so when the solute dissolves in a solvent this is an equilibrium process similar to that of evaporation and we talked about that water evaporating was like playing Red Rover and if you have enough energy you can break through and go into the gas state but then the gas molecules zipping around collide with the surface of the water and recondense so it's an it's an equilibrium so dissolving is also an equilibrium at the beginning the rate of dissolving is the only thing happening because there's no dissolved ions to recrystallize and so we only have dissolving but as we increase the concentration of ions in solution now some of those are going to run into the crystal and get stuck just like condensing water and they're going to recrystallize so that the recrystallization rate will increase as the concentration increases so we get a dynamic equilibrium where the rates of dissolution and recrystallization are equal so sodium chloride dissolving in water we're going to have this double headed arrow meaning this reaction is going both ways so the sodium chloride is being separated into aqueous sodium and chloride ions but these aqueous sodium and chloride ions can get together and form solid sodium chloride as well and it goes both ways so here we have pictures illustrating this process chunka sodium chloride in the water initially all that can happen is that it's gonna dissolve because there's nothing in here that can recrystallize so we've got this solid dissolving and we've got the individual ions going out here making ion dipole force connections with the water molecules so dissolving is happening at a much greater rate than recrystallizing because there's very few ions out here to recrystallize as time goes on the rate of dissolving will go on and that causes the concentration of dissolved particles to be larger and that will increase the rate of recrystallization and that those changes will occur until the rate of recrystallization is equal to the rate of dissolution and then both processes are happening at the same rate any questions so solution equilibrium where we have rate of recrystallization equal to the rate of dissolution and that's a saturated solution this flask over here is currently a saturated solution you see there's liquid in there but there's a ton of solid sodium acetate right now is this a static process that's just sitting here no there's actually a lot of action going on there's sodium acetate dissolving and recrystallizing at the same time you saw me pour out the extra liquid at the top and I called it water it's not just water it's actually saturated sodium acetate but that's that's not exactly dangerous so I just throw it down the sink that's a saturated solution if we were to add more solute to that it would not dissolve they just sit there right if we have an unsaturated solution that means we have less than an equilibrium amount of the solid and so an unsaturated solution if we add more solute it will dissolve because we have not maxed out the rate of this solution and the rate of recrystallization unsaturated solutions are pretty pretty common and then we have what's called a supersaturated solution in a supersaturated solution you have more than the equilibrium amount you have more solid dissolved than can possibly dissolve that doesn't seem right right well it isn't and it's not stable okay and it's a little bit like that supercooled liquid that we had an experiment 12a where we dropped a crystal in there and then all of a sudden it's like telling Roadrunner that he ran off the cliff and then he falls so the supersaturated solution is unstable and if it gets disturbed it will suddenly crystallize and become a saturated solution so I've got a couple of videos here and this is actually the thing I'm trying to do here with the actual physical demo but the videos are pretty cool and that one's going to take a while so we'll we'll get to watch that a little bit later so this is supersaturated study what's that bar I see the spatula in there and it looks like magic happening these crystals growing by disturbing it a person is telling the solution hey your supersaturated you can't do that it was like oh okay sorry I'm gonna crystallize now and it forms all those beautiful spiky crystals this particular solution when it does that this is an exothermic process and so the solutions or experiments carried out between them yeah we don't already hear about that one these are linked to my playlist and my playlist needs to be reordered this one's called fun with sodium acetate it's also called hot ice because it's it looks like it's freezing right but it's getting warm there are 52 degrees that most didn't start out at 52 degrees so they've made a whole bunch of it and then they're messing around all day having some fun and it's warm it's frozen but it's warm that's why I poured off the liquid there cuz it's kind of pennis wish you like a personally melted popsicle this is fun yeah salt castles like this as the saturated sodium acetate hits the plate it crystallizes and so you can make structures out of it and you can even let that come up into the beaker and crystallize inside the deeper I don't like it chemistry at dominoes and suppose trying to spell the word ice hot ice there are still pictures of that process occurring here we have a supersaturated solution just looking at it you can't tell that it's anything other than water right it looks like water how do you make a supersaturated solution how do you get more to dissolve than will dissolve