 Welcome back my little portobello mushrooms. This is a video about lab on density. It's supposed to be a lab on density in specific gravity, but I don't care about specific gravity because it's not, well, the specific gravity calculations that you would be doing would not really make a lot of sense, so we're going to skip it. So I'm going to talk about density, which is what you're going to do for a lab. Density is a feature that all material objects have. It has a mathematical definition. Here is the definition of density. It is the mass of your material divided by the volume of your material. Mass is pretty close to weight, so basically density is how much something weighs roughly that I'm cutting a corner there, but close enough, divided by how much space it takes up, which is called the volume. And the units for mass, usually, you know, if you're using something related to the metric system, it will be grams, kilograms, things like that. And units for volume, liters, milliliters, cubic centimeters, things like that. And those units don't cancel each other, so you have grams, kilograms, some mass unit in the numerator, you have liters, milliliters, cubic centimeters, something related to that in the denominator and they don't cancel. What that means is when you perform a density calculation and you get some number out, called the density, the units will always be some mass number divided by a volume number. A common unit for density is grams per milliliter, but it doesn't have to be that. Those units could be other ones. So all materials have a density. Solids have a density, so if you have a brick, it weighs something, it takes up a certain amount of space, you can figure out its density as long as you can get numbers on that. Gold, lead, ice, all of those things have densities. Liquids have densities as well. And one thing that I want to point out is that for a lot of materials, if you're very careful and you're very specific about the temperature that you do the experiment at, a lot of times the density of your material can be a fingerprint for what the material is made of. So in other words, gold has a very specific density. And if you have a material that has a density that matches the density of gold, then it's a pretty good chance that the material you have is gold. Not guaranteed, but that's usually true, and that's true for a lot of different materials. Lead has a density that's pretty specific to lead, and on and on and on. So I want you to realize that density, a lot of times, is used to help identify what materials are made of. Like I said, liquids have densities as well, because they weigh something and take up a certain amount of space. Even gases have densities. So the air in the room that you're in, it has a density. It weighs something. It might not weigh much, but it does weigh something, and it takes up a certain amount of space. And so if you could figure out those numbers, you could plug them in up here, and you could get the density. So if I gave you the mass and the volume of something, you could figure out the density, just as I'm going to make up these numbers. So pretend I have some little block or something, and I'm just going to keep this really simple. Let's say that weighs 6 grams, and so that's its mass. And the volume, I don't know, 18 milliliters, or 18 cubic centimeters. That's the volume. And I'm just making these numbers up. And if I said, hey, what's the density of this material, you would break out this formula and you'd plug the mass into the numerator, plug the volume into the denominator, so you'd say 6 grams divided by 18 milliliters, and that is going to be 0.3 repeating, but with significant digits that, you know, there's a certain way you're supposed to write this that I don't care about. And like I said, those units don't cancel. So the way that you would write the density is 0.3 grams per milliliter. Just like this. There is no number written next to the milliliter, but there is a number implied to be there, and that's the number 1. That's almost always true. If you see a unit with no number next to it, the number 1 is almost certainly implied to be next to it. So what this density is saying is that for every 1 milliliter worth of my block, that 1 milliliter worth weighs about 0.3 grams. If I have twice as much, if I have 2 milliliters of block, then it's going to weigh twice as much. 0.6 grams, or 0.6 repeating. So that's how you calculate density. So the other thing that I wanted to point out is the density formula. I'm just going to simplify it here, d equals m divided by v. There are three places where you can put a number. You can put a number to the left, put a number in the numerator, you can put a number in the denominator. As long as you know two of the three numbers, you can use algebra to figure out the third. Obviously the problem that we just did, I gave you those two numbers, and I asked you to figure out the number on the left. But it doesn't have to be that way. I could give you the density number, and I could give you the mass, and you should be able to figure out the volume. And basically that, you know, if you want to memorize this you can, but volume is going to equal, screw that up, volume is going to equal the mass divided by the density, and then, well it's the third thing, I could give you the density