Are solubility and dissolving the same thing?

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BlondeChick

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okay guys. so tell me if the following is wrong, right.

2. Use your textbook as a reference to look up structural formulas for the solutes and solvent you used today. Can you explain any solubility trends based upon structure of these substances? How does this relate to the adage that “like dissolves like”?

Organic compounds such as naphthalene and benzoic acid will not dissolve water because the molecular structures of these compounds is such that they are hydrophobic and nonpolar; for a substance to dissolve in water, it must be able to form similar interactions with the water molecules. This is not possible, however, given that water molecules are polar and hydrophilic. Incidentally, the water molecules do not have enough energy to break apart the hydrogen bonds in the naphthalene and benzoic acid; in which case, a homogenous solution will not form. On the other hand, ionic compounds are also polar; when placed in water, there will be ion-dipole interactions between the hydrogen end of the water molecule and the negative ion, and between the oxygen end of the water molecule and the positive ion.


Oh yeah, and tell me if this right?

Furthermore, it is not always possible to tell, on the basis of mere observation, what types of bonds the molecule possesses. For example, a water soluble substance may be either ionic or polar, but only a strong electrolyte will dissociate into its constituent ions. Using a circuit tester, one could differentiate between a semi-metal (poor conducter of electricity) & a metal (ionic, strong electrolyte).

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isn't benzoic acid soluble in water? out of all the functional groups i thought it was the most soluble because of the two oxygen atoms. or is it not soluble because of the benzene ring?
 
Uh, the benzoic acid was insoluble for the two temperatures we did. BUt the naphthalene showed limited solubility in 10 deg C water. Someone whos done Organic can probably answer ur ques better. Getting back to my ques: if the stuff sinks to the bottom of the test tube, tell me if i'm wrong here - and when u stir it, it stays at the bottom, that means the stuff is (I) insoluble, and (II) doesn't dissolve?
 
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BlondeChick said:
Uh, the benzoic acid was insoluble for the two temperatures we did. BUt the naphthalene showed limited solubility in 10 deg C water. Someone whos done Organic can probably answer ur ques better. Getting back to my ques: if the stuff sinks to the bottom of the test tube, tell me if i'm wrong here - and when u stir it, it stays at the bottom, that means the stuff is (I) insoluble, and (II) doesn't dissolve?
Well yes, that makes sense, right? If it doesn't dissolve when you stir...it doesn't dissolve.
 
benzoic acid is not soluble in water at room temp, but is soluble in Sodium Bicarbonate and Sodium Hydroxide. I ran this test a few weeks ago for an ochem lab.

A carboxylic acid will be soluble in water if it has a low molecular weight. I think that means 4 carbons or less.

And looking at OP questions makes me so happy to be done with orgo! sorry I can't give a solid answer.

mdsquared said:
isn't benzoic acid soluble in water? out of all the functional groups i thought it was the most soluble because of the two oxygen atoms. or is it not soluble because of the benzene ring?
 
cool. u guys r real brainy. :cool: got another new question.

3. What methods, including differences in temperature or polarity, could be
used to separate a mixture of sand, copper, sulfate, & naphthalene?

you could filter out the sand using the thingie with the holes in it (or else vacuum filtration). but i dunno about the rest. the only seperation technique we did so far was paper chrom.
 
BlondeChick said:
cool. u guys r real brainy. :cool: got another new question.

3. What methods, including differences in temperature or polarity, could be
used to separate a mixture of sand, copper, sulfate, & naphthalene?

you could filter out the sand using the thingie with the holes in it (or else vacuum filtration). but i dunno about the rest. the only seperation technique we did so far was paper chrom.
Do you mean "copper sulfate" rather than "copper, sulfate"? If so, the copper sulfate is water soluble (it's also a really pretty color of blue, like the hot springs at Yellowstone) while the naphthalene is not, so you would indeed filter out the sand by rinsing the solution with hot water (which should likely dissolve both the naphthalene and the copper sulfate) and then cool the solution to <10 degrees celsius to make the naphthalene precipitate. Filter the cold sultion to remove the naphthalene. If, in fact, the lab wants you to then separate the copper from the sulfate, this can be done by adding sodium hydroxide. Sodium is extremely soluble, but any hydroxide other than one with an alkali earth metal is insoluble, and sulfate is highly soluble. hence, the copper would precipitate as copper (II) hydroxide, while the solution would contain the Na+ and sulfate ions in solutions. At this point, further filtration would separate the copper from the sulfate.

Check out this for some quick solubility rules.

http://www.ausetute.com.au/solrules.html
 
Yeah, that was written incorrectly. It should have read as sand, copper sulfate, caffeine, & naphthalene. Your method makes sense; for the caffeine, I guess I could just use a funnel (since the caffeine is made up of crystals).
 
Cozmosis said:
Naphthalene doesn't have any hydrogen bonding. They don't dissolve because the ring is so freaking nonpolar it is ridiculous.
Indeed. If you refer to it as hydrogen bonding, the professor will almost certainly mark it wrong. Hydrogen bonding is only in places where the hydrogen is polarized. It occurs in water and in other polar parts of water-soluble compounds. It would be more correct to say that the naphthalene is incapable of breaking the hydrogen bonds in the water. The naphthalene is insoluble not because the naphthalene is so fond of bonding to itself, but rather because the water is so fond of bonding to itself that it doesn't want some crappy non-polar stuff like naphthalene coming in a breaking up the party. Naphthalene stays together in this situation because water won't associate with it, much like a pack of nerds grouped together outside a popular club--the nerds don't necessarily like eachother, but they hang together because of their commonality in not being able to get into the party.

Blonde Girl said:
Furthermore, it is not always possible to tell, on the basis of mere observation, what types of bonds the molecule possesses. For example, a water soluble substance may be either ionic or polar, but only a strong electrolyte will dissociate into its constituent ions. Using a circuit tester, one could differentiate between a semi-metal (poor conducter of electricity) & a metal (ionic, strong electrolyte).

Don't confuse an electrolytic solution with a metal. Chloride is not a metal, but it can contribute to the conduction of a current through water nontheless. An ionic solution conducts electricity, as does a solid metal, but in totally different ways. The metal has conjugated electircal orbitals that allow the free passage of electrons through it's conducting "bands" of energy--the atomic nuclei stay put, while the electrons move. In a solution with ions, cations are atracted to the negative electrode, and anions to the positive, but none of these ions necessarily need to be metals at all--it could be a solution of NH4+ and I-, for instance. A curcuit tester could let one differentiate an electrolyte from a non-elctrolyte in a solution, or it could be used to test the conductive properties of metals or non-metals, but this would be a different thing. Solid conducting metals are non-ionic, and they share their electrons freely from atom to atom.
 
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