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- [Lecturer] Almost all[br]the matter in the universe

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is made from a few, about[br]100 different elements,

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kind of like how all the English words

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and sentences in all the books

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and poems are made from just 26 letters,

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and of course, some spaces.

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But what I love about this analogy

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is we can take it one step further.

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Just like how letters can[br]combine in interesting ways

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to form lots and lots of words,

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these elements can combine[br]in some interesting ways

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to form substances,[br]what we call compounds,

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and that's what we wanna[br]talk about in this video.

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Well, first things first,[br]how did we figure out

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that elements can combine[br]to form compounds?

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Well, when we took naturally[br]occurring substances

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with metals in them, which[br]we call ores, for example,

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cinnabar is an ore

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because it contains mercury metal in it.

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And when we heated it, we were[br]able to extract that metal,

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we were able to get that metal,

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so we were able to get[br]the mercury out of it,

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and some other substances as well.

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From this, we were able to guess that,

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hey, look, cinnabar is[br]made from the combination

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of different elements.

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So cinnabar is probably your compound.

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And there are other examples.

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For example, if you take the ore of gold,

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which we call calaverite

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Yeah, these ores have very fancy names.

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Then if you heat that up,

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we will be able to extract[br]metal, the gold out of it

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and some other elements as well.

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And again, you can see[br]therefore calaverite,

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this ore is a combination[br]of these elements,

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and that's how we are able to guess

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that elements can combine[br]together to form compounds.

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And that's also the reason[br]why the periodic table

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does not have these compounds,

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because they are not[br]fundamentally new elements.

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These are the elements,

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and these are formed by[br]combination of elements.

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But of course, let's be very concrete now.

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What exactly are compounds?

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Well, we define compounds are substances

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which are made from chemical combinations

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of two or more different elements.

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For example, if you look[br]at water, it's a compound

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because it's made from[br]the chemical combination

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of two elements, hydrogen and oxygen.

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But what does it mean?

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What does it mean to say

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that they are a chemical combination?

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Well, for that, let's zoom into it.

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If you could zoom into water

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and look at the smallest bit of water,

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then we would see something like this.

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We would find that one[br]oxygen atom is combined,

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it's connected to two[br]other hydrogen atoms.

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This is what we call a molecule of water.

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But what keeps these atoms together?

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Why are they connected like this?

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Well, it turns out that[br]atoms can have a force

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of attraction between them.

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And this force of attraction

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is what we call a chemical bond.

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Again, don't worry too much

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about where this chemical bond[br]comes from and all of that.

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We'll study more about that[br]in high school chemistry.

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But what's important is that[br]when atoms combine like this,

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that's what we call as[br]a chemical combination.

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And so look, water is[br]a chemical combination

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of two or more different elements.

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And so it's made of billions

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and trillions of these molecules.

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Let's take another example.

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If we consider carbon dioxide,

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which we usually find in[br]smoke, then we'll find

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that it's made of two[br]elements, carbon and oxygen.

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And again, if we could zoom into it

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and look at the smallest[br]bit of carbon dioxide,

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this is what we would find.

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We would again find that look,[br]one carbon atom is connected

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to two other oxygen[br]atoms forming a molecule

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of carbon dioxide.

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And again, it's a chemical combination.

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And that's why carbon[br]dioxide is a compound.

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It's called dioxide because[br]there are two oxygen atoms.

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Okay, let's take one last example.

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If you were to look at the[br]smallest bit of oxygen,

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you would see something like this.

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Two oxygen atoms are combined[br]together to form a molecule.

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So do you think oxygen is a compound?

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Well, remember, compounds[br]require chemical combination

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of two or more different elements.

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This is a chemical combination[br]of the same element,

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and therefore this is not a compound.

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Okay, now that we understand[br]what compounds are,

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let's finally see how to[br]write their chemical formulas.

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So if we go back to water,

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we saw that oxygen atom is connected

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to two other hydrogen atoms, right?

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So guess what?

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This ratio stays the same.

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One oxygen atom will always be connected

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to two other hydrogen atoms

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when it comes to a water molecule.

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And so the way we write this

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is we write the chemical formula as H2O.

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Notice that the two over here[br]is written as a subscript,

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which represents that there are[br]two hydrogen atoms connected

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to one oxygen atom.

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We'll not write it as H2O2, or HO.

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This would be all wrong.

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And fun fact, H2O2 is[br]called hydrogen peroxide,

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which is poisonous,

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which is ironic because[br]H2O, which is water,

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it gives us life, but add[br]one more oxygen atom to it,

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it becomes poisonous.

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Okay, similarly, if you were[br]to look at the chemical formula

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for carbon dioxide,

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because I have one carbon[br]attached to two oxygen atoms,

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and that will always be the[br]fixed ratio for carbon dioxide.

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The way we'll write this is CO2.

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Again, notice we have written[br]two over here as a subscript

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because one carbon is[br]attached to two oxygen atoms.

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And we can have much bigger molecules.

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If you have ever put, sugar in lemonade,

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then you worked with a[br]molecule called sucrose.

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It is a big molecule.

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And let's now look at its[br]chemical formula, C11H22O11.

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So this tells us

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that there are 11 carbon atoms

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attached to 22 hydrogen atoms,[br]attached to 11 oxygen atoms.

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This is one single molecule of sucrose.

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All right, putting it all together,

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all matter in this universe

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is made from a few, about[br]100 different elements.

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And when these different elements combine

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together chemically, we get compounds.

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The ratio of the elements is always fixed

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in these compounds,

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and that's represented by[br]these chemical formulae.

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And these compounds can be[br]represented by these molecules,

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which act like a single unit over here.