and I could give you the volume, and ask you to figure out the mass, and so that, that's a third formula. Mass is equal to the density times the volume like that. So if you want to sort of memorize these three equations, you're welcome to do that. There are other ways of reasoning through it as well. There's this triangle symbol that my students use. I feel like they're performing a satanic ritual whenever they break this thing out, and I hate it, but I will show it to you. It makes my skin crawl, but students love it. How does it go? It goes like this. So there's some triangle, and it's broken into these parts like that, where there's a D and M and a V for density, mass, and volume. Basically the idea is, if you don't know the density, then you do mass, and then it's on top of the volume, so you do mass divided by volume. Or you could be in this situation where if you don't know the mass, then you do density times the volume because they're right next to each other. Or the third situation, if you don't know the volume, but you know the other two, well, the mass is over the density, so volume is going to be mass over density. Students love this. I hate it, but I have a feeling that many of you will like it. So there it is, I just sold my soul. Specific gravity, I said I don't care about. Specific gravity is basically, it's related to density. It's when you take the density of one thing and you compare it to the density of some other thing that everybody is using as a reference. Usually people are using water, the density of water as a reference, and the density of water under the conditions that people usually use it is one gram per milliliter. Specific gravity of something would be density of whatever your thing is, density of my stuff divided by the density of water because that's what we're comparing it to, and that's one gram per milliliter, but you're just taking your density number in these cases and dividing it by one, so you end up with the same number, which students feel like you're just wasting their time because you're just asking them to divide by one. There's a little bit more going on there than that, but not much. So we're going to ignore it, blah, blah, blah, and this is what you would have done in an on-ground lab, we're going to skip it. You're going to do something similar to this in the lab handout. You would have also figured out the density of some solids. Again, I'll go over the lab itself in a tiny bit of detail. And there was a bit about density of pennies, you can ignore that too. All right, here's the actual lab. Here's a video where they suggest to watch them. It's probably a reasonably good idea to watch that video. It might help explain anything that I explained poorly. Here you have some data to figure out the density of a couple of different liquids. Calculate the density of number one, number two, which one of them is water. Explain why this is related to me. Remember I said that density of a lot of things can be used to figure out what those things are, so which one is water. You might have to look some things up on the internet. Here's another video. This video is okay too, a little bit. The person talks like they have a poll at their ass, but what are you going to do? Here, basically you're going to try to figure out the density of this block. And remember you need the mass, you need to write it by the volume. So maybe the tricky thing here is you need to figure out the volume of this block. But what they are doing is they're giving you enough information. They're telling you the dimensions of the block. They're telling you how long it is, how wide it is over here. They're telling you how tall it is. And if you remember if you have a simple box like that volume, it's going to be equal to the length times a width times a height. And that's related to, I think, last week's video. So if this doesn't make any sense, you can go back and watch last week's video about how to compute a volume of a box. And so you've got some data there. Tell me the volume. Here's the mass. Figure out the density. Hopefully that makes sense. Then you've got some weird object, right? Here's some weird looking object. There's no simple formula to figure out the volume of that object. You can weigh it. So if you want to figure out the density, you can weigh it. And it's pretty easy to figure out the mass. Volume. No simple formula. You can't break out a ruler and figure out the volume of that thing. But what you can do, and I believe I covered this in a previous video, too, is you can measure the volume of a weirdly shaped object using something called volume by displacement. And in this case, what you would do is you pour some liquid into a graduated cylinder. So here's our liquid in a graduated cylinder. You measure the volume of the liquid all by itself. You throw your weird object into the liquid. Volume goes up by a certain amount. You measure the new volume. And then the difference between the new volume and the old volume is the volume of your weirdly shaped object. And then you basically have the pieces you need for density. So you can go and finish up. Blah, blah, blah. What's the density of the object? What is it made of? You may have to look at another website to figure that out. And that's it. Look at that. No specific gravity. All right, bye-bye, everybody.