[Wongman]

I have a question about what happened in a chemistry experiment. During the thermodynamics unit, we were talking about entropy driving a reaction at high temperatures and how oil and water doesn't mix because it isn't favorable from an enthalpy standpoint. The oil breaking up the intermolecular attractions between other water molecules goes to higher energy because the attractions between oil and water are weak. The guy who I sat next to in AP Chem said that we should add oil and water to a container and heat it so that entropy will drive the reaction and they will mix. To allow the water to reach a sufficient temperature, we would add a really soluble salt such as KI or NaI. So after declining the offer to do it myself, he went ahead and did it, but the oil and water boiled so violently that it splashed out onto him... we're not totally sure how he didn't get scalded... well, we decided to do it as a project, and we made some glass wear for the flask we were working in so it couldn't boil out, and we added bumping stones and more salt. So we re-did the experiment. We had water, lots of NaI, and olive oil, which we heated to around 140-150 oC. At first the water was just boiling with the oil sitting on top, but then for one trial we removed most of the olive oil and heated the flask to 148 oC and then all the NaI just came out of solution. After the experiment, there was this crystal, salt oil sludge in the flask. I tried to repeat the experiment, but I had a hard time getting all the NaI to redissolve because a lot of the water had evaporated off. We then repeated this experiment with mineral oil and got similar results, and it clearly appeared that the oil was dissolving and the salt was no longer soluble. But now, we're not really sure what happened  because the salt shouldn't have come out of solution from what I know even though common sense would tell you that it got forced out by the oil dissolving. However, we ran an experiment were we showed that two salts dissolving with no common ions between them did not significantly affect the solubility of each salt, so we aren't really sure what happened in our experiment... could anyone offer any insight?

[tamim83]

I really think that you did not cause more ofthe oil to dissolve in the water.  The reason why your NaI fell out of solution is because as you heated the salt water solution and lost water as water vapor, less and less NaI was able to stay in the remaining water.  As you probably figured out, it is quite simple to boil off the water in a salt solution and leave the solid behind.  So that is what happened there. 

Your idea was a very interesting one, heating usually does increase solubility.  But I think that the "like dissolves like" principle simply wins out in the end in the case of oil and water, exspecially in the liquid phase where the intermolecular forces between the water molecules are extremely strong.    With out another chemical called an emulsifier (which would allow the oil to break up and somewhat dissolve in the water, this is what dishwashing detergent is), you are not going to be able to get oil to dissolve in water. 

[Yggdrasil]

I have heard that oil won't disolve in water because it is entropically unfavorable.  The reason being that water interacts more strongly with itself than oil, so the water forms a highly ordered solublization shell around oil droplets which maximizes interactions between water molecules and minimizes interactions between oil and water.  In order to minimize the area of water which much form these highly ordered solublization shells, the oil aggregates in large droplets in order to minimize the surface area in contact with the water.  This is aggregation of hydrophobic materials in an aqueous environment is known as the hydrophobic effect.

[gregpawin]

Also, I'd have to point out that you have one of your phases going into the gas phase before you find your alleged 1 phase 2 component system.  If you don't find this phase before the boiling of water, increasing the temperature won't do anything, because obviously the water temperature's not going to increase beyond the elevated boiling point of the solution.

[GCT]

I have a question about what happened in a chemistry experiment. During the thermodynamics unit, we were talking about entropy driving a reaction at high temperatures and how oil and water doesn't mix because it isn't favorable from an enthalpy standpoint. The oil breaking up the intermolecular attractions between other water molecules goes to higher energy because the attractions between oil and water are weak. The guy who I sat next to in AP Chem said that we should add oil and water to a container and heat it so that entropy will drive the reaction and they will mix. To allow the water to reach a sufficient temperature, we would add a really soluble salt such as KI or NaI. So after declining the offer to do it myself, he went ahead and did it, but the oil and water boiled so violently that it splashed out onto him... we're not totally sure how he didn't get scalded... well, we decided to do it as a project, and we made some glass wear for the flask we were working in so it couldn't boil out, and we added bumping stones and more salt. So we re-did the experiment. We had water, lots of NaI, and olive oil, which we heated to around 140-150 oC. At first the water was just boiling with the oil sitting on top, but then for one trial we removed most of the olive oil and heated the flask to 148 oC and then all the NaI just came out of solution. After the experiment, there was this crystal, salt oil sludge in the flask. I tried to repeat the experiment, but I had a hard time getting all the NaI to redissolve because a lot of the water had evaporated off. We then repeated this experiment with mineral oil and got similar results, and it clearly appeared that the oil was dissolving and the salt was no longer soluble. But now, we're not really sure what happened  because the salt shouldn't have come out of solution from what I know even though common sense would tell you that it got forced out by the oil dissolving. However, we ran an experiment were we showed that two salts dissolving with no common ions between them did not significantly affect the solubility of each salt, so we aren't really sure what happened in our experiment... could anyone offer any insight?

First...heating a water+oil mixture is generally not a good idea, water does dissolve oils to certain extents, even in the vapor phase and increases the surface area by the formation of "some oil bubbles" with the cavity formation due to the water in the gaseous state at the high temperature.  So consult a supervisor about this matter before continuing on if you should do so e.g. if you throw some water toward a burning candle, it may result in an small scale explosion.

As for the phenomena you've witnessed it may have stem from a liquid extraction phenomena, if you've ever taken an organic chemistry lab you would remember the concept I'm referring to.  Some NaI does dissolve in the the oil, probably to a greater extent then NaCl due to the relative polarizable nature of an Iodine atom, and when the oil evaporates as some of the posters here have indicated, the NaI gets left behind, at the sides of the glassware for instance where some of the oil vapors may have stuck to during the evaporation process and then completely escaped the reaction chamber.  Also check to make sure that those crystals are actually NaI, you may have some impurities in there also.  Get its melting point for instance and find assess the impurity factor by noting the range in the mp.