Solubility, melting points and boiling points - Chemistry LibreTexts
The difference between the boiling points of pentane (36 °C) and 1-butanamine the boiling point can be defined via the relationship between the evaporation Carboxylic acids with low molecular weights are soluble in water because the. There may well be a small indirect relation. Both boiling and dissolving a solid require that the inter-molecular bonds are broken. Hence if the. Solubility in water. The two most important (and somewhat related) factors are the polarity of the molecule and the presence of hydrogen.
And in our everyday language, salt is table salt. It makes food salty, or sodium chloride. And this indeed is both a salt from the Food Channel point of view and from the chemistry point of view, although the chemistry point of view does not care about what it does to season your food.
The chemistry point of view, the reason why it's called a salt is because it's a neutral compound that's made with ions. So we all know that this is made when you take sodium. Sodium wants to lose its one electron in its valence shell.
Chloride really wants to take it, so it does. Chloride becomes a negative ion and sodium is a positive ion, and they stick to each other really strongly because this guy's positive now, and this guy's negative after he took away his electron.
Imagine your house is too small, so you have to give away your dog to someone who has room for the dog, but now you have to hang out at that person's house all the time because they have the dog you love.
I don't know if that analogy was at all appropriate. But I think you get the idea. A salt is just any compound that's neutral.Intermolecular Forces - Hydrogen Bonding, Dipole Dipole Interactions - Boiling Point & Solubility
The other common ones, potassium chloride, you could do calcium bromide, or I could do a bunch of them, but these are all salts. And what we want to think about is what happens when you try to essentially dissolve these salts in water. So we know what water is doing, liquid water. So let me draw some liquid water. So if that's the oxygen and then you have two hydrogens that are kind of lumping off of it, I'll draw it like that.
I'll draw a couple of them. And then, of course, you have another oxygen here. Maybe the hydrogens are in this orientation because the hydrogen ends are attracted through hydrogen bonds-- we've learned this-- to the oxygen ends because this has a slight negative charge here, a slight positive charge here.
These are the hydrogen bonds that we've talked so much about.
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And maybe you have another oxygen here and it's got its hydrogens there and there. You have some hydrogen bonds there. I could do another oxygen here, and you can kind of see the structure that forms, although what I'm drawing, this is actually more of a-- if you were in a solid state, this would be kind of rigid and they would just vibrate in place. In the liquid state, they're all moving around. They're rubbing up against each other, but they're staying very close. Actually, the liquid state for water is actually the most compact state for water.
Now, when you're dealing with stuff like this-- these are moving around, maybe this guy's moving that way, that guy's moving that way-- and you want to dissolve something like sodium chloride. Sodium chloride's actually quite a large molecule. If you look at the Periodic Table up here, oxygen is a Period 2 element. Hydrogen is very small. We know when it gets into a hydrogen bond with oxygen, it's really just a proton sitting out there because all the electrons like to hang out with the oxygen, while, say, sodium and chloride, they're considerably larger.
I won't go into the exact molecular sizes, but maybe sodium-- let's do sodium-- which actually, just as a review, which is larger. We know that it becomes smaller as you go to the right of the Periodic Table, so sodium is quite a large atom, while chloride is a good bit smaller, but they're both bigger than oxygen and a lot bigger than hydrogen.
Solubility and boiling point | Physics Forums
So let me draw that. So sodium-- I'll do sodium as a positive. Maybe it looks like this. Sodium is positive and then you have the chloride.
Solubility and intermolecular forces
The chloride I'll do in purple. They're still pretty big. The chloride, it'll look like this. And what happens when you put it into water, it disassociates. Even though these guys in a solid state, they're jam-packed to each other.
When you put it into water, the positive cations are attracted to the negative partial charges on the oxygen side of the water, and the negative anions are attracted to the positive sides of the hydrogen. But in order to get, for example, this sodium ion into the water, it has to fit in there. So, for example, I drew this as a liquid initially, but if this was a solid and you had this structure, it would be extremely difficult. In fact, it would be next to impossible to squeeze these huge sodium ions in place to make it soluble into, say, solid ice.
And as even cold water, these bonds are still going to be pretty strong and they're going to be just kind of barely moving past each other because there's not a lot of kinetic energy. So what you need to do is, the warmer the water you have-- I mean, you can fit it into cold water, because at least cold water has some give, but the warmer the better, because you have some kinetic energy, and that essentially gives space.
Or it makes room for this sodium ion that's entering in to kind of bump its way into a configuration that's reasonably stable. And a reasonably stable configuration would look something like this. Sodium would look-- and then you'd have a bunch of-- sodium is positive. It would be attracted to the negative end of the water molecules, so the oxygen end.
So it looks like that, the oxygen end, and then the hydrogen ends are going to be pointing in the other direction. The hydrogen ends are going to be on the other side. And, of course, the chlorine atom is going to be very attracted to that other side, so the chlorine atom might be right over here. So the chlorine atom might want to hang out right here. In order to get as much of the sodium chloride into your water sample, you want to heat up the water as much as possible.
