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- [Instructor] Check this out.

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I have two clear colorless
solutions over here.

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Let's pour them into each other.

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We pour the first one and
we pour the second one.

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And boom, we now get a
white colored solution.

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Here's another example.

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Again, two colorless solutions.

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We pour one into another, and boom!

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We again get a beautiful
yellow colored solution.

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What's going on over here?

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To find out, let's dig a little deeper.

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Here's a more curious
question for us, okay?

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so in the first case, what did we do?

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We poured sodium chloride
and silver nitrate together,

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and that gave us a white
colored solution, right?

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But now if I were to
change just one element,

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instead of silver, if I
had potassium over here,

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everything else is the same,

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so if I had poured potassium nitrate

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and sodium chloride into each other,

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I wouldn't have gotten anything.

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I would've just gotten
a colorless solution.

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It's not that interesting.

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And therefore, I couldn't
find any footage online.

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So this is just an edited
image, but you get the point.

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We wouldn't get anything
interesting over here.

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But the big question is why?

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Why silver nitrate and sodium chloride

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gives us a white colored solution,

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whereas potassium
nitrate, just one change,

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and sodium chloride does
not give us anything?

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Let's look at it. Let's
look at it one by one.

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So in the first case,

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we are reacting sodium
chloride aqueous solution

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with silver nitrate aqueous solution.

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What do we get?

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Well, remember that in aqueous solutions,

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ions usually dissociate.

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So over here, we'll get basically NA+ ions

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and CL- ions.

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And over here, we have
Ag+ ions and NO3- ions.

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So when I pour them together,

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we just get all those ions together, okay?

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Now, because these are together,

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they can form new combination.

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Of course, cations should always
combine with anions, okay?

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So NA can now combine with NO3,

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but there's nothing special because again,

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they will dissociate.

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But Ag can also combine with Cl.

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When Ag combines with Cl,
something interesting happens.

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What? Well, guess what?

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AgCl is insoluble,

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and therefore, it will precipitate out.

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And that's the reason why
this whole thing looks white

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because of the AgCl precipitation.

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So what do we end up with?

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We'll end up with AgCl,
which is insoluble.

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So that's why it's written
as solid over here.

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It precipitates out.

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So let me just share it over here

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to show that it's precipitating, okay?

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And what remains in the aqueous solution?

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Well, sodium ions and nitrate ions,

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so we get sodium nitrate aqueous solution.

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The white color is due to
the AgCl precipitating out.

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Now, if you zoom out and
look at the reaction,

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see what has happened,

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sodium and silver cations
have switched places.

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Sodium has replaced silver over here

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to get sodium nitrate,

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and silver has replaced sodium over here

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to give us silver chloride.

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So since there are two
cations replacing each other,

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there's a double replacement happening.

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This is called, no surprise,
a double replacement reaction.

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We can also call this double
displacement reaction.

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So what we witnessed was a
double replacement reaction

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and one of the products precipitated

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giving us that white color.

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Now, before we look at the other one,

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a quick question for you is,

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can you identify which of the
elements underwent oxidation

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and which ones underwent reduction?

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Pause and think about this.

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Okay, whenever I want to think about that,

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I just look at the
charges on the elements.

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Well, over here, sodium
has a positive charge.

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It's a cation.

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On the other side, well,
it's still a positive cation.

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So no change happened to
the charge on the sodium.

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Nothing happened to it, okay?

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What about Ag?

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No change.

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The same is the case
with the anions as well.

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No change, no change,

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which means look, nothing
is undergoing an oxidation,

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nothing is undergoing a reduction,

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so double replacement reactions
are not redox reactions.

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And you may be wondering,

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why are you excited about the fact

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that it's not a redox reaction?

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I'm excited because I used to think

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that all chemical reactions

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must involve electron
transfers, and therefore,

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all chemical reactions should
have something oxidizing

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and something else reducing.

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But I was wrong. Look,
right in front of our eyes.

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We can see examples of chemical reactions

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where there are no electron transfers,

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where there is no oxidation or reduction,

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so that's pretty cool.

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But anyways, now let's
look at the other one.

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What happens when I
pour these two together?

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Well, let's look at the reactants.

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This time, the reactants
are NaCl and KNO3.

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Both are aqueous solutions.

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I pour them together.

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So just like before, I will now have

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all the four different
kinds of ions over here.

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Na can combine with NO3.

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Remember, cations can only
combine with anions, okay?

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Those are the only new
combinations you can form.

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So Na can combine with NO3-,

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but again, it'll dissociate.

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K+ can also combine with Cl-.

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But what's important over here

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is that KCl, potassium
chloride, is soluble.

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Therefore, when K and Cl combine,

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again, they will dissociate.

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So nothing happens over here.
There's no precipitation.

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I'll just end up with a solution

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where all the four different kinds of ions

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are just floating around together.

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So no chemical changes happened.

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And that's the reason why I
don't get any colorations.

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I don't get anything over here.

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So over here, I get
essentially no reaction.

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So you notice the key difference?

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The key difference was AgCl was insoluble.

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That's why it precipitated out.

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And that's why in order for us

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to get a double replacement reaction,

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we need one of the
products to precipitate.

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If both are soluble and
they form aqueous solution,

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then nothing will happen.

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We'll just get a solution

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with all the four different kinds of ions.

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No chemical change at all.

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So in general, we can now write down

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what a displacement reaction looks like.

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We can say that if you have
an aqueous solution of AB

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reacting with an aqueous solution of CD,

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then a double replacement reaction,

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the two cations replace each other.

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So A will now combine with
D and C will combine with B.

