- [Instructor] Check this out.
I have two clear colorless solutions over here.
Let's pour them into each other.
We pour the first one and we pour the second one.
And boom, we now get a white colored solution.
Here's another example.
Again, two colorless solutions.
We pour one into another, and boom!
We again get a beautiful yellow colored solution.
What's going on over here?
To find out, let's dig a little deeper.
Here's a more curious question for us, okay?
so in the first case, what did we do?
We poured sodium chloride and silver nitrate together,
and that gave us a white colored solution, right?
But now if I were to change just one element,
instead of silver, if I had potassium over here,
everything else is the same,
so if I had poured potassium nitrate
and sodium chloride into each other,
I wouldn't have gotten anything.
I would've just gotten a colorless solution.
It's not that interesting.
And therefore, I couldn't find any footage online.
So this is just an edited image, but you get the point.
We wouldn't get anything interesting over here.
But the big question is why?
Why silver nitrate and sodium chloride
gives us a white colored solution,
whereas potassium nitrate, just one change,
and sodium chloride does not give us anything?
Let's look at it. Let's look at it one by one.
So in the first case,
we are reacting sodium chloride aqueous solution
with silver nitrate aqueous solution.
What do we get?
Well, remember that in aqueous solutions,
ions usually dissociate.
So over here, we'll get basically NA+ ions
and CL- ions.
And over here, we have Ag+ ions and NO3- ions.
So when I pour them together,
we just get all those ions together, okay?
Now, because these are together,
they can form new combination.
Of course, cations should always combine with anions, okay?
So NA can now combine with NO3,
but there's nothing special because again,
they will dissociate.
But Ag can also combine with Cl.
When Ag combines with Cl, something interesting happens.
What? Well, guess what?
AgCl is insoluble,
and therefore, it will precipitate out.
And that's the reason why this whole thing looks white
because of the AgCl precipitation.
So what do we end up with?
We'll end up with AgCl, which is insoluble.
So that's why it's written as solid over here.
It precipitates out.
So let me just share it over here
to show that it's precipitating, okay?
And what remains in the aqueous solution?
Well, sodium ions and nitrate ions,
so we get sodium nitrate aqueous solution.
The white color is due to the AgCl precipitating out.
Now, if you zoom out and look at the reaction,
see what has happened,
sodium and silver cations have switched places.
Sodium has replaced silver over here
to get sodium nitrate,
and silver has replaced sodium over here
to give us silver chloride.
So since there are two cations replacing each other,
there's a double replacement happening.
This is called, no surprise, a double replacement reaction.
We can also call this double displacement reaction.
So what we witnessed was a double replacement reaction
and one of the products precipitated
giving us that white color.
Now, before we look at the other one,
a quick question for you is,
can you identify which of the elements underwent oxidation
and which ones underwent reduction?
Pause and think about this.
Okay, whenever I want to think about that,
I just look at the charges on the elements.
Well, over here, sodium has a positive charge.
It's a cation.
On the other side, well, it's still a positive cation.
So no change happened to the charge on the sodium.
Nothing happened to it, okay?
What about Ag?
No change.
The same is the case with the anions as well.
No change, no change,
which means look, nothing is undergoing an oxidation,
nothing is undergoing a reduction,
so double replacement reactions are not redox reactions.
And you may be wondering,
why are you excited about the fact
that it's not a redox reaction?
I'm excited because I used to think
that all chemical reactions
must involve electron transfers, and therefore,
all chemical reactions should have something oxidizing
and something else reducing.
But I was wrong. Look, right in front of our eyes.
We can see examples of chemical reactions
where there are no electron transfers,
where there is no oxidation or reduction,
so that's pretty cool.
But anyways, now let's look at the other one.
What happens when I pour these two together?
Well, let's look at the reactants.
This time, the reactants are NaCl and KNO3.
Both are aqueous solutions.
I pour them together.
So just like before, I will now have
all the four different kinds of ions over here.
Na can combine with NO3.
Remember, cations can only combine with anions, okay?
Those are the only new combinations you can form.
So Na can combine with NO3-,
but again, it'll dissociate.
K+ can also combine with Cl-.
But what's important over here
is that KCl, potassium chloride, is soluble.
Therefore, when K and Cl combine,
again, they will dissociate.
So nothing happens over here. There's no precipitation.
I'll just end up with a solution
where all the four different kinds of ions
are just floating around together.
So no chemical changes happened.
And that's the reason why I don't get any colorations.
I don't get anything over here.
So over here, I get essentially no reaction.
So you notice the key difference?
The key difference was AgCl was insoluble.
That's why it precipitated out.
And that's why in order for us
to get a double replacement reaction,
we need one of the products to precipitate.
If both are soluble and they form aqueous solution,
then nothing will happen.
We'll just get a solution
with all the four different kinds of ions.
No chemical change at all.
So in general, we can now write down
what a displacement reaction looks like.
We can say that if you have an aqueous solution of AB
reacting with an aqueous solution of CD,
then a double replacement reaction,
the two cations replace each other.
So A will now combine with D and C will combine with B.
