Representing solutions using particulate models
A solution is a homogeneous mixture composed of two or more pure substances. In this video, we'll learn how to represent the relative concentrations of the substances in a solution as well as the interactions between the substances using a particulate model. Created by Sal Khan.
- [Instructor] The goal of this video is to help us visualize what's going on with the solution at a microscopic really at a molecular level. And also to get practice drawing these types of visualizations, because you might be asked to do so, depending on the type of chemistry class you're in. So what I have here are three different aqueous solutions, which means that the solute is dissolved in water. The first one is sodium chloride. Then we have magnesium chloride. Then we have C12-H22-O11, which in other words is sucrose. And each of them are dissolved in water. What I'm gonna do is I'm gonna try to do a drawing of what's happening at a particulate level in the respective rectangles right below them. So, first of all, let's do think about what happens with sodium chloride. So the first thing that you might realize, is that sodium chloride, this is an ionic compound. It's made of up of a sodium positively charged ion or cation, and a chloride negatively charged ion. And if I wanna draw them, and they're going to be in a one to one ratio. For every sodium there's one chloride. And I could think about their relative sizes and to help us do so I'll get out the periodic table of elements, and we can see here that they're both in the third period. And if we were just looking at the atom, a sodium atom versus a chlorine atom, the general trend is as you go to the right and you have more electrons in that outer most shell, the radius tends to actually get smaller. So a chlorine atom is actually smaller than a sodium atom but we're not talking about atoms. We're talking about ions. So positively charged sodium ion that has lost an electron. So it actually has an electron configuration of neon, while the chloride anion has an electron configuration of argon. So it actually turns out that the chloride anion, is going to be larger than the sodium cation. And so what I will do is I will represent the chloride anion looking like this. I'll put a negative charge there, and for the positively charged sodium cation, I will make it a little bit smaller, something like that. And if I wanted to visualize it for every sodium positively charged ion. I would also have to draw a chloride anion. So that's one of them. And then you might say, "okay, well that's the solute, but where is..." And sometimes they actually just ask you just to draw the solute in which case you would be done. But if you're wondering, well how is that interacting with the water? And this will even help us understand what's happening. Why sodium chloride, why ionic compounds dissolve well into water. Well then we have to draw the water molecules. And if we still wanna get the relative sizing right, we can go back to our periodic table of elements. And we know that the sodium cation, has an electron configuration of neon, and oxygen is pretty cool close to that. And when we're talking about water, the oxygen atom is hogging the electrons. The electrons are spending a little bit more time around the oxygen than around the hydrogens. So it's actually going to be similar in size, and oxygen and water and a sodium cation. And obviously these aren't going to be exact drawings, but we can imagine each water might look something like that. So it's an oxygen with two hydrogens. I'll just do it all in this white color, right over here. And then the question is what would their orientation be? And that's really important to get right, especially when you're dealing with water, which is a polar molecule. We know that the electrons spend more time around the oxygen. We talked about this in many videos. So the oxygen end has a partial negative charge, while the hydrogen ends have a partial positive charge. And so the orientation of a water molecule is that the partially negative oxygen end, will be attracted to the positive ions. And then the positively charged hydrogen ends will be attracted to the negative ions. So you might have something like that. That's oxygen and then the two hydrogens. You might have an oxygen, and then you have the two hydrogens. Because the hydrogens, that end of the water molecule has a partially positive charge. They're going to be attracted to this chloride anion. You might have once again, the two hydrogens and then the oxygen. And once again, the oxygen is going to be attracted to the sodium, the positively charged sodium cations. And I could keep filling these in for, in this entire space. But I think you get the idea of what the water would kind of look like and how it would be oriented. Now for these next two, let's just focus on the solutes. What would the solute look like in the solution? Well, magnesium chloride, this once again is an ionic compound. And so it is going to disassociate into its constituent ions for every one magnesium ion, actually going to have a plus two charge. You're going to have two, negatively charged chloride anions. And what's the relative sizing? Well, to help us with that, we go back to the periodic table of elements. And if we're talking about a magnesium two plus or positively charged ion, it's still going to have an electron configuration of neon, and it's going to have more protons than the sodium ion. So it's gonna pull even harder on them. So it's gonna be even smaller than the sodium ion. So we could draw the magnesium ions like this. Maybe I'll do two of them. So I'm gonna do it even smaller than the sodium and I'll write two plus, because it has a positive two charge. I will write two plus again, 'cause it has a positive charge. And for every one of those, you're going to have two chloride anions. So maybe one there, maybe one there, maybe one there and then maybe one there. And then if you were asked to draw the solvent, draw the water, you would orient it similarly. Where the partially positive the hydrogen ends of the water would be attracted to the negative chloride anions. And the oxygen end of the water molecules would be attracted to these plus two charge magnesium ions. Now what about sucrose? It isn't an ionic compounds. So in this situation, it's not going to disassociate. So a sucrose molecule is relatively larger. I'll draw it like that. Maybe we have another one just like that. And the reason why it dissolves in water well is, a sucrose molecule has parts of the molecule that have polarity to it. It has a lot of OH groups. So there's parts of the molecule that are partially positive and other parts of the molecule that are partially negative, other parts that are partially positive. And so that's able to be attracted to the various ends depending on whether it's partially positive, partially negative, of the water molecule. And so I'll just write it like this. C12-H22-O11. C12-H22-O11. For the sake of time, I haven't drawn the water molecules here and to actually make, to draw them intelligently, you would have to know what parts of this larger molecule have a partially positive or partial negative charge. But as we'll see the fact that you can either disassociate into ions that clearly have charge, or then you have a larger molecule where parts of it have a partial charge or have some charge associated with it. That's what allows it to dissolve well into a polar solvent like water.