with solutes that also have hydrogen bonds. It also has the ability to bond to ionic materials because of the dipole-dipole bonds that give Hydrogen a delta-plus charge and oxygen a delta-minus. So the salt solution by itself is Na+ and Cl- atoms bonded to water ions, and the vinegar by itself is water bonded to ethanoic acid molecules. Mixing the two means you have a huge excess of water, so both substances stay in solution because there are (by a huge factor) many times more water molecules than solute ones. In addition, both NaCl and Ethanoic acid are highly soluble, so there isn't chance for competing dissolution power to displace one or the other molecule.
2)Why is the cell membrane situation so different?
The reason that your system is going to be very, very difficult to model is that the phospholipid bilayer (in real life) is held together and transfers molecules using the fact that, at such small scales, both water and other materials behave very differently; at cell level, the surface tension of water makes it essentially a gel, so the forces involved in transferring solutions across a membrane are totally different to the forces acting on a large-scale model. Incidentally, it is this difference that allows such a membrane to achieve what you were trying to model in the first place. Carrier proteins, fatty acids, etc are also used to a great extent in the transfer of materials across such membranes.
If you want to model the phospholipid system (which we often do at the uni I teach at), you could a) Make 3 jellies of different densities, dyed a different color; this is physically more realistic than trying to create a large-scale system that models the nanoscopic.
b) This paper http://cnx.org/content/m15254/latest/ suggests a model more appropriate for the advanced biology lab, using the only everyday situation where water forms structures more akin to those experienced in a cell; soap bubble.
Hope this helps, again, follow-ups are welcome.
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QUESTION: Is there anyway I can saturate vinegar?(Adding salt perhaps?) Would that make a difference? If not, is it possible that I can use other liquid to accomplish what I desired?(other than oil of course) If nothing works I am going to go with your plan of jello.
Answer Thank you for your follow-up.
There are two problems with the vinegar-salt idea; 1) Vinegar is mostly water, so in essence, all you do when you add salt is to make a solution of salt in water. You're not saturating the solution, because to saturate vinegar, you'd have to add more concentrated ethanoic acid (in fact, although this would increase the concentration of the vinegar, it wouldn't affect the mixing of the two liquids to any great extent).
To reiterate, the main problem is that you have two aqueous solutions on a large-scale setup; when you mix the two, you just get a big mixed solution.
The second major problem is that mixing salt and vinegar produces a chemical reaction in addition to a physical, forming a dilute solution of HCl. So what you put in to your density column will not be the same as what forms once the two solutions react.
Overall, I can't see any way, on a large scale, of combining the two (salt solution and vinegar) to form distinct, separate layers, which I believe is what you need? Remembering what I said about the difference between cell-scale and large-scale reactions, what you have to do is to get the same effect (layering) in a different way (e.g. using jellies).
When we do this at our place, our procedure is as follows: (using 3 different jelly flavors to get 3 different colors):
1) In a jug, mix the first packet of jello with hot water (see the packet for amount and safety advice).
2) Stand the container you want to make your model in (large glass tube or beaker probably) on a tray in a domestic refrigerator.
3) Pour the hot jello in to the container in the refrigerator, and leave it to set. Obviously, depending on the size of your container, you may have to use a larger / smaller amount than the manufacturers recommend.
4) Wait until the jello sets, then repeat the pouring/setting/next color procedure for the next two layers. Each setting may take an hour or more, depending on the amount of jello and the efficiency of the refrigerator.
The following page has a picture of jello made by this method. In this case, the person has taken the cast jello out of the mould rather than leaving it in, but it gives you a good general idea of the effect achieved. http://www.sliceofawesome.com/2008/03/
There are other liquid systems you could use to demonstrate layering, but these are banned in most areas because they rely on tetrachloromethane (Carbon Tetrachloride, CCl4) as a base. Overall, it's simpler and clearer to go with Jello.
Hope this helps, again, follow-ups are welcome! Thank you.
