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  • Hi. It's Paul Andersen and this is video 001 in the chemistry essentials series.

  • The history of chemistry goes way back to the Alchemists. The Alchemists searched for

  • what is called the philosopher's stone. It was this magical stone that they thought could

  • turn something like lead into gold. Now they never discovered the philosopher's stone but

  • they did start to discover some unique properties of elements and molecules. And the whole thing

  • eventually lead to this atomic theory. This idea that all matter is made up of smaller

  • units. And those are going to be called atoms. And so in this video I'm going to talk about

  • mostly molecules and elements. And so all matter can be broken down either into mixtures

  • or pure substances. And those pure substances are either elements or molecules. So an example

  • of an element could be pure gold. An example of a molecule could be water. An example of

  • a mixture could be dirt. And so if we take those elements and molecules, they're each

  • made of atoms. In other words gold is just going to be made of gold atoms. And then water

  • is just going to be made up of oxygen and two hydrogen atoms. And so if we were to take

  • a pure sample of it. It's only that element, we would find that no matter how big the size

  • of that object is, it's going to have the same average mass. Likewise if we were to

  • take a pure sample of a molecule, like water. We're going to find that there's the same

  • ratio of the average masses. In other words the average mass of the oxygen and the average

  • mass of the hydrogen. But let's say we find something else that's made of oxygen and hydrogen.

  • A good example could be hydrogen peroxide which is actually two oxygens and two hydrogens.

  • We would find since it has a different atom number it's going to have different ratio

  • of average masses. It's still going to have a consistent ratio through all different size

  • of the objects, but it's going to have a different ratio compared to that of water. And so what

  • do we mean by pure sample again? A pure sample is just going to have those atoms from either

  • that element or molecule inside it. So this is a pure sample of gold. If we were to look

  • at a pure sample of silicon it's going to look like this. Or lead is going to look like

  • this. Or uranium. In other words if we were to dig through it we would find just uranium

  • atoms in this pure sample. Likewise if it's a molecule, like water, we're just going to

  • find two hydrogen and one oxygen connected together. And we're just going to find that

  • repeating over and over and over again. Or if we were to look at something like dry ice,

  • which is simply solid carbon dioxide, we're going to find that it's going to be the same

  • atoms in a specific ratio over time. And so again one of the big points is that the average

  • mass is going to stay the same no matter how big the sample. In other words the average

  • mass of this, which is the largest gold bar ever created. It would be worth about $11,000,000.

  • So it's about 500 and some pounds. If we were to take the average mass of that whole gold

  • bar, or the the average mass of a section of it or a smaller section or a smaller section,

  • it doesn't matter how small the section is, it's going to have the same average mass.

  • And that's because it's made up of these atoms. And so let me kind of explain that. Let me

  • use analogy. Imagine I'm building something out of Legos. I'm using that standard 2 x

  • 4 lego brick. Well that weighs about 2.5 grams. One of those Lego bricks. And so let's say

  • I build something that was made up of 4 them. It's going to weigh 10 grams. Or let's say

  • I build something that had 32 of them. It's going to be 80 grams. But if we were to look

  • at the average mass of those objects, we're going to get around 2.5 grams per brick. So

  • it doesn't matter how large or small the sample is. If I had a big structure that was the

  • size of this room made out of these red Lego bricks it's going to still have the same average

  • mass. And we find the same thing in matter. That's because it's made up of these atoms.

  • And so it doesn't matter if we have 4 atoms of gold or 32 or billions, billions, billions

  • of atoms. It's still going to have that same average mass. Now this is just elements we're

  • talking about. But the same thing applies to molecules. So if we're looking at water,

  • small and smaller and smallest sample of water are still going to have the same average mass.

  • And how does that work? Well imagine we go back to the Lego analogy. If we've got one

  • Lego brick that weighs 2.5. So we could think of that like the oxygen molecule. And then

  • we had two of these little blue bricks. Each of those weigh 0.25 grams. Then we're going

  • to have a total mass of 3.00 grams. And so it doesn't matter if we have one of those

  • units or 4 of those units or 32 of those units. It's still going to have the same average

  • mass if we divide by the number of units like that. So that's an example of water. But let's

  • say we were to try using the same atoms in this analogy. So just the red and the blue.

  • But we were to put it together in a different structure. And so let's say our building blocks

  • look like this. So instead of building with 1 red and 2 blue, what if we were to build

  • with 2 red and 2 blue? It's going to be a lot more like hydrogen peroxide. Well now

  • we're going to get an average mass that is going to be higher, because we're adding more

  • units to it. And so if we were to look at two molecules that have the same atoms, oxygen

  • and hydrogen, but they have them in a different ratio, then we're going to get a different

  • ratio of their average masses. And so let's kind of review. Could you pause the video

  • and fill in these little blanks? What goes here, here, here and here? Well up at the

  • top, so those are going to be elements and molecules. They're made up of atoms. They're

  • going to make the same ratio of average masses and then they're going to have a different

  • number of atoms down here on the side. And so this is just an introduction to chemistry.

  • What did you learn? Well you should have learned, that was my intent, that the ratio of masses

  • in a pure sample is identical on the basis of atomic theory. And that's going to apply

  • for elements and molecules. And I hope that was helpful.

Hi. It's Paul Andersen and this is video 001 in the chemistry essentials series.

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B1 average sample pure ratio hydrogen gold

Elements and Molecules

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    李掌櫃 posted on 2014/03/31
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