PROFESSOR CIMA: We're gonna talk about solutions today.
A year or so ago, I ran across this great picture here.
This is a scanning tunneling microscope image of a
copper (100) surface.
And I think you can see the atoms of the copper right here.
It's a square lattice--
you can see it right there--
as it should be for a (100) surface of a FCC cubic material.
But you can see there's some disturbances here.
They don't look like anything.
They look like kind of like blobs.
And what those are are iridium atoms that are just below the surface.
They are substitutionally placed in the copper crystal.
And of course because they're bigger than the copper atoms, they distort,
strain the lattice.
And so when you run the STM needle over the surface, you see that
distortion here, both in the placement of the atoms-- and it turns out the
way this STM works is tunneling of electrons.
And their ability to tunnel changes dramatically in regions of strain.
So this is an illustration of a true solution.
In other words, these locations that, more or less randomly here, are
iridium atoms sitting on those sites.
And so when we say solutions, we mean at the either atomic--
as in this example--
or molecular scale.
They're different than, let's say, an emulsion like milk, right, where if
you look under the microscope you'll actually see regions of fat and then
regions of water.
And that's different.
That's not a true solution.
Although we'll talk about those another time.
Just so everybody's on the same page, we will--
of course we've been using these all along.
We have sort of measures of concentration.
So you are all familiar with mole fraction.
Let's say we're talking about a solution of A and B. So the mole
fraction of A is just the moles of A divided by moles A and B, moles A plus
B. We'll also use--
And when we talk about kinetics, we use the concentration.
And you'd see me often write it like this or like this.
Today we use this notation the square brackets of A to mean molar
concentration.
So that's moles of A per volume of solution.
And then of course there's the mass concentration which is the mass of A
over the volume of solution.
So those are pretty common methods.