INTERVIEWER: Let's talk about the factors that affect whether or not something is soluble in something else. Right here I've got two graduated cylinders. The first one is filled with distilled water, and the second one is filled with hexane. And you're probably familiar with the molecular structure of water, and here is a representation of a molecule of hexane. These black spheres represent carbon, and these white spheres represent hydrogen. So this is a hydrocarbon. It's hexane, it's got 6 carbons. These little sticks in between the little spheres represent a region of electron density, a bond. And between carbon and carbon, you wouldn't expect that region of electron density to be very polarized. It's going to be pretty uniform across that particular bond. If you look at the hydrogen, hydrogen and carbon actually have about the same preference for the electrons. So this is not a very polar bond either. In the water case, however, the electrons will be spending most of their time around the oxygen. Oxygen is very electron-rich. So the question is, what do you think would happen when I mix hexane and water? Let's go ahead and mix these. Let's put the water in first. And the hexane. So, what we notice inside this flask is that there are two distinct phases. They did not dissolve in one another. But we could have predicted this based on what we saw in the molecular structures. Here we had bonds that were not very polar. But in water we had bonds that were polar. And that's how we come up with this general rule of thumb, like dissolves in like. So, here we could have predicted it. But let's look at another example. Here I have a volumetric flask filled with 50 milliliters of water. This is a volumetric flask filled with exactly 50 mils of ethanol. This is a molecular model of water, and this is a molecular model of ethanol. And in water, again, we said we had polar bonds. If we look at ethanol, it's kind of like a combination of both of the last two. Here we have polar bonds between the oxygen and the hydrogen, but we also have a part of the molecule that has two carbon atoms in it. So what would you predict would happen here? Will ethanol dissolve in water? Let's do the experiment. We're putting in the ethanol. And now we'll put it in the water. What's strange here is if we look at the glassware, this is a volumetric flask, and this line represents 100 milliliters. We put in 50 milliliters of water, and 50 milliliters of ethanol, but it didn't add up to 100 milliliters. Why is that? Well, it has everything to do with the types of interactions between water and ethanol. Water and ethanol both have polar bonds. And because they both contain oxygen and hydrogen, they're able to do a special type of bonding called hydrogen bonding. Where there's a strong attraction between this oxygen and this hydrogen, and overall, as that process occurs of these two molecules mixing, overall we have a smaller volume than we started out with. So the forces between the molecules has a strong effect on whether or not something is soluble. Another factor that affects whether or not things are soluble in solutions is temperature. Here we have a beaker that has solid potassium nitrate in the bottom. And there's also a magnetic stirrer bar in the bottom. There's so much potassium nitrate in this solution that we've actually-- we have solid potassium nitrate at the bottom. This solution is saturated. There is more potassium nitrate in this beaker than the solution can hold. So, let's turn on the magnetic stirring plate, which will interact with the stir bar. And we'll turn on the heat. And even though it's saturated right now as we increase the temperature, more solute will dissolve. OK, so a couple of minutes have passed and all of that solid has dissolved, and that's a really hot solution. It's also important to note here that not all solids behave this way. Calcium acetate is one notable exception. In that when you heat up a saturated solution of calcium acetate, less would dissolve. You would actually see a precipitate. So it's one exception to the rule. But in general, solids' solubility tends to increase as you increase the temperature.