we'll talk about that in a minute so disturbing this giving the crystals a place to form by either dropping a crystal into it or by touching it with a stirring rod or something with an irregular surface causes crystallization to begin and once it starts it's like an avalanche and it will just continue until we've reached a saturated solution and then you've got your just regular saturated solution so the solubility of solids depends on temperature and for most solids if you increase the temperature of the water you can get more of it to dissolve so this is a solubility chart showing solubility in grams of solute per hundred grams of water as a function of temperature they don't show sodium acetate on whoo that would be useful but they don't here's lead nitrate calcium chloride potassium nitrate we see that as you increase the temperature for most of these the solubility goes up sodium chloride is is fairly level and then we've got some weird ones like cerium sulfate where the solubility actually goes down as you heat it up that's very unusual not many things do that so the way you make a saturated sodium acetate solution is you heat the solution and that's what I'm doing at the flask over here I'm going to heat it to a higher temperature where it becomes a saturated or slightly unsaturated solution and then if I allow it to cool down slowly this is like the roadrunner running off the cliff right and he just runs off the cliff and he's not gonna fall until he notices that he's run off the cliff so we're gonna let that cool down very carefully we're not gonna bump it or shake it or drop anything into it it's gonna cool down and when it's room temperature then we'll say hey you you ran off the cliff I'll poke something in there and it'll all crystallize anybody have any questions so recrystallization is a technique that we can use to purify a solid so do you remember that experiment we did with the gooc crucibles the pink liquid that we heated it became yellow and then we filtered is kind of messy and all that we were trying to make calcium oxalate and one of the things that's important is that we we want our crystals to be pure because when a crystal when a solid like that precipitates quickly it can incorporate solute particles or extra ions from the other reagents used in the process it can be impure it can trap things within it so the recrystallization process is where we allow the solid and the saturated liquid to be in equilibrium and to hang out together and then what happens is we get dissolving of the crystal and recrystallizing and when the recrystallization happens slowly it's going to happen in a more regular way it's going to give us beautiful large crystals and these crystals will naturally exclude impurities because any impurity that's going to get in there is going to disrupt its crystal lattice and so it won't be the lowest energy and so those will get excluded and so recrystallization you can just allow it to sit there like we did boiling our liquids or you can take the solid and dissolve it in a solution at a higher temperature and then allow it to cool slowly that's also a way of doing recrystallization this is a picture of rock candy which is made by recrystallizing table sugar that's something you can do at home but I would encourage you to look up instructions for how to do it because I remember doing that as a kid and it didn't work because I didn't know what I was doing you have to have enough sugar in your water or it's never going to crystallize it's a lot of sugar so increasing the temperature of a solid solution tends to increase the solubility for the for a gas dissolved in a liquid the opposite is true and solutions of gases in water are common the the water that comes out of the tap whether it's di water or a regular tap water is going to have dissolved air in it the the water in a river has dissolved oxygen in it that's what the fish and other aquatic life are breathing the solubility of gases in a liquid also depends on temperature but it's the opposite the solubility decreases with increasing temperature and if you have a hard time remembering that just think about pouring cold soda and warm soda the warm soda is going to fizz up much more than the cold soda will this fizzing is caused by the carbon dioxide coming out of the solution because at a warmer temperature the carbon dioxide is not as soluble and so it will come up more rapidly than will cause a lot of foaming and this is you know when your cup overflows and stuff you can also do a comparison where you take maybe two two glasses of soda and you put one in the refrigerator and you leave one out on the counter looks better in the summer when it's not maybe cold in your kitchen but the one in the refrigerator will stay fizzy longer than the one that's sitting out at room temperature because the solubility of the gas at room temperature is less than at the cold temperature inside the refrigerator this also has effect on oxygen levels in lakes and rivers or in aquariums so the temperature of an aquarium or a lake or a river is important because that has a direct effect on how much oxygen is dissolved in the water so tropical fish are accustomed to a lower concentration of ox then Coldwater fish we had when we first moved to the Central Valley we got a goldfish it you know one of those carnivals so he had put him in a bowl of water and it's like fish seems to be having a hard time breathing right he's come to the surface so I you know hooked up some stuff online it's like