>> host: We all know that adding taco sauce to a taco makes a big difference
in how it tastes. In fact, mixing the types and amounts of different ingredients
can completely change the flavor of any food. But did you realize that many recipes
are basically instructions for causing chemical changes in the food we eat? In the world of chemistry,
these changes are caused by something called bonding. Chemical changes occur whenever you break
a chemical bond or form a new chemical bond. And bonding is what we'll learn about in this unit
of Chemistry Matters, Unit 4. But let's get back to the taco sauce to show you exactly what I mean.
You see these dull pennies? Believe it or not, I can make them shine again with the help of taco sauce...
and chemistry. First you have to realize that the pennies aren't dull simply because they're dirty.
As you recall from Unit 2, this dullness is partly the result of oxidation, a chemical reaction
that occurs when atoms or ions lose electrons, changing the composition of that substance.
In this case, the copper has interacted with oxygen and lost electrons, making it dull.
To see exactly how taco sauce interacts with the copper in the pennies, I'm going to run
a controlled experiment with the individual ingredients found in typical taco sauce-- salt, vinegar,
and tomato sauce. I'll put salt on this first penny.
And a little water to dissolve the salt.
On the second penny, I'll put some vinegar.
The third will get tomato sauce.
And, taco sauce on this last one.
Now I'll wait a couple of minutes before wiping off the ingredients.
Let's start with the penny with the salt on it to see if anything happened. Nope, still dull.
Now we'll try the penny with the vinegar?
Also, still dull.
How about the one with the tomato sauce.
Also, still dull.
Now let's take a look at the last one with the taco sauce.
You see how much brighter that is?
Especially when you compare it to the back, which had no taco sauce.
So how did that happen? Why did the taco sauce shine the penny when the ingredients
by themselves didn't? We'll find out in this unit of Chemistry Matters!
♪♪
>> host: Welcome back to Chemistry Matters, Unit 4: Bonds. Are you wondering exactly
what made the penny with the taco sauce shine? We know it wasn't the individual tomato sauce,
or the vinegar, or the salt. But since those are the three ingredients in our taco sauce,
what was it? Let's try this... a mixture of the salt and vinegar
that are found in taco sauce. Could it be that a combination of ingredients did something
they couldn't do alone?
We'll give this combination a couple of minutes.
And look at that! The salt and vinegar side of the penny is shiny! So our evidence tells us
that it's the combination of salt and vinegar. Or more precisely, it's the reaction between
the salt, vinegar and copper in the pennies that causes the change. The vinegar, which is
the common name for acetic acid, begins to remove the tarnish, or oxidation. The salt,
which is sodium chloride, dissolves into sodium and chloride ions in the vinegar,
and those chloride ions are responsible for much of this process. When chemicals react like this,
it's because the bonds between them are breaking and new bonds are forming.
Chemical bonds are one of the most, if not the most, important concepts in chemistry. Chemical bonds join atoms
together to form gases such as air, as well as water, metal, your car, the stars, you, me--
any matter you can think of in our physical world. That's pretty important stuff! In this unit,
we're going to learn about different types of bonds and how they form. So let's go out
to our classroom, where we'll start by asking our students to explore some of the facts
and misunderstandings about chemical bonds, so we can get a handle on what we already know
and what we need to learn.
>> professor: Let's look at some statements about chemical bonds and see if you can tell whether
they're true or false. You might need to think back to your middle school science classes.
But don't worry. We'll be able to explore any questions you have. Larissa, why don't you
try the first one. >> student: Okay.
I don't think that's true because atoms aren't really "trying" to do anything! >> professor: That's right.
Atoms aren't human, so that's not a good way to describe them. It's actually electrical forces
that cause atoms to move toward each other and away from each other. We'll get into more detail
on those forces later in this unit. Nefkrishna, try the next one. >> student: Sure.
The only types of bonds I remember studying were back in 8th grade, and they were covalent
and ionic. So that's probably true. >> student: Well, I think you might be wrong,
because I know I've heard of something called metallic bonds. >> professor: Good.
The important thing to remember here is that there's actually a spectrum of bond types,
rather than bonds fitting into narrow categories. And we'll learn more about many different types of bonds
in this unit as well.
>> host: Our students did a nice job of getting you started on this exercise. In our Chemistry Matters
Toolkit, you'll find more statements like these that you can investigate and discuss with your teacher.
So go ahead and do that before continuing to the next video. When you're done,
you'll be better prepared for the rest of this unit, and the next video on the unit 4 playlist
about bonding.