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Chemical and Physical Changes

Pearson
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An important feature of any study of chemistry, is change. So we want to talk about two different kinds a change. Physical changes and chemical changes. In a physical change, a substance has changed its appearance in some way, but remains the same substance. In a chemical change that's not the case. It becomes something new through the process of a chemical reaction. And that's important for you to know because the properties of materials depend on their physical state for example. So let's look at an example of a physical change. What I'm going to do is I'm going to heat up this iron nail right here. And I'm going to heat this to the point where it is red-hot. So this is a physical change because we've changed its appearance, but it's still iron. Iron is a metal. Let's look at another metal. This metal is gallium. What do you think will happen to gallium, which has a lower melting point, when I put it in this hot water? Now, what you can see is that the gallium metal has melted. Is that what you would have predicted? Gallium is one metal, iron is another. Let's look at another kind of metal. This metal right here is a strip of magnesium. We're gonna heat this one up as well. It had that metallic sheen before we started this chemical reaction. Its appearance has definitely changed. Now it's flaky, it's white. It doesn't have that metallic sheen. Do you think this is a chemical change or a physical change? In fact this is a chemical change. It's changed its appearance but its identity has also changed. It is now magnesium oxide. There's been a reaction with oxygen in the air. And there are several ways that you can detect if a chemical reaction has happened. Let's take a look at those. If a chemical reaction evolves a gas, you know that a chemical change has occurred. If there's a change in color, that's another indicator. A loss or gain of heat. Or if light is emitted. These are ways you can know that a chemical change has happen. So let's look at a couple examples of that. So if I take acid, pour it in this beaker, and I add a piece of caulk. Immediately there's the evolution of a gas. A chemical change has happened. In this next chemical reaction, I'm going to show you a change in color. In this case we're not going to worry about the details of the chemicals involved, but more the evidence that a chemical change has occurred. Here when we mix these three chemicals, a very interesting chemical reaction happens. There was definitely a color change. And now another color change. And if we wait long enough, it will change color again. There's a chemical reaction that is continuing to happen. This is a chemical change. In this next example of a chemical change, I'm going to show you what it looks like when a chemical reaction emits light. This is actually the chemistry of glow sticks. That color will slowly fade over time. Another important example of a chemical change is when there's heat lost or heat gained in the process of a chemical reaction. This is hydrogen. We're going to ignite this hydrogen balloon and it will serve as an example of heat lost or heat gained in a chemical reaction.
An important feature of any study of chemistry, is change. So we want to talk about two different kinds a change. Physical changes and chemical changes. In a physical change, a substance has changed its appearance in some way, but remains the same substance. In a chemical change that's not the case. It becomes something new through the process of a chemical reaction. And that's important for you to know because the properties of materials depend on their physical state for example. So let's look at an example of a physical change. What I'm going to do is I'm going to heat up this iron nail right here. And I'm going to heat this to the point where it is red-hot. So this is a physical change because we've changed its appearance, but it's still iron. Iron is a metal. Let's look at another metal. This metal is gallium. What do you think will happen to gallium, which has a lower melting point, when I put it in this hot water? Now, what you can see is that the gallium metal has melted. Is that what you would have predicted? Gallium is one metal, iron is another. Let's look at another kind of metal. This metal right here is a strip of magnesium. We're gonna heat this one up as well. It had that metallic sheen before we started this chemical reaction. Its appearance has definitely changed. Now it's flaky, it's white. It doesn't have that metallic sheen. Do you think this is a chemical change or a physical change? In fact this is a chemical change. It's changed its appearance but its identity has also changed. It is now magnesium oxide. There's been a reaction with oxygen in the air. And there are several ways that you can detect if a chemical reaction has happened. Let's take a look at those. If a chemical reaction evolves a gas, you know that a chemical change has occurred. If there's a change in color, that's another indicator. A loss or gain of heat. Or if light is emitted. These are ways you can know that a chemical change has happen. So let's look at a couple examples of that. So if I take acid, pour it in this beaker, and I add a piece of caulk. Immediately there's the evolution of a gas. A chemical change has happened. In this next chemical reaction, I'm going to show you a change in color. In this case we're not going to worry about the details of the chemicals involved, but more the evidence that a chemical change has occurred. Here when we mix these three chemicals, a very interesting chemical reaction happens. There was definitely a color change. And now another color change. And if we wait long enough, it will change color again. There's a chemical reaction that is continuing to happen. This is a chemical change. In this next example of a chemical change, I'm going to show you what it looks like when a chemical reaction emits light. This is actually the chemistry of glow sticks. That color will slowly fade over time. Another important example of a chemical change is when there's heat lost or heat gained in the process of a chemical reaction. This is hydrogen. We're going to ignite this hydrogen balloon and it will serve as an example of heat lost or heat gained in a chemical reaction.