The Science of Everything Podcast - Episode 31: Solutions and Mixtures

Episode Date: March 31, 2012

An analysis of liquids and their behaviour when mixed with other substances, incorporating a look at solutions, mixtures, colloids and suspensions. Special emphasis is placed on solutions, with a more detailed discussion of solubility, saturation, molarity and bubble formation.

You're listening to The Science of Everything podcast, episode 31. Solutions and Mixures. I'm your host, James Fodor. In this episode, we're going to look at an analysis of liquids and their behaviour when mixed with other substances, including a look at solutions, mixtures, colloids, and suspensions, all of which are various types of liquids mixed with other substances. Although mostly we'll look at solutions,

because they're the ones that come up most often in chemistry and other locations, including an examination of topics including solubility, saturation, molarity, and the formation of bubbles, which is actually quite interesting. Okay, so before we get into those details, we'll start off with an introduction of, and look at the question of what is a liquid. Now, from previous episodes, or general knowledge, you may know that liquid is one of the three classical states of matter, the other two being gas and solids. Liquid is in sort of temperature range between gas and solids, so it's sort of in the middle range. However, liquids, pure liquids are relatively rare on, at least at Earth,

temperatures that are found on Earth's surface, like around standard room conditions. Water is one of the few substances, few substances that is a liquid at room temperature. Liquids form when the intermolecular bonds between the molecules, holding a solid together, break apart, and therefore the molecules are able to begin sliding past each other. However, in liquids, the intermolector bonds don't completely break apart, so they sort of mostly, but not quite break apart, so that the molecules are still held together to retain the basic shape. The difference between the liquid and a gas though is that in a gas the molecules, the bonds between the molecules completely break apart and the molecules are free to

just move completely separate from each other, whereas in a liquid, the molecules are still held together by weaker bonds, but they're still held together to some degree, and so the molecules merely slide past each other. That's why liquids will change shape to fill their container or to fill whatever gap they're in, but they won't expand in volume to fill their container, which a gas will. The density of a liquid is, you is usually fairly high, similar to that of a solid. In the case of water, in fact, the density of liquid water is higher than a solid water, which is why ice floats in liquid water.

That's unusual, though. Usually liquids are a little bit less dense, but around about the same as the solid, and much higher in both cases than the density of a gas. So that means, in practical terms, that the molecules in a liquid are clumped much closer together, and held in a much denser volume than they are in a gas, which once again is a reflection of the fact that the molecules are still held together by bonds of a weaker extent than in a solid, but they still held together to some degree, unlike in gases where the molecules are completely free to move around each other, bounce off the walls of the container and so on.

Okay, so that's just a bit of a revision about what a liquid is. Now, a solution, which is what we'll be focusing on mostly in this episode, a solution is a homogenous mixture composed of two or more substances. So when I say homogeneous, that means it's the same throughout. So if you take a little sample from here, from there, and from this other place inside the solution, the samples that you take will have the same consistency. So most macroscopic objects, like objects that we deal with in the world, are not homogenous. If you just think of...

Consider, for example, a bowl of soup. That's likely not going to be homogeneous, because within the soup, parts of it are just going to be mostly water and other parts. You'll have the meat or the vegetables or whatever's in the soup, which has a different consistency to the soup as a whole or the merely liquid parts. Consider a chair, for example. different, the exterior covering, and then inside it's whatever it's made of, and then it's got the frame and the pillows on it. Each of those different parts of the object have different

consistencies, and so the object as a whole is not homogeneous. So the key thing about a solution is it is homogeneous throughout. All parts of it are the same, at least down to a very low sort of molecular level. Reasonably, sizable sample of the solution will be the same throughout. Now, in a solution, a solution's made up of, I said it's composed of two or more substances. There are basically two types of substances that go into make a solution. One is called the solute and the other is called the solvent. So solute, solvent. They sound similar but don't get confused because they're sort of opposites. The solute is the one that is dissolved into the solvent. So for example, when you put sugar into a drink of coffee or milk or something like that, the sugar

and, you know, stir it in, the sugar will be the solute and the water or, or, you know, or milk or coffee or whatever, will be the solvent. So the solvents and the solutes don't have to be of the same phase of matter. That is, you can have solutions with both the solvent and solute are solids, where both of them are liquids, where one is a solid and one is a liquid, and even where one or more of them are gases. So you can have different combinations. A lot of the time when people think about the solutions,

they think of the solvent, the solvent being the liquid, and the solute being the solids. For example, that's your salt or sugar-deserved in water or something along those lines. And that's a common type of solution, but it's not the only type of solution. We'll talk about some other different types of them later on the episode. An aqueous solution is the most common type of solution. That's just when something is dissolved in water.

