Representing solutions using particulate models | AP Chemistry | Khan Academy

The goal of this video is to help us visualize what's going on with the solution at a microscopic level, 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 C12H22O11, which in other words is sucrose. Each of them is dissolved in water. What I'm going to do is I'm going to try to do a drawing of what's happening at a particular level in the respective rectangles right below them.

So first of all, let's think about what happens with sodium chloride. The first thing that you might realize is that sodium chloride is an ionic compound. It's made up of a sodium positively charged ion, or cation, and a chloride negatively charged ion. And if I want to draw them, they're going to be in a one-to-one ratio. For every sodium, there's one chloride.

I can think about their relative sizes, and to help us do so, I'll get out the periodic table of elements. We can see here that they're both in the third period. If we were just looking at a sodium atom versus a chlorine atom, the general trend is that as you go to the right and you have more electrons in that outermost 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. The positively charged sodium ion has lost an electron, so it actually has an electron configuration of neon, while the chloride anion has an electron configuration of argon. It turns out that the chloride anion is going to be larger than the sodium cation.

So what I will do is I will represent the chloride anion looking like this; I'll put a negative charge there. For the positively charged sodium cation, I will make it a little bit smaller, something like that. If I wanted to visualize it, for every sodium positively charged ion, I would also have to draw a chloride anion. That's one of them.

You might say, "Okay, well that's the solute, but where is..." Sometimes they actually just ask you to draw the solute, in which case you would be done. But if you're wondering, "Well, how is that interacting with the water?" this will even help us understand what's happening, why sodium chloride, why ionic compounds dissolve well in water.

Then we have to draw the water molecules. If we still want to get the relative sizing right, we can go back to our periodic table of elements. We know that the sodium cation has an electron configuration of neon, and oxygen is pretty close to that. 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; an oxygen in water and a sodium cation. Obviously, these aren't going to be exact drawings, but we can imagine each water might look something like that. It’s an oxygen with two hydrogens. I'll just do it all in this white color right over here.

Then the question is, what would their orientation be? 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've talked about this in many videos.

So the oxygen end has a partial negative charge, while the hydrogen ends have a partial positive charge. 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—this oxygen and then the two hydrogens. You might have an oxygen, and then you have the two hydrogens because the hydrogens at that end of the water molecule have 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. Once again, the oxygen is going to be attracted to the sodium, the positively charged sodium cations. I could keep filling these in for 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 it is going to dissociate into its constituent ions. For every one magnesium ion, you're actually going to have a +2 charge; you're going to have two negatively charged chloride anions.

What's the relative sizing? Well, to help us with that, we go back to the periodic table of elements. If we're talking about a magnesium 2+ 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 going to pull even harder on them. Therefore, it's going to 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 going to do them even smaller than the sodium and I'll write 2+ because it has a positive 2 charge. I will write 2+ again because it has a positive charge. 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.

Then if you were asked to draw the solvent, draw the water, you would orient it similarly, where the partially positive 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 +2 charged magnesium ions.

Now what about sucrose? It isn't an ionic compound, so in this situation, it's not going to dissociate. A sucrose molecule is relatively larger; I'll draw it like that. Maybe we have another one just like that. The reason why it dissolves in water is that a sucrose molecule has parts of the molecule that have polarity to it.

It has a lot of -OH groups, so there are 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 or partially negative of the water molecule.

So I'll just write it like this: C12H22O11, C12H22O11. For the sake of time, I haven't drawn the water molecules here, and to actually 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 dissociate into ions that clearly have charges, or that 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.