oh well yeah he would like an average temperature more like 65 degrees and it's like 78 in the house because it was summer right when we're running the air conditioning but I'm not gonna cool the house to 65 for the goldfish right the free goldfish so we put ice cubes in his water periodically just to keep him cool and eventually goldfish will acclimate to that warmer temperature and be okay but that's why they're gasping they appear to be gasping because there's not enough oxygen in the water and so you can have thermal pollution of lakes and rivers so you've got a manufacturing company and they're dumping warm water into the river the water may be perfectly clean nothing contaminated in it but just the fact that it's warm increases the temperature of the river decreasing the oxygen and causing the death of aquatic life Lake turnover [Music] yeah and mix it up to get the yeah yeah there's a lot of interesting effects of this yeah let's think you got something from the book but it's it's later pressure also affects the solubility of a gas so here we have two soda cans this one's sealed up and this one's opened and we're all familiar with that sound that when you open a soda can registers and that's the excess pressure escaping as soon as you break that seal and hear that noise your soda begins to go flat it's going to take a while but it starts to go flat because the solubility of this gas is dependent on the pressure of that gas above the liquid an increased pressure of co2 here causes the solubility of co2 in the water to be higher so like with a clear bottle of soda you can look at it and you'll see there's there's no bubbling or anything going on and you haven't shaken it up you haven't done anything to it and then you you crack open the lid and all of a sudden bubbles start coming up it's because you release the pressure and so the solubility has dropped and now the carbon dioxide is going to be coming out of solution so releasing the pressure causes co2 to come out of solution you can also increase the pressure and cause it to go into solution it's an equilibrium process much like evaporation so here we have this at equilibrium we've got dissolved co2 in the water and we also have co2 in the gas state above the water if we put pressure on this squeeze it down now we've decreased the volume of the co2 and we have the same number of co2 particles so that is in essence a higher concentration of co2 above the liquid it's also an increased pressure of co2 that's going to cause the rate of co2 dissolving to increase because there's more particles here and that increasing dissolving will go on until the equilibrium is reestablished and at this new equilibrium we have more co2 dissolved in the water than we did over here the opposite happens when you release this release the pressure or increase the volume any question so there's an interesting there's an interesting story in the in the textbook about some a couple of lakes in in Africa that are on top of volcanoes and so there's a lot of natural co2 that's coming up from the bottom of the lake and at the bottom of the lake there's a lot of pressure because of the water we talked about that earlier every ten feet you go down it's an increase one atmosphere of pressure so at the bottom of these large deep Lakes the pressure is quite high and so the solubility of co2 is much higher well you get a disruption in that and the co2 comes out of solution that forms a big bubble and goes to the surface which is reducing the pressure around it and it causes sort of a cascade effect and you get these clouds of co2 that can be given off by these Lakes they like burp right big clouds of co2 well big deal well it actually is a big deal it's killed hundreds of people because co2 is more dense than air a big cloud of co2 rolls down the hill it's down where the people in the cattle are and everybody suffocates it's very tragic so you know they happened twice in two different lakes and so instead of just allowing this to continue they came up with a way to vent the co2 from the bottom of the lake so this wouldn't happen but very weird phenomenon so of course we can quantify the effect of pressure on the solubility and this is known as henry's law the solubility of a gas is proportional to the partial pressure of that gas it's not the total pressure it's the partial pressure of that gas and so KH here is a solubility constant and you can look those up in tables that the solubility is usually listed as moles per liter and the pressure is usually in atmospheres so the Henry's law constant are usually in molarity per atmosphere so determine the solubility of oxygen and water at 25 degrees Celsius exposed to air at one atmosphere assume a partial pressure for oxygen of 0.2 one atmospheres well we need Henry's law the solubility we're talking about oxygen the solubility for oxygen is going to equal to be equal to the Henry's law constant for oxygen times the partial pressure of oxygen here was given the partial pressure and so we need to look up the Henry's law constant on the previous slide for o2 it's 1.3 times 10 to the minus 3 so one point three times ten to the minus three molarity per atmosphere times point to one atmosphere yes yes conveniently cancel out and we get 2.7 times 10 to the minus 4 moles per liter so in 1 liter of oxygen of oxygen in one liter of oxygen there's a liter of oxygen in a liter of water there would be two point seven times ten minus four point zero zero zero to seven moles of oxygen dissolved questions