So in that case, the solvent will be water, and the solvent, of course, being water, will be in the liquid state, usually. Some solutions are made up of, that is, the solutes in the solution, are made up of charged ions. Well, that's what an ion is. It's a charged molecule or atom, which therefore enable the solution to carry an electric current. So salt, well, most types of water, as long as it has a few salt ions dissolved in it,

will be able to carry a current of electricity. Electrolite solutions are used in things like batteries and so on, because they have to carry a charge throughout the solution inside the battery, and we'll talk more about that in a future episode because it's quite interesting area of chemistry, actually. Now, before we get into the nitty-gritty details of solutions, I just want to have a moment to talk about why solutions are so important. Often when you perhaps think about chemistry, you think about, you know, the white lab-coated scientist with his fly,

of chemicals and dissolving one and the other or putting some crystals or whatever compound into a flask and dissolving into it. And that is indeed what happens a lot in chemistry, if you're doing experiments or trying to synthesize some compound or whatever. A lot of it does relate to dealing with solutions. Perhaps you might think that that's a little strange, because solution is only one rather small subset of all the different types of matter or arrangements of matter that you can have, especially referring to aqueous solutions. Why are they so common in particular? Because water is just one substance that you can have in one phase of matter. and why do we have so many aqueous solutions in chemistry?

One of the big reasons is that for chemical reactions to occur, the reacting particles must first collide frequently enough for the reaction to continue, and also with the sufficient velocity to overcome the initial energy barrier for the bonds to be broken and the reaction to take place. There's more on that, the energetics of chemical reactions in the episode on chemical reactions. Particles and gases move around freely and swiftly and produce a lot of collisions, so you might think those would be good for chemical reactions, and some of them are, of course.

The trouble is that reactions in the gas phase, well, first of all, it's often hard to get many substances in the gas phase because you need very high temperatures. And because of that, they can often be difficult to control and sometimes dangerous, can be explosives, or it's difficult to contain their volume, or in order to contain the volume of the gas, need very high pressures, which can be dangerous. So, gases can be good for reactions, but they're difficult, they're tricky in a practical sense to deal with. On the other hand, solids, the particles in them aren't moving around enough. They're too stuck in place. that's sort of the definition of what a solid is and so you're not going to be able to get the particles to mix and combine with each other and to react as well as you want to be able to have interesting reactions

occurring in chemistry. So liquids sort of represent the ideal compromise between the gases and the solids having enough reactions and energetic enough interactions and collisions between the particles to get reactions to occur but not too energetic and too hard to control as gases can be. And so that's why solution is so

important because, as I said, you have them in liquid form because that's easy to deal with and you get good reactions. Water is a very convenient solvent to use because it's, you know, everywhere. And then you just dissolve what you want in the solution and the reaction can take place relatively easily. Okay, so that's done with the introduction parts about what are liquids and water solutions and why they're so important. Now we're going to go through more details about solutions about how and why they form, and we'll look at solubility, how solubility changes with temperature. We'll also talk about molarity. Okay, so first of all, why solutions form. The basic idea of why solutions form,

that is why the substances mixed together is because of probability. In other words, it's more likely that two substances will mix together than simply by chance stay separated. So you can imagine if you put a bunch of, let's think, salt particles into a glass of water, it would just be exceedingly improbable if they all... Actually, let's think of a cuba salt, you put a cuba of salt or cuba sugar or whatever, a cube of those crystals into the water. It would be very unlikely if those would just by chance stay all clumped together

and the water all stay separated from it, just as long as you leave it in there. Because the particles within the sugar, all the particles are moving around and bumping into each other and jiggling and vibrating and so on, especially what's happening on the edge of the solute, that is on the edge of this little lump of sugar that we've put into the water.

The water molecules are continually striking and bouncing off and vibrating onto the edges, onto the edges of the clump of sugar. And every now and then a collision dislodges one of the particles, one of the sugar particles, from the edges of the, of the, solute, causing it to then break away and join the solution. Sometimes, of course, the reverse happen. A sugar particle is just sort of floating along or vibrating around, and it happens to

strike the edge of the clump of sugar, and forms a bond with some of the surrounding sugars, and rejoins the solute. However, if you think about the probability of the situation, when you initially put the cube in there, it's much more likely that a sugar particle of sugar is going to be dislodged away from the solute, that is out into the rest of the solution, then the reverse to happen, because the rest of the solution has a much larger volume than the small little bit of solidate that you've put in there. So when you first put the solute in there, the lump of sugar, there's relatively lots of sugar particles being dislodged away from the solid, but, well, basically none that are going into rejoin it, because there's basically none

the rest of the solution. As the amount of dissolved sugar grows over time, as more and more is dissolved, the number of sugar particles going back in and rejoining the solute increases, because that's dependent upon how many of these things there are floating around in the solution. Similarly, as the number, as time passes and the more and more of the sugar is dissolved, there's fewer sugar particles to be dislodged away from that lump and join the rest of the solution, and dissolve in the solution. So that means that the number of, essentially the number of sugar particles going out goes down over time, and the number of sugar particles going into the solution increases over time.

