In this video, we’re going to talk about how things dissolve in water to make aqueous
solutions. We are going to look at an atomic view of how water molecules dissolve solute
particles, a process known as solvation. And we’ll see that solvation occurs differently
for covalent and ionic solutes. Let’s start by making an aqueous solution, one you’ve
probably made yourself many times. We’ll dissolve a sugar cube in water. Here, the
sugar is the solute, and the water is the solvent: we’re making an aqueous solution
of sugar. Now, if you’ve ever put a sugar cube in
water, you know it takes a little bit of time to dissolve. It’s not instantaneous. Let’s
take a closer look at what’s happening during this dissolving process.
Here's our sugar cube. Let’s imagine that we could zoom into
it many times, and see the atoms and molecules.
We’d see that the sugar cube is made of trillions and trillions of molecules that
look like this. Table sugar is actually a compound called “sucrose.”
Here's its chemical formula. It’s made of carbon, hydrogen, and oxygen atoms. Because these
elements are all nonmetals, you know sugar is covalent.
Its atoms are connected by covalent bonds
made by sharing electrons. There are different ways we could show this molecule. So here,
we’re representing the sugar molecule with
a 2D Lewis Structure that shows the covalent bonds.
If you made that into a “ball and stick” model, it would look like this. But a more
accurate depiction of sugar is actually closer to this.
We call it the 3D model. It’s a bit blobby, and kinda hard to see all the different atoms. But
we’re going to use this model for now, because it will better illustrate what we’re talking
about here, which is how molecules interact.
Now let’s look at water, which is our solvent. Water has a chemical
formula of H2O. And here are some 3D models of water molecules.
Now we wanna look at dissolving
here. So let’s zoom in on our sugar cube in the water. We’ll look at this edge right
here. So here’s this edge, zoomed in millions and millions of times. The sugar molecules
are all clumped together. Now this looks pretty confusing, so we’re going to clean it up
a little. We’ll reduce the number of sugar molecules and show them in neat little columns.
See? This is easier to look at. Just remember, things are more messy in real life. Also,
remember that this sugar cube is sitting in water. So
now we’ll add the water molecules that are
on the edge of the cube, over here. There we go: so we’ve got the water, and the edge
of the sugar cube. Now…for the water to dissolve the sugar, water molecules have to
bump into sugar molecules, so they kinda gotta bang into the sugar like this. They can only
do this at the surface of the sugar, so it takes some time. Then, after bumping
into the sugar, the water molecules kinda work their way in between sugar molecules and
surround them, you see that happening here.
Once a sugar molecule is surrounded by water molecules, they can
carry it into solution. Here, you can see
that one of the molecules of sugar is surrounded by
water molecules, and it’s carried away. Now keep your eye on this one here at the
top. And there we go, it got surrounded and carried away. As the water molecules carry
the sugar molecules into solution, new water molecules can reach the undissolved sugar
over here, and the process continues. I think this one’s next… There we go: another
one gets surrounded and starts to float away. And this is what’s happening at the atomic
level, as sugar dissolves. All of those sugar molecules are coming apart from each other,
they’re getting surrounded by the water molecules, and they’re floating off into
the solution. Now notice one thing here, about how the sugar
molecules float away. This is super important The sugar molecules separate from each other,
OK? but the whole molecule stays together. The molecules don’t break apart into their
atoms. In other words, the dissolving process doesn’t break the covalent bonds inside
a sugar molecule. The atoms in the molecule stay together as the sugar dissolves.
So as you can see, we have three molecules that have come off the surface so far. And
this process will keep happening, pulling more and more sugar molecules into the solution.
Let’s zoom back out and look at our glass. Here we are. We were just looking at one side
of the cube, but this dissolving process is happening all over, on all the different sides,
anywhere that’s touching water. Now, after the dissolving is complete, all the sugar
molecules will have been pulled away from the cube, and the cube is gone. And if we
could zoom in and look at the liquid, we’d see that the sugar molecules are mixed in
with the water molecules, spread all throughout. So this is what a solution of sugar dissolved
in water would look like at the atomic level. Now as we’ve seen,
a big part of dissolving is that the molecules
of solute get surrounded by molecules of solvent. There’s a special name for this process.
We call it solvation. And, we use the term “solvated” to refer to a solute
that is surrounded by solvent molecules. Now,
if the solvent is water, like it is here, you can be extra
precise, and say that these molecules are “hydrated.” When we say a solute particle
is hydrated, it means the same thing as “solvated,”
but we know the solvent is water.
