 Okay, a couple of new topics. This first new topic is going to be very brief. I'm going to talk about the word salt or salts. For most people on the planet, the word salt means table salt, N-A-C-L or sodium chloride. But for chemists and biologists, table salt is just one kind of salt. So I'm going to give you the chemistry definition of what a salt is. Table salt is anything that's electrically neutral but is made of ions. So table salt is kind of the classic example of this. It's made of a sodium ion with a positive charge and a chloride ion with a negative charge. So it's made of ions. Remember, ions are anything with an electrical charge. But the electrical charges cancel each other out. So sodium chloride is electrically neutral, even though it's made of things that are charged. And what that means is sodium chloride is a type of salt. There are many other salts. This is calcium chloride. It's made of a calcium ion with a charge of plus 2. And it's made of two chloride ions. Each one has a charge of minus 1. If you put them all together, they are electrically neutral because the plus 2 charge over here gets canceled by the two negatives on the chloride. And on and on and on, magnesium sulfate, MGS04, there's another type of salt. I want you to know what the definition of salt is. The next new topic, and we're going to spend most of the rest of the lecture of this collection of videos talking about this, is something called solute concentration. So if you remember, solute is the stuff you dissolve in a solution or into a solvent. So in this case, my solvent is going to be my Dunkin' Donuts coffee. And the way that I usually introduce the idea of concentration is to talk about putting Splenda or some other kind of sweetener into my coffee. So first question I ask is, if I put one packet of Splenda into my coffee, does it taste sweeter than no Splenda at all? Students usually say yes, and that's correct. If I put two packs of Splenda into my coffee, will it taste even sweeter? Students say yes. You can do this a lot, right? So if you put four packs of Splenda in, will it taste even sweeter still? The answer is probably yes. So what we're doing is we're saying, look, we have a certain amount of liquid and we have one Splenda divided by a certain amount of liquid, and that is a certain amount of sweetness. And if we have four Splendas divided by a certain amount of liquid, that's even sweeter. And what we're doing is we're describing how concentrated the sweet stuff is when it's dissolved in the liquid, that's called solute concentration. And if there's more of the stuff dissolved in a certain amount of liquid, then the concentration goes up. But what I want to avoid here is I want to avoid saying that, oh, one Splenda is a little bit sweet and four Splendas is really, really sweet. I want to actually put numbers on this. I want to get more precise. And so what we're going to do is we're going to do a little bit of math and I'm going to show you some formulas that can let you put numbers on how much solute is dissolved in a liquid so that you can avoid saying, my liquid is really, really sweet, or it's super, super duper sweet, or I have another liquid with table salt dissolved in it and it's really, really salty. We want to actually use numbers. So a little bit more background before we get to the numbers. I believe this video is not going to do any numbers at all. We're just going to sort of talk about the ideas in general. So here's another situation. So this is my regular cup of coffee from Dunkin Donuts. And this is an espresso cup. And I know that espresso coffee is not the same as regular Dunkin Donuts coffee, but I want you to pretend same coffee is in my espresso cup. The espresso cup is smaller. And then what I say is I ask the students, if I take one Splenda, dissolve it in my regular Dunkin Donuts coffee, and I take another Splenda and I dissolve it in my espresso cup of coffee, which one is going to taste sweeter, assuming they have the same type of coffee in each one. Usually the students say this one will taste sweeter. And so the students who answer that this situation is sweeter over here are correct. And what they're doing in their head is they're sort of getting a general idea of what concentration is. So concentration is, you know, for a Splenda, it would be the amount of sweetness dissolved by the amount of liquid. And in the coffee cup situation and in the espresso cup situation, the top numbers are the same because I threw just one Splenda into each one. So the amount of sweetness that I dissolved in is going to be the same. That top number will be the same in both situations. But in the espresso cup situation, the bottom number is going to be smaller than it would be in the coffee cup situation. And because the bottom number is smaller, you know, the amount of sweetness divided by a smaller number is going to make the number get bigger. And that means the concentration of whatever you're dissolving, in this case the Splenda, the concentration of the Splenda will be higher than the concentration of the Splenda when the bottom number is bigger, which is the situation with the Dunkin Donuts coffee. So that's what concentration is in general. It's the amount of stuff you dissolve divided by the amount of liquid that's doing the dissolving. And this is the general equation that you get when you try to describe concentration of people. It turns out that there are many variations of this equation, and I'm going to show you, I think four different versions of this general equation. Three of them are very similar to each other. The fourth one is a little bit weird, and that's the one that students usually don't like, but three out of the four aren't too difficult to understand. And those three of those four are coming in the next video. So see you in the next video.