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  • \\f0\\fs28 \\cf0 Hi. It's Mr. Andersen and today I'm going to be talking about changes. These

  • changes are physical and chemical changes. And students sometimes struggle telling the

  • difference between chemical and physical changes. And teachers love to kind of ask you hard

  • questions like, let's see. If water is boiling is that a physical or chemical change? Or

  • if a moth ball vaporizes over years, is that a physical or a chemical change? Or if you

  • mix vinegar and baking soda, is that a physical or chemical change? So hopefully by the end

  • I'll give you a few clues that will allow you to answer that. This picture, I decided

  • to start with this picture right here. I live in Montana and so in Montana the burgers that

  • we have our pretty good. The steak that we have is really good in Montana. But we don't

  • have In and Out Burger. And whenever I go to California I love to get a nice In and

  • Out Burger. Because the fries are perfect. Get a shake. Oh my goodness. I'm already getting

  • hungry. But when I eat that burger, that burger is going to go through two changes. The first

  • changes it's going to go through are physical. In other words, I'm going to start chewing

  • up that burger and making it into smaller little bits. And then it's going to go through

  • my digestive system and I'm going to start to chemically breakdown that burger. Now the

  • cool thing about a burger is that that burger will eventually will become you. In other

  • words the amino acids that are in a burger are going to be broken down. And then we're

  • going to use them up inside our body to actually build up the proteins inside our body. And

  • so it's important that you understand chemical and physical changes and the difference between

  • them because you need to understand how you are made from a burger. Okay. So let's get

  • started. Let's start with physical changes. My definition for physical changes occur when

  • the appearance changes the the substance does not. And so if we were to look at this pen

  • for example, is it undergoing change? You'd say no. It's not changing at all. But let's

  • say that this pen were to change it's appearance but it were still to be the parts of a pen.

  • That would be a physical change. And so how could that happen? Well if we were to heat

  • this pan up and so it were to melt slightly, it still would be a pen. If we were to break

  • it in half or bend it, it would still be a pen. In other words, as long as it doesn't

  • combine with another chemical, as long as that substance stays the same, it's a physical

  • change. In other words if you start with something like water and we go from water that is say

  • a solid, like in this down here. And then we end up with water that is a, let's say

  • a liquid. In other words it starts to melt. Or even if it's H2O that forms a gas, a lot

  • of this H2O inside this ice is actually going to sublimate. So it's going to vaporize and

  • turn into a gas. Well it's still H2O. And so it's going to be a physical change at each

  • of those points. And so here are some things on the side that I said you could check off

  • if it's a physical change. If it's just melting. If it's boiling. I think I have a picture

  • of that. So if we're boiling water, this always blew me away as a teacher. A lot of people

  • don't know why water boils. In other words they say okay you heat it up and it's going

  • to boil. But they don't know what's inside the bubbles inside boiling water. And so what

  • is inside the bubbles inside boiling water? Well it's not hydrogen and oxygen gas. That

  • would be bad. Because if it was hydrogen and oxygen gas, hydrogen and oxygen gas once they

  • come out would quickly combust. Hydrogen's going to combust and explode. And we know

  • that doesn't happen. And so what's actually in the bubbles in boiling water? It's simply

  • water vapor. In other words it's water that is in the state of a gas. And so we would

  • say not a chemical change. That's a physical change. Or if we were to for example break

  • these bricks in half or keep breaking these bricks in half and half and half and half

  • and half and they're still bricks. Then we call that physical. Or let's say we were to

  • cut things in half. So if we were to use this torch and cut this metal in half, the metal

  • here and the metal here would still be the same. And so we'd call that a physical change

  • to this pipe. Now right along the point at which you're cutting, if we combine that with

  • oxygen, right along that edge, then it's going to be oxidized. So we call that a chemical

  • change. And then another tricky one that science teachers like to trick you up with is dissolving.

  • In other words if I were to take a little bit of sugar and add it to this tea as that

  • sugar dissolves in other words, as it goes from this solid to a more surrounded by water

  • kind of a state, we wouldn't call that a chemical change. We'd call that a physical change.

  • And so it's not a chemical change. And if it's physical change then we don't have any

  • kind of a chemical reaction taking place. And we don't have a formula. We simply have

  • a change in its state. Or a change in its appearance. And so there's a lot of stuff

  • that's not really a chemical change. So let's get to what a chemical change actually is.

  • Chemical change is when you have a substance that changes. And so for example if we go

  • back to water again. If I were to breakdown water into its gases, H2 and O2, then you'd

  • know that I don't have a water anymore. I have a hydrogen gas and I have oxygen gas.

  • And so we'd know that a chemical change has occurred. In other words the substance on

  • one side yields, that's what this sign means, yields two new substances on the right. And

  • these would be the products over here. And so we'd say a chemical change has occurred.

  • Now what are some clues that tell us a chemical change has occurred? Maybe it produces bubbles.

  • And so this right here is mixing acetic acid with sodium bicarbonate. Otherwise known as

  • mixing baking soda with vinegar. And what we're getting is bubbles forming. And those

  • bubbles are going to be a new gas. Now you might be confused thinking, yeah but you just

  • talked about boiling water and how once that turns into bubbles then it's a physical change.

