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BABATUNDE OGUNLADE:
Hello, everyone.

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Today we'll be working
through Goodie Bag

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3, which is on ionic solids.

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In order to work
through this Goodie Bag,

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you'll need a conductivity
meter, some small measuring

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cups, some medium sized cups, a
small scale, a writing utensil,

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some nitrile gloves,
some stirrers,

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and four unique solids.

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Today, I'll be using sodium
chloride, magnesium sulfate,

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magnesium oxide, and sucrose.

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Our main objective today is to
use solubility and conductivity

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measurements to determine
if a given solid is

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an ionic or covalent compound.

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And as we do this, I'd like you
to think about two questions.

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First, how do solids
dissolve in water?

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And second, what factors
may influence how conductive

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a given solid is in water?

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So first we're going to look
at the solubility of our solids

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in water.

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And to do this, we're going to
dissolve 1 gram of each solid

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in 30 milliliters of water.

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I've already weighed
out my solids here.

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And I've also already
weighed out my water.

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Notice how I'm wearing a
pair of nitrile gloves.

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The compounds that
I'm working with today

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aren't that dangerous, but
just as general safety,

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it's good to wear gloves
when handling compounds.

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So I'm going to pour each
solute into each respective cup

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of water and stir and mix for
about a minute to two minutes

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and see if the
compound dissolves.

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So after one to two minutes
of stirring and dissolving,

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you can see that our magnesium
oxide right here has not

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dissolved in our water.

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You can see a very clear
boundary between the solute,

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which has settled at
the bottom of the cup,

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and the water, which is on top.

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If you compare that to
our sodium chloride,

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though, if you look
very carefully,

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you can see that our
solute has dissolved

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very well in our water.

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So the next thing
we'll be doing is

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measuring the conductivity
of our compounds

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when dissolved in water.

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And to do that,
we're going to be

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using a handy dandy conductivity
meter that I have right here.

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So I have some cups
of water prepared here

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to be used as both a
baseline correction

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when we're measuring our
conductivity of our compounds

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and as a water bath to clean our
probe in between measurements.

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OK, so first I'm going to turn
on my conductivity measure.

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And I'm going to make
sure that my units are

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set to microsiemens
per centimeter,

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or some equivalent unit.

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And first I'm going to measure
a baseline of just regular water

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here.

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I have tap water here.

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And so when I do
that, I see that I'm

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measuring around 900
microsiemens per centimeter.

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So that is the connectivity
of my water as is.

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And I'm going to use
that as a baseline

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when I'm measuring
the connectivity

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of my other compounds.

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So since this is water,
it's already clean,

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and now I'm going to measure
my sodium chloride, which

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is right here.

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And so right now I'm reading
around 4,700 microsiemens

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per centimeter for
my sodium chloride.

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I'm going to take note of that.

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And eventually, when
I'm collecting my data,

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I'm going to subtract my water
baseline from this value here.

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Make sure in between
each measurement

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that you dip the probe in
the water bath to clean it,

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and you can continue on with
the rest of your measurements.

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So the last experiment
we're going to do today

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is to measure the
solubility of our compounds

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in water, except with 2
grams instead of 1 gram.

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So I've measured out an
additional 1 gram of each solid

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here, and I'm just
going to pour each one

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into each respective
cup, mix and stir.

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And after one to
two minutes, I'm

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going to see if each
compound is dissolved.

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So again, we have
our magnesium oxide

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after round two of solubility.

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Again, we can see that
our solute has not

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dissolved in water.

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There's a very clear boundary
between the water on top

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and the solute on the bottom.

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So in this case, round
one solubility and round

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two solubility look
identical, but it may not

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be the case for our
other three solutes.

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All right, so now that we've
finished our last solubility

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test, let's put all our data--

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our first solubility test,
our connectivity test,

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and our second solubility test--

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on the board.

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So here we have a chart of all
the data from our experiments.

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I have the compounds that we
use going down right here.

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And I have solubility here
at 1 gram and 2 grams,

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and conductivity after
water subtraction here.

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So before going into
this data, first thing

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I would like us to notice
is that for sodium chloride

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and for magnesium sulfate,
these conductivity values

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are much lower than I expected.

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This should be close to
the 10,000 microsiemens,

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and this should be closer
to 8,000 microsiemens

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per centimeter, but
the fact that we're

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able to measure conductivity
for these respective compounds

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gives us insight into what
type of compounds they are.

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So first, if we look
at sodium chloride,

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we could see that at
both 1 gram and 2 grams,

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our sodium chloride was
able to dissolve in water.

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And we were able to measure
conductivity readings for this.

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This means our
sodium chloride was

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able to dissociate into ions
when dissolved in water.

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And this is an
extra check for us

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to determine that sodium
chloride is indeed

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an ionic compound.

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Next we can look
at magnesium oxide.

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And we see that for magnesium
oxide, at 1 gram and 2 grams,

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we weren't able to dissolve
the magnesium oxide in water.

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And not only that,
because of that,

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we weren't able to
measure any conductivity.

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But we know that magnesium
oxide is an ionic compound

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just from our
understanding of bonding.

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So we had to think more closely
about why this is the case.

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So magnesium oxide
and sodium chloride

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are both ionic compounds,
but magnesium oxide

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has a much higher lattice
energy than sodium chloride.

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This means that when magnesium
oxide is placed in water,

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it's more energetically
favorable for it

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to stay as a solid than it
is to dissociate into ions

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of magnesium and oxygen.
So magnesium oxide is still

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an ionic compound, but
it's lattice energy

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is high enough such that it does
not dissolve and does not give

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us conductivity measurements.

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So it's still ionic.

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Now we can look at
magnesium sulfate.

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And we see that for 1 gram,
we do get some solubility.

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And at 2 grams, we
get no solubility.

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It was not able to dissolve.

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And we're able to measure
some conductivity.

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This means that magnesium
sulfate dissociated

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into respective ions and
we're able to measure

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some conductivity
while in water.

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But magnesium sulfate
has a limited solubility

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in water, which
is why at 2 grams

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it wasn't able to
dissolve completely.

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So magnesium sulfate is
still an ionic compound.

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Finally, if we
look at sucrose, we

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see that we were able
to get 1 gram of sucrose

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to dissolve in water,
and 2 grams of sucrose

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was able to dissolve in water.

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But we were not able to
get any conductivity.

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This means that sucrose
dissolved in water,

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but did not dissociate
into respective ions.

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So if you think about
the structure of sucrose,

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sucrose has a bunch of polar
groups on it, which make

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it able to dissolve in water.

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But sucrose does not
dissociate into ions,

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which is why we were not able
to measure any conductivity.

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So sucrose is a
covalent compound.

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So by using our knowledge of
bonding and our solubility

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and conductivity data,
we're able to affirm

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the identity of our compounds.

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That is, whether or not
they're ionic or covalent.

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So today we looked
at how the type

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and strength of
bonding of a solid

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influences its properties.

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Specifically, we looked at
how these aspects of bonding

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influence these solubility
and conductivity

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of these solids in water.