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.