And eventually those two processes will exactly balance each other out, so that the number of sugar particles rejoining the little cube of solute is exactly equal to the number being dislodged. And when that occurs, it's called a dynamic equilibrium, and the solution is said to be saturated. No more solute can be dissolved in the solvent. So in this case, no more sugar can be dissolved in the water. Saturation, that literally means like the maximum amount of solute in the solvent has been reached. If there is still some solute, that is if there's still a little lump of sugar around when saturated, When saturation is reached, then not all of the sugar will be dissolved. That is, you'll still have a little lump of sugar there.

Some of it will have dissolved off, and there'll be sugar particles around in the rest of the solution. But basically, the reason the last bits of it don't dissolve is that for every sugar particle that's dislodged away from the remaining solute, another one comes in to take its place, essentially, because you've reached that equilibrium of outs to inns of the sugar particles. And so the overall size of the solute doesn't change. If, on the other hand, there was, say you put in less sugar, and now all of it, is able to be dissolved before the saturation point is reached, then you don't have any of that leftover solute. All of it is dissolved in the water. That is the two processes, you know, the outs of the

sugar particles and the ins of the sugar particles don't reach each other before all of the sugar is already dissolved. So that's the basic reason for why solutions form. It's fundamentally statistical, that it's just more likely that the two substances will be spread out throughout each other rather than remain in discrete places. It's sort of the same reason that gas fills a container of its size rather than just all sitting clumped over to one side. It's just extremely unlikely that that would happen. Now, the ability of one compound to dissolve in another, or the ability of a solute to dissolve in and mix throughout a solvent,

is called solubility. The solubility of different substances depends upon the relative strength of the intermolecular bonds between the particles in the solvent compared to those in the solute. So remember when I just previously in the last section said that it's very unlikely that the two substances when placed in the same container or the same area

will just arbitrarily stay separated from each other. It's much more likely that they'll mix together. That's true, but only if the intermolecular bonds or the forces between the particles in each of the substances are roughly similar, in which case, essentially, that is if the intermolecular forces are similar, one particle can move throughout substance A, then into substance B and back into substance A and so on, with impunity. It doesn't make much difference which substance it's in,

in terms of the intermolecular energetics of bonds. It doesn't preferentially bond with substance A or substance B. In our sugar case, for example, the sugar particle doesn't really preferentially bond to another sugar or to water molecules in the liquid or the solid phase. It doesn't make much difference. The bonds are of similar strengths, and so it just sort of goes from one to the other. In that case, if the bonds are of similar strengths, you'll tend to have a high degree of misability, or in other words, the two substances will mix together very well, and therefore which of one

is the solute will be highly soluble in the solvent. However, if one of the two substances, it doesn't actually so much matter if it's the sole utal solvent, but if one of them has a much higher intermolecular bond bonding force than the other one, then the stronger bonds will tend to preferentially bond to each other, pushing out the weaker bonds, and therefore the two substances will tend to remain separate. This is essentially the reason why oil and water don't mix, because oil, if you remember back to previous episodes on chemistry, including the cleaning one where I introduced the different types of biochemical molecules, oil molecules

are essentially carbon change surrounded by hydrogen, so they're hydrocarbons, and then non-polar, which means that they don't have a partial charge at one end or the other of the molecule, whereas water molecules are highly polar. In fact, they experience hydrogen-hydrogen bonding, which is a very strong form of polarity. So that means that the strength of a bond between one water molecule and another is going to be much stronger than the strength of bond between one water molecule and one oil molecule. So therefore, the water molecules preferentially bond to each other, and they sort of push out the oil molecules from around them. And so because they're being pushed away from the water, the oil molecules themselves will clump together and bind with each other.

So because of this substantial difference in intermolecular bonding strength, the water and oil tend to separate out and clump out, and therefore the oil is not soluble in the water. This doesn't just happen for oil and water, but that's a good example of it. The bigger the difference in the relative intermolecular bonding strength, or strength of the forces, then the less soluble, the one substance will tend to be in the other. There's a shorthand to help remember this, which is basically that like dissolves like, which means polar molecules tend to dissolve other polar molecules, so water tend to dissolve other polar molecules, and non-polar molecules, like oils, for example, and fats tend to dissolve other non-polar molecules.

So just to flesh that concept out a bit more, you can imagine, instead of putting a cube of sugar in our glass of water, we've now put a drop of oil. In the previous case, we said that the water molecules surrounding the sugar would smash into it, periodically just because of random molecular fluctuations and motions and dislodged some of the sugar particles which would then enter the solution enabling more sugar molecules to be dislodged and therefore the sugar gradually dissolves into the water. In the case of the oil however it's still the case that the water molecules are going to be dislodging oil molecules off which will then sort of start to move into the water however as soon as that