But that'll only be the case if one of them is insoluble
and precipitates out.
Precipitation, sorry, is the key
to having double replacement reaction.
So look, if you pour any two aqueous ionic solutions,
do not expect to get a double replacement reaction.
You'll only get them if one of the products is insoluble.
But now we'll be wondering,
how do we know whether a particular salt
is soluble or insoluble?
I'm glad you asked that question
because that brings us to the solubility chart.
A solubility chart is basically that,
it tells us whether a salt is soluble or not.
So here's how we can read it.
If you wanna look at potassium chloride,
here's potassium cation,
here's the chloride anion, sorry.
And now we can just say, hey, this is where they meet
and so this is the solubility of potassium chloride
and you can see it is soluble.
But what about silver chloride?
Silver is here.
Chloride is here.
Again, try to make them meet. And what do you notice?
Silver chloride is insoluble.
And what about this yellow slightly soluble?
Well, don't worry too much about that.
We'll only work with the soluble
and the insoluble ones, okay?
And just by looking at this chart,
you can see some trends.
For example, you can see salts of lithium,
sodium, potassium, and even ammonium.
Almost all are soluble.
Of course, there are some exceptions,
but they're all soluble.
In contrast, salts of lead are almost insoluble.
You can also see salts which have nitrate ions
and acetate ions, pretty much soluble.
Anyways, now equipped with this solubility chart,
we can predict whether certain double replacement reactions
are gonna happen or not, okay?
So let's check that. Here's the first one.
We're gonna pour lead two nitrate aqueous solution
and potassium iodide aqueous solution together.
What will we get?
Pause the video and try to do this yourself.
First, think about what the potential products are
by swapping the cations
and then check whether one of them is insoluble.
If it is, then it'll precipitate it out.
We'll get the reaction.
If both are soluble, we'll get nothing.
So pause and try.
All right, here it goes.
So one of the potential products is
lead cation combines with iodide ion.
So before writing, let me just check over here.
Where is lead?
Lead is over here
and iodide lead cation, okay.
Iodide is over here.
So if you look at that, there you go. It's insoluble.
So I know immediately,
lead
iodide,
and I need to be careful,
lead has a +2 charge and iodine over here has a -1 charge.
So to compensate, I have to put two over here.
So I get lead two iodide. That is insoluble.
So that will precipitate out.
And what else will I get?
Well, potassium can combine with nitrate.
And again, we can check for it. Where is potassium?
Potassium is here.
Nitrate is over here.
So if I go down, go over here, look, it's soluble.
So I'll get potassium nitrate, which is soluble,
charge is +1, -1, okay, so I'll just get this.
So I'll get an aqueous solution.
And, of course, I'll have to balance it out.
Let's quickly do that.
So I have two iodine over here,
so I'll put a two here.
So two potassium, so I'll put a two here.
And that balances everything out.
And this is the experiment that we saw earlier.
We are pouring potassium iodide into lead two nitrate.
What is that yellow color?
That's basically the lead two iodide being precipitated.
And now the aqueous solution
contains potassium and nitrate ions.
All right, why don't we try another one?
This one looks a little bit intimidating,
but the idea is the same.
So why don't you pause the video and try this again.
All right, we start by thinking about
what the potential products are.
How do we do that? We swap the cations, okay?
So ammonium cation, let's combine them with the acetate ion.
Again, before writing it,
let's just look over here.
So where is ammonium?
Here's ammonium, and acetate is over here.
So let's look at that. Oh yeah, that is soluble.
So this one is soluble.
The other one would be sodium and sulfate ions.
So sodium is here, sulfate is here.
What do we get? Oh, that's also soluble.
Nothing is insoluble over here.
What we'll get is soluble, so nothing precipitates out,
which means we'll just end up with a solution
where you have all these four kinds of ions.
So that means we will get no reaction.
All right, so the final thing
is that there's a special kind
of double replacement reaction,
which we call acid-base neutralization.
Now, we'll talk about what acids and bases are
in detail in future videos,
we'll look at all the cool properties and everything,
but for now, think about acid
as basically an ionic solution,
which has hydrogen cation and some other anion,
and base as an ionic solution,
which contains a hydroxide anion.
And, of course, some metal cation.
For example, consider HCl,
which is an acid because it has a hydrogen cation,
reacting with sodium hydroxide,
which is a base because it has a hydroxide anion.
What will happen?
Well, we just swap the cations.
So sodium will combine with chlorine
to give me sodium chloride, and that is soluble,
so I'll get an aqueous solution.
But the interesting part over here is
what happens when hydrogen combines with OH-?
What do we get? This is no longer an ionic salt.
This is H2O. This is water.
Water is covalently bonded.
So we now end up with a covalently bonded molecule.
So we will get water, H2O.
And since it's no longer an ionic solution,
we just write as liquid.
So look what we get in general.
When you combine acid with a base,
they neutralize each other to give us a salt and water.
So this is a special kind of double replacement reaction
because there are no precipitates over here,
but the reaction happens
because we get a covalently bonded liquid water.