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- Now that we know what a solution is, let's think a
- little bit about what it takes to get a molecule to be
- soluble into a solution or into a solvent.
- So let's say I start off with a salt, and I'll do a little
- side here, because in chemistry, you'll hear the
- word salt all the time.
- Let me right it down: salt.
- 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.
- 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.
- 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.
- So let me draw that.
- So sodium-- I'll do sodium as a positive.
- It's pretty big.
- Maybe it looks like this.
- Sodium is positive and then you have the chloride.
- 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.
- So temperature goes up, solubility goes up.
- For example, if you were to take some table salt, and you
- could experiment with this.
- It doesn't seem too dangerous and not too expensive because
- salt is reasonably cheap.
- Keep putting it into a glass, and at
- some point it'll dissolve.
- You could shake it a little bit, just to make sure.
- You could think about what's happening at the molecular
- level while you shake it and why does that help to shake or
- stir things?
- But at some point, you're going to end up with-- if this
- is your glass of water, the salt will keep going in there,
- but at some point, you'll have salt crystals at the bottom of
- your glass.
- At that point, your water is saturated with salt at the
- temperature that you're trying to deal with it.
- Now, right when you start seeing that, if you were to
- put it in the microwave or if you were to heat it up, you
- would see that even these guys are able to be absorbed in the
- water, and that's because the extra kinetic energy from the
- temperature is making it more likely that these guys are
- going to be able to bump out of configuration for just long
- enough for these guys to bump in.
- And just a little side note, when you take these salts,
- which are just ionic compounds that are neutral, they're made
- of ions, but they cancel each other out.
- When you put them in water, these compounds by themselves
- aren't normally-- when they're in the solid state, they don't
- normally conduct electricity.
- Even though they're charged, they're very closely stuck to
- each other, so there's not a lot of room
- for movement of charge.
- But once you disassociate them in water or dissolve them in
- water, now, all of a sudden, you have these floating
- charges in the water, and this does conduct electricity, so
- it becomes quite a reasonable conductor of electricity.
- So the general rule of thumb is, if you're dealing with a
- solid in a liquid solvent, lowering the temperature will
- decrease the solubility, because it's harder to jam the
- molecules in there, and increasing the temperature
- will increase the solubility.
- But what about a gas?
- What if you make some soda and you want to dissolve some
- carbon dioxide into, let's say, water again?
- So here, the way to think about it when we did it with
- salts, these are ionic compounds.
- They had some natural attraction to the different
- polar ends of the water molecule.
- But gases, for the most part, do not have
- strong attractive forces.
- That's why they're gases, especially at room
- temperature.
- They like to be free.
- A gas, they have a good bit of kinetic energy, but more
- important, the bonds between them, for example, in ideal
- gases we talked about it, they just have their London
- dispersion forces.
- They have very weak bonds, and that's why at, say, the same
- temperature and pressure that water would be a liquid, a lot
- of these gases are gases.
- They jump away from each other because they don't want to
- touch each other.
- Now, when you put this in liquid, and this is at least
- my intuition, so let's just say this is a bunch of water
- molecules here.
- If you were to dissolve-- let's say it's carbon dioxide.
- You can ignore this stuff up here.
- If you were to dissolve carbon dioxide in water-- so if you
- were to dissolve this in water, so those are some
- carbon dioxide molecules.
- I'm just drawing the whole molecule as a circle.
- What do these molecules want to do?
- It's natural state is a gas and it is a gas at let's say
- the standard pressure, so it really wants to escape from
- this water, but it just can't do it that easily because
- there's water molecules all around it, right?
- This guy right here, he might want to bump out, but he's
- surrounded by water molecules.
- So what would help him bump out?
- Well, if you raise the average kinetic energy of the system,
- if you made all of these guys, that these guys were moving
- faster, and especially if the carbon dioxide molecules
- themselves had more kinetic energy, then maybe
- they could break out.
- And as you have from personal experience with Coke bottles,
- you could also shake the system, because if you shake
- the system, it just moves everything around enough that
- these guys can escape.
- So when you're dissolving a gas inside of a liquid
- solvent, when the solute is a gas, it actually has the
- opposite effect, that rising temperature.
- So when temperature goes up, solubility goes down because
- these guys want to escape.
- They want to be free.
- They want to be away from other molecules and they want
- to bounce around in open-- I shouldn't use the word air--
- in open space.
- And so anything that lets the system move around more,
- they're going to go up.
- And likewise, if temperature goes down, solubility goes up.
- The other factor, and it's not as big of a factor when you
- talk about a solid solute, but when you talk about a liquid
- solute-- let me just do it again.
- So those are the carbon dioxide molecules and then you
- have a bunch of water molecules-- they should all be
- the same size-- that it's dissolved in.
- I think you get the idea.
- Pressure is also a big factor.
- I already said that these guys, their natural state is
- to roam free.
- They want to get out.
- They want to somehow bounce out of the water.
- But if you have a really high pressure up here-- just the
- atmosphere up here has just tons of molecules bouncing
- really hard down on the surface of our solution-- so
- if there's just tons of molecules bouncing really hard
- off the surface, it'll be harder for
- anything to escape upwards.
- And that's why, when you have pressure going up, or at least
- this is the intuition, when pressure goes up, solubility
- of a gas also goes up.
- And this is for a gas.
- So just the interesting thing to remember is that when you
- think about solubility, solids do the inverse of gas.
- Temperature is good for solid solubility, right?
- We said when you put salt or sugar in water, it's good to
- increase the temperature.
- You'll be able put more in there.
- On the other hand, with a gas, it's the opposite.
- You want colder temperatures to put more gas into the
- solution, or you want higher pressure to keep it-- at least
- in the way my mind works-- from escaping out the top.
- Anyway, hope you found that useful.