happens the preferential bonding of the water molecules for each other essentially pushes the water, sorry the oil molecule back into the rest of the clump of And so for every oil molecule that's dislodged in a sense it's pushed back almost immediately into the clump of oil. And so it never dissolves, or at least to no significant extent, does it dissolve into the water? I'm simplifying a bit there because there's always going to be some degree of mixing in pretty much any within two substances. It's never going to be 100% dissolve or not dissolve, but to a very great extent oil will not dissolve in water because of that massive difference in intermolecular bonding strengths. Okay, so that's solubility. There's a sort of a subset of the solubility property, which is the relationship. between solubility and temperature. As the temperature of a liquid increases,

solubilities of gases in that liquid generally decrease. So if you have solutes that are gases, their solubilities generally decrease, while the solubility of solids and liquids tends to increase of the temperature, especially solids. So there's an opposite relationship there. As you heat, think of those glass of water, as you heat the water, you can get, you can dissolve more and more solids into it, so more and more sugar in it, for example. That's why if you have a hot drink, it's easier to dissolve sugar or Milo or whatever else you're trying to dissolve in it. It's easy to do that when it's hot because you can dissolve more in it because of the increased temperature, and I'll explain that why that is in a second.

But for gases dissolved in liquids, so for example, soda cans, when you heat them up, it will actually reduce the solubility. So that's why you'll find that if a soda can is, say, left in the sun, try and open it, you'll find that it's more likely that you'll have the gases shooting out of the solution and therefore the can will froth up and make a mess. That's because the increased temperature has reduced the solubility of the gas, in that case carbon dioxide, in the liquid, therefore causing it to push outwards, building up pressure on the top of the can, which is then released when you actually open it. Now, understanding why there's this different response of gases versus liquids with changes in temperature is a little tricky, perhaps the best way of thinking about it is that when the solute increases in temperature, well, the whole system, but we'll just think of the solute. So when the solute increases in temperature, it's going to tend to adopt a more disordered, highly dispersed,

state. That's sort of consistent with the second law of thermodynamics. As you add energy, it sort of makes the system more dispersed. So, you know, as you heat liquid, it goes from being nice orderly structure in an ice to less orderly structure in a liquid to the least orderly structure when it's in a gas and the what molecules are completely separated. The same sort of thing's going to happen even if the substance are in solution. Now, in the case of a solid, when you heat that up, it's still a solid, and so the more disorderly state that it will adopt is being more dispersed throughout the liquid and therefore it becomes more soluble. Another way of thinking about that is that in the case of our sugar cube in the water example, increased kinetic energy on behalf of the water

molecules and also on behalf of the sugar molecules makes it easier for the sugar molecules to be dislodged because they were sort of already vibrating, barely kept in place as it was and therefore it's easier for them to be dislodged out of the lattice out of the, and join the solution. On the other hand, if we had a bunch of gas in our solution of water, say carbon dioxide, and then you increase the temperature of that gas, then it would be more difficult for the water molecules to sort of keep that gas in place, to keep it in the solution, because it's only the bonding forces between, the intermolecular bonding forces between gas molecules and the water molecules that's keeping the gas in the solution in the liquid in the first place. As the gas becomes more energetic, it's harder

and harder for those intermolecular bonding forces to overcome the large kinetic energies of the gas molecules, and therefore the gas molecules increasingly are able to escape from the liquid. And therefore, are more easily able to escape. They are less soluble in the liquid solent because they can't really be dissolved in it. So that's sort of why you get the difference between the behavior of solid solutes in solution versus gas solutes in solution when you vary temperature. Another interesting phenomenon related to solubility and saturation is the concept of supersaturation. So super saturation is basically when you have more solute than a solution can handle. That is, you get to saturation point, and then you sort of keep dissolving solute in the solution so that you go beyond saturation point.

So in other words, you've got like 110% of the solute dissolved in the solute in relation to saturation. Now, that might sound nonsensical, because if I just said that saturation is the maximum amount of solute you can have in a solution, and then I'm now saying you can have more than the maximum amount. But, in fact, super saturation can only occur, or it's only manifest in certain circumstances. Common example, so probably the most common situation where you'll have super saturation, saturation occurring, is if you have a, say, in this example, we'll say we've got our sugar that we've dissolved in our water at a relatively high temperature. So remember when we

increase the temperature, solute, in this case the sugar will be more easily soluble. We'll have a greater solubility in the water. But then, so we've dissolved that in, and we've dissolved a fair bit in because we increase the temperature of the water. But then we, suppose we reduce the temperature of the water, we just let it to cool or put it in the fridge or something. That's, as the temperature of the water falls, the sort of maximum amount of solute that it can hold in it, that is sort of the saturation level, declines, because as we said, solubility falls with temperature in the case of a solid. However, it takes time for a solute to come out of a solution, even if, say, it's now fallen below the saturation level that previously would have prevented more solute from

