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Name: Dan B.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: Saturday, November 16, 2002


Question:
Yellowish Corn oil for cooking floats nicely on White vinegar with a drop of food coloring. This is an easy and cheap 2 layer liquid density demonstration. Three layers would be much more impressive. What is a relatively cheap and easy to get liquid that is insoluble in vinegar AND Oil?


Replies:
Dan,

Use corn oil, dyed water, and then drop a few mothballs into the system. Insoluble in either corn oil or water, they'll sink to the bottom because their density is greater than that of the oil or water. Mothballs are made of naphthalene. Moth crystals are para-dichlorobenzene. Use mothballs.

Regards,
ProfHoff 518


HOW ABOUT 4, 5,OR 6 LAYERS?? This makes a great science demo. You can even make it a class competition to see who can get the most layers. I will give you an outline of how to do this, but the details you will have to work out depending upon what materials you have available. The principle is to have alternating layers of immiscible solutions with decreasing density starting from the bottom and going to the top. Use a long glass tube: A large test tube is O.K. but the larger the tube, the more dramatic the show. Hint: When adding the solutions, dribble them down a stirring rod to avoid turbulence. Now here is the approach:

You want to start with the most dense SOLUTION (either aqueous or organic liquid) you can find. An example might be a saturated solution of p-dichlorobenzene (moth balls) in xylene -- do not know the solubility off hand, but it is large. It may be so great that the density of the saturated solution may even be greater than the density of solid p-dichlorobenzene itself, if the spacing of crystal lattice is large (I just do not know.) Both are available at a hardware store. Or you can start with the most dense aqueous solution you can find. A saturated solution of Mg(SO4) (Epsom salts) or CaCl2[density of sat.soln.= ~1.4gm/ml], (sidewalk de-icer) available at pharmacies or hardware stores, copper sulfate (available at farm supply stores) You can color any aqueous layer with a drop or two of vinegar, HCl, NaHCO3, or sodium hydroxide (Draino)[the density of saturated NaOH is also about 1.4, but you have to handle this carefully], etc. to change the pH without changing the density, and of course a drop of your favorite pH indicator (use several if you have them) to get different colors at the different pH's. If you have some KI available, a drop or two of tincture of iodine (from the pharmacy) will give an intense blue color from the I3(-1) ion. Even iodized salt may contain enough iodide to work, but I've never tried that. Now you just layer the immiscible layers, decreasing the density of alternate layers by decreasing the concentration of the heavy component (p-dichlorobenzene or Mg(SO4)) if you select those two components to adjust the density. Also, you can use glycerin, ethylene glycol, or propylene glycol to reduce the density of aqueous phases, but you will have to be careful that the organic solute is insoluble in a contiguous layer. You can also check the density of urea/water solutions since urea is Note that the high concentration of salt in the aqueous phases dramatically decreases the solubility of adjacent water-insoluble phases.

Obviously, the challenge is to find the most colorful combinations of alternating aqueous and water-insoluble solutions. You can see that using SOLUTIONS rather than pure liquids gives you a much better handle on adjusting the density without having to find a new liquid. If your chem lab has other solvents available, so much the better. You can find the density of many of the various aqueous combinations in the Handbook of Chemistry and Physics.

Hint: Students should practice layering three solutions (A/B/C), (B/C/D), (C/D/E),... at a time before putting together (A/B/C/D/E/...) to make sure the densities are correct and that no unexpected mixing makes the layering fall apart, and that the layers are stable over time (hours or days). You can check the stability by just letting the tri-samples age. An indicator such as I2 and pH indicators can diffuse through several layers because it is soluble in both aqueous and non-aqueous solvents. You should be aware of that. If the layers are at least 2-5 cm thick diffusion across adjacent layers will be slow enough.

HAVE FUN!!!

Vince Calder



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