Physical & Chemical Changes

recall that chemistry is the study of matter and the changes that undergoes now remember, these changes can either be physical changes or chemical changes. Now, when we're talking about physical changes here, it's changes in the physical state of a substance without a change in composition. So that basically means that the identity of our original substance stays intact. Now some common examples of physical changes include dissolving off Salyut. Now we'll talk about in greater detail about the term Salyut in later chapters. But just realize that a salute is the substance that gets dissolved within a liquid. Then what we can have next is we can have the mixing off substances. These air keyword that you should be on the lookout for when dealing with physical changes. Next. The next four terms are pretty similar to each other. We can have the chopping, cutting, tearing or breaking of material and similar to breaking. We can have the crushing off a substance. For example, you have a can of soda. You take that can of soda and you crush it. Although it looks different at the end of the day, it's still a can. It's just a crushed can. So a physical change makes a physical change within the state of a substance. But the identity stays intact. I had a can of soda cannon beginning. I crushed it, but I still have a soda can at the end of the day. Now that we've understood the basics of physical changes, let's move on to our example question.

So here it says, which change is a physical change. Remember, a physical change does not change the original identity of my substance. A wood burning. Well, when I burn a piece of wood, what's gonna be left at the end will no longer be would. It will be so good. It'll be ash will be something completely new and different. So it's identity has been changed, and this is not a physical change iron rusting. It's a natural phenomenon that happens every day. If you ever look at old beat up cars, you might see some rust spots on it. Well, that's when the metal surface of the car has interacted with oxygen from the air. The oxygen on the in the air has actually bonded to the metal surface and created a metal oxide. We've changed the identity of the metal completely into something new. Remember, because we're changing its identity. Rusting would not be a physical change. Dynamite exploding. If I set off some dynamite, I can't go back out and wreak and recollect what's left over and try to set it off again. It's forever been changed from the explosion. All the chemical bonds have been altered so that identity is different and this would not be a physical change. The answer here is deemed because dissolving sugar in water counts as a physical change. So the sugar acts as the salute. It's what's being dissolved by the liquid water. I can regain that sugar. All I have to do is boil off, the water heated up enough, the water will evaporate, and I'll get left at the bottom of my off my pot. The original sugar it's identity hasn't been changed. Its just been dissolved in water. I can always go back and recollect that sugar if I really wanted so just remember physical property. Physical changes do not change the identity of my original substance.

so chemical changes, chemical changes or changes in the chemical composition that creates a new chemical bond or bonds and product or products. So, basically, ah, chemical change changes the identity of your original substance. Now, some common types of chemical changes involved the rusting of metals. So the example that we had up above of iron, rusting or old cars rusting is a prime example of a chemical change. The metal is transformed into a metal oxide by reacting with the oxygen in the air. Next we have the burning of materials. So if you're burning, if you are baking, if you are cooking you're doing me. You're breaking down old bonds and creating new connections, thereby changing the identity of your original substance. Next, we have the metabolism of food. So when you're eating food goes to your digestive track, your body absorbs nutrients. It becomes something completely different. Next, chemical reactions. Chemical reactions involve our original substance undergoing some new chemical changes to create chemical bonds and then finally last to weaken visually see, and that is when we have a color change, a substance might be clear, and then it changes to blue or a color that we can see or an odor change. Something might smell great and then undergoes a chemical reaction. And it becomes horrible in terms of smell. Or it could smell really bad. And it does a chemical change and start smelling okay or lose it Smell altogether. Now remember, of chemical change is basically us changing the identity of a substance by changing its chemical bonds and connections.

So if we take a look here at this example question, it says which the following is a chemical change? Remember, a chemical change will change the identity of the original substance. For a we have the melting of wax, so when we're melting wax, it's just going from its solid form towards liquid form. But at the end of the day, it's still wax. We haven't changed its identity, so it is not a chemical change, but a physical change for the next one, we have cooking an egg. Remember for burning, cooking or baking. Then that means that we're changing the actual chemical bonds in our original substance. To create something new, you can tell if you're cooking an egg. It looked completely different from its original substance. Egg yolk is clear, but once you start cooking it, it's going to solidify. It'll change colors, so we're going to say that this is a chemical change. For see, we have the condensing of water vapor, so condensation is a term you might have heard off in high school or in grade school. We know that condensation on Windows is just water in its liquid form. That's because the water that's in its gaseous state in the air touches the surface of the window, and it cools down so much that it liquefies here. In both instances, whether it's a gas or liquid, it's still water. It's identity hasn't changed, so it is not a chemical change. Then finally, we have carving a piece of wood. If we're carving wood, I could carve it into a beautiful statue. But the substance of the statue is still would. I haven't burned the wood, so it's not ash or soot. It's still would. So that would just be a physical change. So, out of all the options here, Onley Option B represents a chemical change.

Which of the following is a physical change?

• A. Milk sours when left out of the refrigerator
• B. Tums (containing CaCO₃) neutralizes stomach acid.
• C. Sugar caramelizing when heated on a skillet.
• D. Tearing a piece of paper.