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But that'll only be the case
if one of them is insoluble

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and precipitates out.

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Precipitation, sorry, is the key

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to having double replacement reaction.

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So look, if you pour any
two aqueous ionic solutions,

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do not expect to get a
double replacement reaction.

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You'll only get them if one
of the products is insoluble.

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But now we'll be wondering,

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how do we know whether a particular salt

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is soluble or insoluble?

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I'm glad you asked that question

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because that brings us
to the solubility chart.

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A solubility chart is basically that,

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it tells us whether a
salt is soluble or not.

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So here's how we can read it.

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If you wanna look at potassium chloride,

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here's potassium cation,

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here's the chloride anion, sorry.

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And now we can just say,
hey, this is where they meet

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and so this is the solubility
of potassium chloride

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and you can see it is soluble.

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But what about silver chloride?

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Silver is here.

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Chloride is here.

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Again, try to make them
meet. And what do you notice?

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Silver chloride is insoluble.

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And what about this
yellow slightly soluble?

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Well, don't worry too much about that.

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We'll only work with the soluble

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and the insoluble ones, okay?

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And just by looking at this chart,

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you can see some trends.

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For example, you can see salts of lithium,

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sodium, potassium, and even ammonium.

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Almost all are soluble.

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Of course, there are some exceptions,

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but they're all soluble.

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In contrast, salts of
lead are almost insoluble.

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You can also see salts
which have nitrate ions

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and acetate ions, pretty much soluble.

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Anyways, now equipped with
this solubility chart,

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we can predict whether certain
double replacement reactions

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are gonna happen or not, okay?

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So let's check that. Here's the first one.

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We're gonna pour lead two
nitrate aqueous solution

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and potassium iodide
aqueous solution together.

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What will we get?

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Pause the video and try
to do this yourself.

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First, think about what
the potential products are

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by swapping the cations

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and then check whether
one of them is insoluble.

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If it is, then it'll precipitate it out.

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We'll get the reaction.

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If both are soluble, we'll get nothing.

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So pause and try.

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All right, here it goes.

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So one of the potential products is

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lead cation combines with iodide ion.

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So before writing, let
me just check over here.

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Where is lead?

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Lead is over here

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and iodide lead cation, okay.

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Iodide is over here.

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So if you look at that,
there you go. It's insoluble.

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So I know immediately,

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lead

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iodide,

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and I need to be careful,

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lead has a +2 charge and iodine
over here has a -1 charge.

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So to compensate, I have
to put two over here.

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So I get lead two iodide.
That is insoluble.

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So that will precipitate out.

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And what else will I get?

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Well, potassium can combine with nitrate.

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And again, we can check
for it. Where is potassium?

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Potassium is here.

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Nitrate is over here.

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So if I go down, go over
here, look, it's soluble.

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So I'll get potassium
nitrate, which is soluble,

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charge is +1, -1, okay,
so I'll just get this.

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So I'll get an aqueous solution.

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And, of course, I'll
have to balance it out.

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Let's quickly do that.

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So I have two iodine over here,

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so I'll put a two here.

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So two potassium, so I'll put a two here.

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And that balances everything out.

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And this is the experiment
that we saw earlier.

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We are pouring potassium
iodide into lead two nitrate.

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What is that yellow color?

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That's basically the lead two
iodide being precipitated.

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And now the aqueous solution

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contains potassium and nitrate ions.

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All right, why don't we try another one?

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This one looks a little bit intimidating,

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but the idea is the same.

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So why don't you pause the
video and try this again.

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All right, we start by thinking about

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what the potential products are.

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How do we do that? We
swap the cations, okay?

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So ammonium cation, let's combine
them with the acetate ion.

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Again, before writing it,

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let's just look over here.

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So where is ammonium?

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Here's ammonium, and acetate is over here.

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So let's look at that.
Oh yeah, that is soluble.

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So this one is soluble.

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The other one would be
sodium and sulfate ions.

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So sodium is here, sulfate is here.

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What do we get? Oh, that's also soluble.

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Nothing is insoluble over here.

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What we'll get is soluble,
so nothing precipitates out,

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which means we'll just
end up with a solution

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where you have all these
four kinds of ions.

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So that means we will get no reaction.

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All right, so the final thing

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is that there's a special kind

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of double replacement reaction,

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which we call acid-base neutralization.

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Now, we'll talk about
what acids and bases are

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in detail in future videos,

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we'll look at all the cool
properties and everything,

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but for now, think about acid

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as basically an ionic solution,

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which has hydrogen cation
and some other anion,

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and base as an ionic solution,

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which contains a hydroxide anion.

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And, of course, some metal cation.

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For example, consider HCl,

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which is an acid because
it has a hydrogen cation,

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reacting with sodium hydroxide,

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which is a base because
it has a hydroxide anion.

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What will happen?

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Well, we just swap the cations.

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So sodium will combine with chlorine

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to give me sodium chloride,
and that is soluble,

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so I'll get an aqueous solution.

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But the interesting part over here is

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what happens when hydrogen
combines with OH-?

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What do we get? This is
no longer an ionic salt.

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This is H2O. This is water.

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Water is covalently bonded.

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So we now end up with a
covalently bonded molecule.

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So we will get water, H2O.

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And since it's no longer
an ionic solution,

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we just write as liquid.

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So look what we get in general.

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When you combine acid with a base,

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they neutralize each other
to give us a salt and water.

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So this is a special kind of
double replacement reaction

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because there are no
precipitates over here,

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but the reaction happens

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because we get a covalently
bonded liquid water.