coming in. Now, the reason it takes time is because it's not exactly, it requires certain processes to occur before the solute can actually come out of solution. In particular, it requires that the the different solute molecules, say the sugar molecules in the water, that they, enough of them sort of clumped together, just sort of by random chance of their fluctuations, as they're sort of swimming around in the water, enough of them must clump together so that they can form a little precipitate, which then builds up as other molecules hit it. And therefore, as more and more of the sugar molecules hit the expanding precipitant, or the expanding clump of solute, well, it's not solute anymore, it's the precipitant, more and more sugar's coming out of solution. And therefore, the expanding precipitant, the expanding clump of solute. And therefore, the expanding, the, um, the expan you're moving away from the level of saturation. But anyway, that has to happen. The sugar molecules have to come together and then the precipitants has to build. If that doesn't happen, perhaps because the water is relatively still

and doesn't have too many interactions, say, with impurities in it or anything like that, you can sort of delay the onset of precipitants forming and therefore you can keep more solute in the solution than you would normally be able to once it's fallen below that temperature. So a good example of this is if you, say, heat up some water in the microwave, then dissolve a bunch of sugar in it so that you reach saturation point, then allow it to cool down. And at least if you do it right, what you can get is a super saturated solution where if you allow the water to sit for long enough, sort of the currents in the water sort of subside. And if you have a clean glass and there won't be impurities in it and so on, there won't be sort of little sights or little, there won't be sights of the impurities or bits of dirt or something like that, where the precipitants can congregate and form up their bunch, and also if the water's not moving around very much, then it's harder for the sugar molecules to come together. So if you get it just right, you can have it so that the

solution becomes super saturated, which means that as soon as you disturb the solution in any way, perhaps you put a spoon in the cup or just knock it or something, generally the solid will precipitate out of the solution quite rapidly, or at least some of it will precipitate out such that the solution once again falls below the saturation point. So that's sort of an interesting phenomenon, super saturation. It's sort of like cheating by getting beyond the saturation point and then cooling down back below it without actually dissolving out without actually precipitating out the solute.

Another concept that I want to talk about is molarity. Now, molarity is basically just a measure of concentration of a solute in a solution. A commonly used unit for molarity or molar concentration is the mole, or specifically moles per litre. A mole is just an amount of substance. generally you can sort of think about it. Most substance... I mean, it depends on the mass and or density of the substance,

but a mole of many substances is something around a handful. So it's a macroscopic amount that we can talk about. That's why we use the concept of a mole. Because if you say, we've got 10 molecules of sugar, that's nothing. We can't really deal with 10 molecules. So in chemistry we have this concept of a mole. When talking about molarity, as I said, we have another measure, which is moles per litre,

which means the number of moles of solute dissolved per... liter of solution. So if I dissolve one mole, that is say, I don't know, a small handful of sugar in one liter of water, that solution will now have, will now have a concentration of one molar, one mole per litre, so one molar. If I dissolved two moles of sugar in the one liter of water, I would have a two molar solution and so on. Now I actually just told a lie there, because the definition of molar is the number of moles of solute per liter of solution, not liter of solvent. So, in fact, when I'm adding the one-mole or two-mole or whatever of sugar to the water, I'm actually increasing the total volume of the solution,

although water is going to be most of the volume, because it takes up much more space than the, and there's more of it, than the solute that we're putting into it, the solute is still going to add somewhat to the volume of the whole solution. Therefore, if I wanted to make a one-molar solution of sugar and water, say, I would have to have slightly less than one liter of solvent, that is the one liter of water, so a bit less than that. and then when I added one mole of sugar to it,

the total volume of that solution would then have to sum to one litre, and then if there was also one mole of solute in it, the one mole of sugar, then I would have a one molar solution. So the higher the molarity, or the greater number of molars, a solution is the more concentrated is the solution said to be. Obviously, this is related to the concept of saturation, because the saturation can essentially be measured as the number of moles of the solute you can put in a given solution.

So maybe, I don't actually know what the figure is, but suppose that water at room temperature can go up to five molar when normal table sugar is softened. Beyond that, you can't dissolve any more sugar in the solution and therefore it's resaturation. One situation we may have seen or heard of the concept of molarity or molars is in terms of acids, it can be used to measure the strength of acids

or how dangerous they are. It doesn't exactly measure that. We'll talk about that in a later podcast, but it's sort of part of the equation in terms of measuring how dangerous an acid or really just any dangerous chemical substance, the bigger the molars, basically, the more concentrated, so probably the more dangerous it is.

And if you see the vials or the containers of the substance, they might be marked with a like, some number with a capital M afterwards, which will mean 5.0 molar or something, or however, such and such molar. The high of that number, the more concentrated the solution is, and so possibly the more dangerous it is. Okay, one final topic that I want to talk about

before we move in our solutions section of the podcast, before we move on to other mixtures. A soap bubbles, which might seem like an old thing to talk about, but they're kind of interesting because they do relate to solutions. A soap bubble is just a thin film, so thin outer layer, of soapy water, enclosing air. I guess it could enclose other things as well, but generally we think about them enclosing air, which then forms a hollow sphere. So they've got the air in the middle, soapy water,

