Hi. It's Mr. Andersen and this is chemistry essentials video 32. It's on chemical change.
If you were to take your mass every night before you go to bed and when you wake up
you would find that on average you're going to lose about a half a pound of mass every
night. Where does that mass go? Well you're going to lose some of it physically. You're
going to evaporate some of that water off the surface of your skin. And so we're just
going to change that water that's a liquid into water that's a gas. So that would be
a physical change. But we're also going to have a lot of chemical changes going on. In
other words you're taking in food and oxygen. There's going to be a chemical, a series of
chemical reactions going on called cellular respiration. And then you're going to breathe
out water and carbon dioxide. And so you're losing mass every time you breathe out. And
so there's an excellent Veritasium video that talks about this and I'll put a link in the
video description down below. But this video is about chemical change. And so change that
we see in the laboratory remember is change that we're observing at the macroscopic level.
Or that the very big level. So we can't really see the particulates. We can't see the atoms
and the bonds as they break and reform. And so we kind of have to infer that through laboratory
work. And so changes can either be chemical or physical in nature. If we have a chemical
change what we're doing is forming a new substance. In other words we're breaking bonds. Forming
new bonds. And so we're creating something that wasn't there before. In other words at
the particulate level we're creating a new substance. And a physical change we still
have the same substance we had before. It's simply just in a new form. And so an example
of a chemical change would be converting that food into carbon dioxide and water. A physical
change might be taking that water and turning it into water vapor. And so what's some evidence
that we can collect that would show that a chemical change is occurring? In other words
macroscopic evidence? Well we might start to see a change in the odor. Or we might see
a change in the color. Perhaps we could have production of a gas. We might have change
in heat. Or we might have a change in the precipitate or the formation of a solid. What
kind of evidence are we going to see that a physical change has occurred? It's simply
going to be a change in the state. And so in the laboratory what we're going to do is
collect macroscopic data and then try to take that to interpret what's going on at the particulate
level. And it's hard. We have to gather a lot of evidence. And sometimes we'll figure
out it's chemical. Sometimes it's physical change. And sometimes we don't know. It's
just ambiguous. And so again, chemical change, this is going to be our evidence, if we see
an increase in the gas or odor or heat or color or precipitate. We know a chemical change
has occurred. But if it's simply a change of state, it's physical. So let me give you
these examples. You may want to pause the video at this point and try to figure out
which of these are going to be chemical changes. And which of these are going to be physical
changes. So let's start with rotting fruit. Rotting fruit. Do we have production of gas?
Maybe. Do we get a change of color. For sure. Is it easy to go from rotting fruit back to
fruit? Not really. And so what would we call that. That would be a chemical change. Let's
say we're rusting iron. Or rusting a metal. What's that going to be? We would call that
a chemical change. What's some evidence? I would see a change in color. When you rust
things you're actually going to see a change in heat as well. And it's hard to reverse
that. So that would be a chemical change. Let's say we take a pill of aspirin and we
crush it up. Would that be chemical or physical? Well I don't see change in color or heat or
odor. No gas is being produced. So that's going to be a physical change. What about
burning paper? What are we doing? We're actually combining it with oxygen. It would be hard
to go back to paper again. And so I would call that a chemical change. What about boiling
water? We're adding heat to it. But the reaction is not generating heat. And so we would call
that a physical change. This is a hard one. What about dissolving sugar in water? Well
when we dissolve the sugar in the water, it's still sugar inside the water. It's simply
separated all those sugar molecules. And so we'd call that a physical change. What about
melting butter? That would also be a physical change. It's still butter. We're just changing
the form. It's now in a liquid. If we were to burn it, it's combing with oxygen and so
we would call it a chemical. And so what are some chemical changes that you should get
used to measuring in the laboratory? One would be acid-base changes. And so this is a wonderful
picture here. What we're doing is taking just cabbage juice, red cabbage juice. And we're
adding different pH of a substance to it and you can get color change here. And so that
color change is going to indicate that we have chemical change as well. What would be
another one? Formation of a precipitate. Or formation of a solid would be indication that
we have a chemical change. And then redox reaction. This right here is going to be a
titration or redox titration. Those are going to be other changes that we can see. It could
either be an acid-base titration where we're seeing a color change based on an indicator.
Or it could just be the reduction of a substance that's causing that color change. And so again,
did you learn the following? To evaluate macroscopic, in other words at the large level changes?
And then classify them as physical, chemical or I don't know? Well the big things are to
look at evidence for chemical change. And I hope that was helpful.