Because what that does is it allows these bonds to not be taken as seriously and these relatively huge atoms to kind of bump their way in. So, in general, if you think about solubility of a solute in water-- or especially if you think of a solid solute, which is sodium chloride-- into a liquid solvent, then the higher the temperature while you're in the liquid state, the more of the solid you're going to be able to get into the liquid, or you're going to raise solubility.
When you try butanol, however, you begin to notice that, as you add more and more to the water, it starts to form its own layer on top of the water. The longer-chain alcohols - pentanol, hexanol, heptanol, and octanol - are increasingly non-soluble. What is happening here? Clearly, the same favorable water-alcohol hydrogen bonds are still possible with these larger alcohols. The difference, of course, is that the larger alcohols have larger nonpolar, hydrophobic regions in addition to their hydrophilic hydroxyl group.
At about four or five carbons, the hydrophobic effect begins to overcome the hydrophilic effect, and water solubility is lost. Now, try dissolving glucose in the water — even though it has six carbons just like hexanol, it also has five hydrogen-bonding, hydrophilic hydroxyl groups in addition to a sixth oxygen that is capable of being a hydrogen bond acceptor.
We have tipped the scales to the hydrophilic side, and we find that glucose is quite soluble in water. We saw that ethanol was very water-soluble if it were not, drinking beer or vodka would be rather inconvenient! How about dimethyl ether, which is a constitutional isomer of ethanol but with an ether rather than an alcohol functional group?
We find that diethyl ether is much less soluble in water. Is it capable of forming hydrogen bonds with water? Yes, in fact, it is —the ether oxygen can act as a hydrogen-bond acceptor. The difference between the ether group and the alcohol group, however, is that the alcohol group is both a hydrogen bond donor and acceptor.
The result is that the alcohol is able to form more energetically favorable interactions with the solvent compared to the ether, and the alcohol is therefore more soluble.
Here is another easy experiment that can be done with proper supervision in an organic laboratory. Try dissolving benzoic acid crystals in room temperature water — you'll find that it is not soluble. As we will learn when we study acid-base chemistry in a later chapter, carboxylic acids such as benzoic acid are relatively weak acids, and thus exist mostly in the acidic protonated form when added to pure water.
Acetic acid, however, is quite soluble. This is easy to explain using the small alcohol vs large alcohol argument: Now, try slowly adding some aqueous sodium hydroxide to the flask containing undissolved benzoic acid. As the solvent becomes more and more basic, the benzoic acid begins to dissolve, until it is completely in solution. What is happening here is that the benzoic acid is being converted to its conjugate base, benzoate.
The neutral carboxylic acid group was not hydrophilic enough to make up for the hydrophobic benzene ring, but the carboxylate group, with its full negative charge, is much more hydrophilic.
Now, the balance is tipped in favor of water solubility, as the powerfully hydrophilic anion part of the molecule drags the hydrophobic part, kicking and screaming, if a benzene ring can kick and scream into solution.
If you want to precipitate the benzoic acid back out of solution, you can simply add enough hydrochloric acid to neutralize the solution and reprotonate the carboxylate. If you are taking a lab component of your organic chemistry course, you will probably do at least one experiment in which you will use this phenomenon to separate an organic acid like benzoic acid from a hydrocarbon compound like biphenyl.
Similar arguments can be made to rationalize the solubility of different organic compounds in nonpolar or slightly polar solvents. In general, the greater the content of charged and polar groups in a molecule, the less soluble it tends to be in solvents such as hexane. The ionic and very hydrophilic sodium chloride, for example, is not at all soluble in hexane solvent, while the hydrophobic biphenyl is very soluble in hexane.
Decide on a classification for each of the vitamins shown below. Hint — in this context, aniline is basic, phenol is not! Solutions Illustrations of solubility concepts: These are most often phosphate, ammonium or carboxylate, all of which are charged when dissolved in an aqueous solution buffered to pH 7.
Some biomolecules, in contrast, contain distinctly nonpolar, hydrophobic components.
The lipid fat molecules that make up membranes are amphipathic: In a biological membrane structure, lipid molecules are arranged in a spherical bilayer: The transport of molecules across the membrane of a cell or organelle can therefore be accomplished in a controlled and specific manner by special transmembrane transport proteins, a fascinating topic that you will learn more about if you take a class in biochemistry. A similar principle is the basis for the action of soaps and detergents.
Soaps are composed of fatty acids, which are long typically carbonhydrophobic hydrocarbon chains with a charged carboxylate group on one end, Fatty acids are derived from animal and vegetable fats and oils. In aqueous solution, the fatty acid molecules in soaps will spontaneously form micelles, a spherical structure that allows the hydrophobic tails to avoid contact with water and simultaneously form favorable London dispersion contacts.
Micelles will form spontaneously around small particles of oil that normally would not dissolve in water like that greasy spot on your shirt from the pepperoni slice that fell off your pizzaand will carry the particle away with it into solution. We will learn more about the chemistry of soap-making in a later chapter section