  • Well think about it. Did we have bubbles inside the vinegar? Did we have bubbles inside the

  • sodium bicarbonate? No. And so we've created something new. We've created a gas and that's

  • what's actually forming the bubbles. And that's what makes the volcano explode when you did

  • this in elementary. Let's say we mix two chemical together. So you've got chemical A and chemical

  • B. And we mix one chemical in the other one which normally was clear. And it starts to

  • get kind of cloudy. So we'd say a chemical change is probably occurring. If we ever have

  • clouds showing up, either cloudy liquid or clouds in the air that usually just means

  • a new particle is being formed. And so you know a chemical is being formed. Okay let's

  • say we get a temperature change. A temperature change is going to indicate a chemical reaction

  • is taking place. And so the simplest one would be like in methane. Methane is natural gas.

  • It's in a bunsen burner. And so when we combine that with oxygen, O2 gas, we get some carbon

  • dioxide. We get some H2O. And we get a lot of energy. And so we're creating energy or

  • giving off energy that was stored in the chemical bonds of the methane. And so we would call

  • that a chemical change. In other words we've increased the temperature. So this right here

  • is pretty cool. We've got two different salts that are probably burning in alcohol. And

  • they're giving off colors depending on what kind of atoms they are. And we would call

  • that a temperature change. The salts by themselves are white. And then as we add enough energy

  • you get combustion with the oxygen. And so this is how fireworks are formed. We have

  • salts that are mixed up and then we give them combustion or enough of an oxidizer and we

  • can get all of these different colors. So color change would indicate that we've got

  • a chemical change going on. And then the last one that would indicate that we have a chemical

  • change is if we've got. Oh, here's a cool one. This is bioluminescence. This is adding

  • a couple of chemicals that actually glow. And so we see this in life a lot. For example

  • we've got jelly fish that can produce this glowing protein. So that's chemical reaction.

  • We're mixing chemicals and giving off light. And the last one that I was trying to get

  • to is bread. If you've ever smelled baking bread, there is a chemical change that is

  • going on inside the bread. So all the products by themselves didn't smell that way, but we

  • mix them together. We get chemical reactions going on. And so we've got a smell or a change

  • in state. Now tasting bread is totally safe. But tasting chemicals in the lab is not so

  • safe. And so I would stay clear of that. This would be a sparkler. And so what do you think?

  • Chemical change? Physical change? Yeah. That would be chemical. Or going back to a couple

  • of those answers at the beginning that I posed. What about boiling water? Chemical or physical?

  • Right answer would be physical. Or what about taking moth balls that you put in your drawer

  • to keep the moths off from eating your sweaters? If that vaporizes over time, what's that going

  • to be? It's going to be a physical change. Okay. And so the last thing I want to leave

  • you with is if we ever have a chemical change then we have a chemical reaction. And so in

  • a chemical reaction you're taking these things which are the reactants. This would be the

  • first reactant. The second reactant. And then we're making products from that. So if we

  • mix hydrogen gas and oxygen gas and create water, then we've had a chemical change. And

  • we usually have to add a little bit of energy to that. Now you should know this. The reactants

  • are on the left side. The products are on the right side. And this arrow stands for

  • the word yields. And so if I were to sketch this out for a second. Let's do the hydrogen.

  • Hydrogen is going to look like this. Hydrogen is two hydrogen molecules attached together.

  • And so this 2 right here, this is called the subscript, that tells me there are 2 atoms

  • of hydrogen in a molecule of hydrogen. This 2 in the front means that we have two molecules

  • of that. And so on the left side we've got two molecules of hydrogen gas on the side

  • but each of those molecules are made up of 2 atoms of hydrogen. If we look over here

  • at the oxygen. Oxygen, I'll draw that a little bit bigger, oxygen attached to itself, there's

  • two oxygen atoms but there's only one molecule of that. And so now let's look at the product.

  • Well if we look at the product over here on the right side, we're going to have one water.

  • So water looks like this. It's got a hydrogen and a hydrogen. It's got another water here.

  • Hydrogen, hydrogen. And so if we count them up we should have the same if it's a balanced

  • equation. So how many hydrogens do I have on the left side? 1, 2, 3, 4. How many hydrogens

  • do I have on the right side? 1, 2, 3, 4. So that's balanced. If we look at the oxygens

  • on the left side we've got 1, 2. Oxygens on the right side we've got 1, 2. And so that's

  • a balanced equation. And so I've got a podcast on balancing equations. So you can take a

  • look at it if you don't know how to do that. The key thing is that you can always change

  • the coefficients. This would be a 1 right here. You can always change what comes before

  • the formula, but you can never change the subscripts. Because if you're changing the

  • subscripts, it would be like breaking that oxygen in half. And we know that that doesn't

  • occur. So I hope that's helpful. Those are chemical and physical changes.}

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B1 中級

物理的・化学的変化 (Physical and Chemical Changes)

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    Yrchinese に公開 2021 年 01 月 14 日
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