These little clusters of water molecules surrounding
a solute particle are called hydration shells.
You may also hear them
referred to as “solvent cages.” They make this little cage
or shell around the solute. Just keep in mind they surround the entire solvated
particle, so they are actually 3D, even though we’re just drawing them in 2D here. So that’s
hydration shells or solvent cages. Now let’s switch gears here and see how ionic compounds
dissolve. Sugar, which we looked at earlier, that’s a covalent compound. But the process
of solvation works a bit differently for ionic compounds like sodium chloride, or table salt.
We know that sodium chloride is an ionic compound because it’s made of a metal, sodium, and
a nonmetal, chloride. Here’s a grain of salt, and if you could zoom down to the atomic
level, you’d see that a salt crystal is made of trillions of tiny ions packed together.
Sodium ions have a positive charge, and chloride ions have a negative charge. These ions are
bonded together by electrostatic attraction.
It's kind of like trillions of tiny magnets, all stuck
together So let’s see how solvation happens with an ionic compound. We’ll zoom in even
closer to get a better look. When you put sodium chloride in water, the water starts
bumping into the ions at the surface of the crystal. And then the water molecules surround
the ions, and pull them apart and carry them into the solution.
Now check this out! What we’re seeing
here is that for an ionic compound, the individual ions get pulled apart during dissolving.
This is different from what we saw with sugar. This process, we call it dissociation:
dissociation is how ionic compounds separate into ions in solution, when they dissolve.
So when salt is dissolved in water, it dissociates, it comes apart,
into ions. Let's zoom out here, remember that dissolving happens all over the
surface of the salt. Once the dissolving is complete, the ions spread throughout the water.
So let’s stop
to compare how water dissolves the two different compounds that we saw.
Sugar is a covalent compound. It’s made of molecules.
When sugar dissolves, the molecules move away from each other into the solution,
but they remain intact. The water doesn’t break the bonds between the atoms
in each molecule. But on
the other hand, salt is an ionic compound. It’s made of ions, when it dissolves,
it dissociates, it splits apart, into positive and negative
ions. Now let’s look at
dissociation here a little more closely. One thing you might have noticed about the diagram
is that the water molecules are surrounding the ions a bit differently. For sodium, their
oxygen atoms face the ion. But for chloride, over here,
the hydrogen atoms are facing the ion. Why
does water form these two different types of hydration
shells? Well, we have to mention something
about water here. You may know that water is a “polar” molecule. That means
that a water molecule has partial charges—very weak electrical charges at different ends
of the molecule. The two hydrogens here are partially positive,
and the oxygen is partially negative. These polar charges are much weaker than full
charges on ions, but they’re still important. This polarity has an effect on solvation.
The partial charges are attracted to the ions. When water molecules approach a positive ion
like sodium here, the partially-negative oxygen is attracted to the positive ion, because
opposite charges attract,
and the partially-positive hydrogens are repelled, because like charges
repel. The exact opposite happens when water molecules
approach a negatively-charged ion like chloride. Here, the partially-positive hydrogens surround
the negative ion, like this. And the oxygens are repelled by the negative ion, because
they have a partial negative charge. This pattern also shows why water is really good at
dissolving ionic compounds. The partial charges are attracted to the ions, the water surrounds
them, and then carries the ions away into solution.
Now finally, we’ve seen that sugar molecules
dissolved in water are organized differently than the sugar molecules in a sugar cube.
And sometimes it’s important for chemists to show the difference between a substance
when it’s a solid and when it’s dissolved in water. We can use state symbols for that.
State symbols tell us the state of the chemical. For instance, here is the formula
for sugar. If it’s solid sugar, we can add “(s)”. But an aqueous
solution of sugar gets the “(aq)” state symbol. The same goes for salt. When you have
solid salt crystals, it’s an s. Salt dissolved in water, aq. Just remember, when an ionic
compound dissolves, it dissociates into ions. So NaCl(aq) actually means Na+(aq) and Cl–(aq).
So we’ve seen how aqueous solutions are made using covalent
and ionic solutes. The water molecules bump into the particles of the solute, knock them
loose, surround them, and carry them into solution. This is called solvation.
If the solute is covalent, the solvated molecules remain intact. But if the solute
is ionic, the water causes the compound to dissociate into separate positive and negative
ions! When particles are surrounded by solvent,
we call them solvated, or we can call them hydrated if the
solvent is water. And water molecules surround ions in particular patterns, because the partial
charges on water are attracted or repelled.
And finally, we can use the state symbol (aq) to show when something is dissolved in water
to make an aqueous solution.