a thin film of soapy water surrounding it, and then just the rest of the water all around it. The reason that soap bubbles are a sphere is because a sphere has the smallest amount of surface area relative to its volume, and it's energetically favourable for this thin film of the soap bubble to have a smaller surface area as possible. Essentially, the reason for that is because the soap bubble is stabilised by the surface tension of the water molecules surrounding it, or the soapy water molecules surrounding it, surface tension referring to the sort of pulling force or the traction force that surface molecules in the water have for each other. So in other words, the water around the soap bubble prefers

to bond to other water molecules than it does to bond with air molecules, and therefore or I guess it prefers to bond to other water molecules than to nothing, because the air is probably a gas, and so it's going to be difficult for the water to bond to it. But anyway, the water molecules preferentially bond to each other, and so in sort of each of them jostling to try and bond with other water molecules, they spread themselves around in a circle or in a sphere in three dimensions. I think we've talked about that in the previous episode.

So if you want more on surface tension, go back to whatever episode that was. Just a note to anyone interested, I just looked that up. That's episode 27 into molecular. bonds and phase transitions where I talk about surface tension. Now, generally the surface tension of water is too great for bubbles to form. So in other words, the water is just pulls another water molecules so much that bubbles are unstable and the air just floats to the surface. However, if you add a detergent or like a soapy material called a suffactant, that reduces the surface tension of the water molecules essentially because the, say, the soap or the detergent molecules, they get in the

way and disrupt the bonding between the water molecules, so therefore reducing that surface tension between them. That reduction in the surface tension of water stabilizes the bubbles by essentially preventing the water molecules from pulling towards, pulling inwards on themselves too much. Now if you added too much detergent, the surface tension would decrease too much and then the bubbles would be out of form in the first place. So having soap bubbles is about the right mix of having just the right level of surface tension in the water to stabilize the bubbles without destroying them. And that's why you need detergents or soaps to form bubbles in water and they generally form, certainly not stable, in just ordinary water. That's actually, it's actually a

relatively complex subject to soap bubbles or bubbles generally, and I've only given just a brief overview on. I mean, I'm sort of waving my hands a bit when I'm explaining how surface tension stabilizes the bubbles there. We may do a more full analysis of that in the future episode, but it relates to surface tensions and solutions, so I just thought I'd throw it in here. Okay, so now that we've talked thoroughly about solutions, I just want to briefly cover some other mixtures. That is just essentially some other conglomerations of matter. So in chemistry, a mixture is a material, it is just a system of materials made up by two different substances, or two or more different substances that are mixed together, but not chemically bonded to each other.

So if I have an oxygen, if I have H2O, a water molecule, hydrogen and oxygens, those atoms are chemically bonded to each other. So that molecule, or a bunch of those molecules, is not a mixture. It's just molecules chemically bonded to each other. When you have different substances that are sort of in the same place or mixed together, but not chemically bonded, that is a mixture. Mixes are generally the product of like mechanical blending or mixing of substances, but without having a chemical change or without chemical bonds forming. So as we just discussed, a solution is a type of mixture.

So a solution is a mixture, but it's a special type of mixture, namely it's a homogenous mixture. Remember, that's the same throughout. In a homogenous mixture, that is a solution, the particles are broken down right down to the molecular or ionic level. So, you know, to the naked eye or really to any analysis, the solution looks homogenous. So think of it as like your... Actually, we'll talk about salt because that might be easy to visualise. We've got salt, which can be in a lattice with all of the, you know, the positive and negative ions bonded together in a big three-dimensional structure.

And then we've got our water with a bunch of H-2-O molecules just scattered around the place. And we dissolve the salt in the water, the bonds between all of the positive and negative ions in the salt. lattice are pulled apart and each of those ions, or perhaps it's an ionic molecular compound, it could be, but we'll just think about it as an individual atom. Each of those are separated and dissolved throughout and sort of mixed throughout the bunch of water molecules, forming a homogenous solution. But no chemical bonds have been formed. I guess chemical bonds have been broken in the act of dissolving the lattice structure, but certainly none have been formed. The two substances have just been mixed together. The two substances retain their separate

property, so you've still got salt and you've still got water, they haven't changed into something different, like happens when, for example, you combine the hydrogen and the oxygen to form, sorry, the hydrogen and the oxygen to form water in the first place. That's a chemical reaction that completely changes the properties of the reactants. In the case of dissolving salt and water, you just have two substances and you mix them together. So that's a homogenous mixture. There are also heterogeneous mixtures, that is, mixtures that are not the same throughout. A chair is an example of a heterogeneous, or your pizza or soup, the things I mentioned earlier, but they don't have special chemical names as such.

Some other interesting substances that do have, that do have specific chemical names and behavior that we can talk about, are called colloids. That's C-O-L-L-O-I-D-S, if you see it written down, a colloid. It's not nearly as sort of well-known as a solution, but they're actually quite common. So a colloid is a substance microscopically dispersed evenly throughout another substance. So that probably sounds like what I just said a solution was,

which is one substance mixed evenly in another. The difference, however, between a colloid and a solution is that, at a microscopic level, collides are heterogeneous. At a microscopic level, solutions are still homogeneous. So, macroscopically, solutions and colloids probably look sort of similar, or at least they look homogeneous. But when you sort of zoom in, look through microscope or something like that, solutions still look homogeneous, colloids don't.