So now we can talk about reversible and irreversible changes to matter. We're going to say here that a reversible change is a change that could be reversed to restore the original substance off a given compound. Now what it classified as a reversible change. We have phase changes. We have the dissolving of compounds and liquids and, of course, physical changes. If we take a look here at this example, we have carbon dioxide, which is CO two s. Here means it's in. It's solid form, it undergoes some process and now it's gas. G here means gas. What? It went through this process, but I didn't change the identity of the original substance. It's still carbon dioxide. It's just in its gashes form. So we're going from the solid form to the gashes form. Phase change, we said our common type off reversible change. And when we're talking about these reversible changes, that means we're going through the different phases of matter. Now when we're talking about bond forming reactions, that means that the molecules within our substance are coming closer together and forming connections. Typically, when we're talking about bond forming, we could talk about going from a gas to a liquid that would be called compensation. Remember, we talked about compensation of water vapor on windows. We could go from liquid to solid liquid to solid. We put water into a nice tray and put it in the freezer. And what happens to it? It freezes so liquid to solid is freezing. But we can also go straight from gas to solid. This is a term not all of us may know, but going from a gas toe a solid, you're depositing a solid. So this is called deposition? Yeah. Now the opposite side of that, If I can form bonds by going through the different phases of matter, then I should be able to break bonds by going through the different phases of matter. So if we're going from a solid to a liquid, we're going to be doing melting now in chemistry. Another name for melting is fusion. So we can say melting or fusion fusion is that's the fancy way of talking about it. Now. Fusion is an interesting word because it could mean different things, depending on what area and chemistry you're talking about when we're talking about the concept of melting we can use fusion, but fusion can also be involved with nuclear chemistry, nuclear chemistry. We're talking about fusion. We're talking about different types of elements, combining together to make a bigger element. But for right now, don't worry about nuclear chemistry because that's several chapters later. For now. When we say the term fusion, we're referring to it in terms of melting now you could also go from liquid to gas. If you're going from a liquid to a gas, that's evaporation or vaporization. So we're talking about vaporization here. Then we can also go straight from solid to gas. Solid to gas is called sublimation. So just remember, in a reversible change, we can go backwards and restore our original material. We haven't truly changed the identity of our substance. Common examples are face changes dissolving and, of course, our physical changes. Now that we looked at reversible changes, let's move on to irreversible changes

in an irreversible change. This is a change that is permanent and cannot be undone to restore the original structure. So when we undergo are irreversible change, we can't go backwards, were stuck with whatever we make. Now the most common example off irreversible changes deal with chemical changes. Remember, chemical changes change the identity of our original material. Ah, great example here is we have a nitrogen gas, hydrogen gas and together they combined to give us an H three gas. They have chemically bonded to each other a end to an H two to form our compound of NH three. And because it chemically bonded this way, it's going to be irreversible. Process. Okay, so we're going forward in one direction. Once we've made that NH three group, we can't go backwards and remake R N two and H two by normal conditions. So just remember, an irreversible process. Create something brand new, and it forever alters the way our original material waas. We can't go back to it at least not by normal means. All right, now that we've seen the difference between reversible and irreversible processes, let's click on the next question and let's tackle some example problems

So if we take a look at this example question it says are the following process is reversible or irreversible. Remember, we tend to connect to the term reversible physical changes and what physical changes. We can undergo the change without changing the identity of the original material. Irreversible changes. We connect to chemical changes. So if we take a look here, it says, First we're dissolving sugar and water. Remember, we said earlier, when you're dissolving your sol Ute, then that represents a physical change because all I have to do here is boil off the water, and what will be left in my pot would be the original sugar. So this would be an example of a physical change, which would mean it's reversible next, adding lemon juice to cabbage juice. It causes the color to transition from purple to deep red. We said earlier that color change is an example of a chemical change, and because it's a chemical change, it would be irreversible, adding Citrus to baking power. Now, this 1 may not be as clear. Um, some people may not have done this little home ec type of experiment, but if you take Citrus like lemon and you squirt it on some baking powder. It's going to start to fizz and bubble. That's because an actual chemical reaction is being done. Now if a chemical reaction is being done, that is an example of a chemical change. And if it's a chemical change, it's an irreversible change. Lastly, if we have the mixing of oil and water we've talked about before, that oil and water don't mix, so they don't really interact. That means they hold on to their individual identities without really changing. We have our original substances being themselves, and because we have original substances, that's a physical change. We could also say earlier that mixing we set of substances is a prime example of a physical change, so this would be reversible. So this is a reversible process. So just remember the key terms that we covered when discussing physical changes in chemical changes remember that these physical changes can be seen as reversible changes, and our chemical changes can be seen as irreversible changes

Which of the following represents a reversible change?

• A. Melting a chocolate bar then re-solidifying it in the freezer.
• B. Changing water vapor into oxygen and hydrogen gas.
• C. Turning a potato into french fries.
• D. Baking a cake from a recipe.