So that's sort of the key difference. Now, remember, if we zoom in close enough to any mixture, we eventually see that it's not homogeneous because we see the individual atoms or molecules, say we see the individual H2O versus molecules versus the sodium ions or whatever. But that's zooming in too far. That's zooming into sort of the molecular atomic level. At that level, nothing is homogeneous, unless it's just a single element, I suppose. So we're not talking about that far. We're just talking about, I don't know exactly what the definition of the definition is, but when you're looking at an atomic level, that's beyond microscopic. That's sort of nanoscopic. Microscopically, solutions are

homogeneous, but microscopically, colloids are not homogenous. In colloids, the particles are small, which is why you can't see them with the naked eye, and so it looks homogenous, but they're not broken down right to the molecular or atomic level. So you can imagine we've got three levels. Substances that are broken down right to the atomic or molecular level. That's a solution. Then at the top level, we've got substances

that are not even broken down at a macroscopic level. Like we can see the chunks of this and that, just with the naked eye. That's heterogeneous substances like your pizza. And then in the middle we've got colloids where the different parts of the things that have gone into the substance, or the mixture have been broken up to a large extent. And so at the naked eye, it looks like it's all mixed together.

But when we zoom in at a microscopic level, we can see the small particles have been broken up in separate. It's sort of like, perhaps you can imagine that if you had a very fine sand, if you just looked at it sort of from a distance, it would just almost look like water. Like it would just look like one mass of substance that could be divided as much as you like. But when you look at it closely, you actually see that it's not homogeneous because it's composed of individual grains of sand, which are actually separate and distinct from

each other. So that might be an analogy you can think of. Now, there are actually many different types of colloids, depending, especially upon the different phase of the solute versus the solvent. There are many different ones. Many of them you probably won't have heard of. I'll just mention some that people that you probably have heard of, or at least the words that people use, but may not actually know what they mean. So the one that most people have definitely heard of is an aerosol. People talk about aerosol cans all the time. And aerosol is actually a colloid. Specifically, it's a colloid of fine solid or liquid droplets in a gas.

So think of it as like you've got solid particles or liquid particles, or liquid droplets, dissolved in a gas. Now, they're not exactly dissolving, because remember, this isn't a solution. And so they're not broken up at the atomic or molecular level, but very small solid particles or very small liquid particles, and they're dispersed throughout a gas. So a classic example of this is a class. cloud, clouds are not actually gas, or in other words, they're not actually made of water vapour. If they were, you wouldn't be able to see. Clouds are actually either water ice or liquid water, plus a few other bits and pieces, but mostly water, but very fine particles of ice or water, which are dissolved, or not

dissolved, but suspended in the air, and they kept there through up currents of warm air and so on. We'll talk about that when I finally get around to doing some Earth Science podcast, which you may have noticed are conspicuously absent, but anyway. So, clas are an example of an aerosol, although I don't think I've ever heard anyone refer to them as such in ordinary speak. Air pollution like smog and smoke are also aerosols, so you've got the small particles of carbon or carbon dioxide or whatever other pollution you've got there, which you can see. So once again, these are not broken down to the molecular or atomic level, but they're broken down into fine enough particles so that they can stay in the air or whatever other gas they're in, and also so that from a macroscopic level, it looks homogenous to us. Other examples of aerosols are, of course, the stuff in aerosol cans that we're more familiar with,

like scents and odourants, deodorants and stuff like that, where you essentially have small droplets of liquid, which are suspended in the air. Another common type of colloid that people have come, that you may have come across, is a foam. Foam is a substance made by trapping a gas in a liquid or solid. So you've got little pockets of gas inside the liquid or solid region. So foam is sort of the opposite of an aerosol. Aerosol, you've got tiny liquid or solids dispersed throughout a gas, In a foam, you've got tiny gas bubbles dispersed throughout a liquid or solid.

Examples of foams include shaving, cream aerogel and pumice. Pumice is actually a type of rock that's got heaps of air dissolved in it, well, not dissolved, suspended in it, which makes it really light. And I think pumice actually floats on water, or at least the particularly light forms do. It's kind of cool stuff. So, yeah, whenever you talk about a foam, what's actually going on there is there's tiny little bubbles of air or some gas, it's often air or carbon dioxide, mixed in inside a solid or a liquid. Now, the difference between a foam and, say, carbon dioxide dissolved in a flavored liquid,

which would be a soda drink, the difference is how finely broken up the gas is. If the gas particles are essentially individuals that carbon dioxide molecules dispersed evenly throughout the solution, that's a solution. But if the little bubbles of gas are actually sort of pockets of gas, there's a bunch of gas molecules all around each other, but they're just really small, and so you can't see them with the naked eye, then it's a foam. So hopefully you're seeing the distinction here between the solution and the colloid. From my reading, it seems that there's not actually, in practice, the distinctions are not always so clear, and it's sort of hard to differentiate the... So it's a bit of a fuzzy line between when you have really fine particles and a colloid and sort of a not-so-great solution.

It's a bit of a fuzzy line there, but conceptually it's still useful. Another type of colloid is a soul, s-o-l, aerosol, except just the sole part. That's a colloidal suspension of small solid particles and in continuous liquid medium. So blood is an example of a soul. Blood is actually, it's not a solution because you've got red blood cells and white blood cells and platelets and proteins and other bits and pieces. So it's clearly not different, it's clearly not broken down to an atomic or molecular level because those things are much bigger than molecules. However, macroscopically, it looks homogenous. You can't see any of that with the naked eye. So that would be an example of a very good example of something that's macroscopically homogenous, but microscopically when you're pulling the microscope, it's not homogenous because you can see the different large proteins or cells and so on that are inside the blood.

Paint and ink are also examples of souls where you have the pigments, the particles that absorb certain frequencies of light suspended throughout the solution. Final example is an emulsion. Another way you may have heard before, an emulsion is a mixture of two or more liquids that are normally immiscible, that is normally can't be mixed. Milk is an example of that, as is liquid soap. So normally you wouldn't be able to mix the two liquids, but in emulsion through various physical processes, they've been mixed. So they're not broken down on a molecular or atomic level, so they're not forming a solution, likely because the bonding forces between the solute versus solvent, or potential solute and solvent, are two different. So you've got your liquid soap example, you've got the fatty acid molecules in the, or similar molecules in the water.

Normally those weren't mixed. But if you put it through the right physical process, you can break up the soap and the water molecules. Well, the water molecules already break up, but you can at least break up the pockets of water, and soap enough and mix them together so that it from the naked eye looks homogenous, even if you zoom in, you actually see that at a microscopic level, the two substances aren't really in solution with each other. So an emulsion is kind of like a pseudo-solution. It's too liquid, is it kind of in solution, or at least it looks like they're in solution,

but when you zoom in, they're actually not in solution. And an emulsifying agent or an emulsifier is any substance that keeps the parts of an emulsion mixed together, so those can be used in cooking. Okay, so that's all the coal, as I want to talk about. Just briefly, we've got the aerosol, which is gas or liquid, sorry, solid or liquid, in a gas. Foam, which is the opposite of an aerosol, small bits of gas in a liquid or solid. A sole, which is small solid particles in a liquid like blood, and an emulsion, two liquids mixed together that normally can't be mixed. So those are type of colloids.

Macroscopically homogenous, microscopically heterogeneous, whereas solutions, once again, are homogenous right down to the, at the microscopic level, until you get to the molecular level where nothing's homogeneous. Suspensions are mixed just where the particles are sufficiently large that you can see the heterogeneity of the mixture to the visible eye. So once again, so an example of a suspension would be sand in water, for example, or mud, or flower in water. You can mix the water and say the sand together, so you don't just have the sand sitting at the bottom of your glass and all the water above it. So you can sort of mix them together so they're a bit mixed. Even with the naked

eye, you're still going to see that what you've got is sand in water. They're not going to be homogeneous even at the macroscopic level. And certainly you don't have a solution. So kind of On the scale, from more mixed to less, you've got solutions, then collars in the middle, and suspensions are the far end. And I guess, sort of even above that, you've just got completely separate, like your pizza, for example,

which they're not even, they're not even tending to be mixed, like in a suspension where you've got one thing inside another. They're just completely separate. So that's how you maybe differentiate suspensions from just other stuff. Dust suspended in air is another example of a suspension. Suspensions will eventually settle if they're left understood.

So if you leave the flour in the water or the sand in the water, eventually it'll just settle down to the bottom and they'll completely separate out again. to become that completely differentiated substance. A coloid will generally not settle out, or at least I suppose it may settle out if it's the right temperature and pressure and so on, but it would take a very, very long time because the particles are broken up at a very small scale,

and so it's going to be very hard for them all to come out of that and separate again. And solutions, if they're stable, you don't change temperature and so on, will never settle out. That's a stable form of the substance. Okay, so that's really all I wanted to cover in this episode. Hopefully you've got a better understanding of solutions and solubility and mixtures and how all that works. And maybe next time you are talking to someone about deodorant, you can tell them what an aerosol is, that it's actually a colloid.

So that's it for this episode. If you enjoy this podcast, then please let some people know how great this podcast is or comment on iTunes. Give the podcast a rating, that is. Visit the website at fods12.podbean.com, where you can see past episodes and leave a comment. You can also email me if you have any feedback you would like to give or suggestions or even suggestions to future episodes. Topics for future episodes. My email address is FODS12, that's F-O-D-S-1-2 at gmail.com. Thanks for listening, and I'll talk to you next time.