Physical & Chemical Changes - Online Tutor, Practice Problems & Exam Prep

Chemistry involves studying matter and its changes, categorized as physical or chemical. Physical changes, like dissolving or crushing, maintain the substance's identity. In contrast, chemical changes alter the substance's identity, forming new bonds, as seen in rusting or burning. Changes can be reversible, such as phase changes, or irreversible, like chemical reactions that create new compounds. Understanding these concepts is crucial for grasping the nature of matter and its transformations in various chemical processes.

Physical and Chemical Changes

Recall that chemistry is the study of matter and the changes it undergoes. Now remember, these changes can either be physical changes or chemical changes. 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. Some common examples of physical changes include the dissolving of solute. We'll talk in greater detail about the term Solute in later chapters, but just realize that a solute is the substance that gets dissolved within a liquid. Then what we can have next is the mixing of substances. These are keywords that you should be on the lookout for when dealing with physical changes. 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 of a substance. For example, if you have a can of soda, you take that can of soda and 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. If I had a soda can in the 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.

Physical & Chemical Changes Example 1

So here it says, which change is a physical change? Remember, a physical change does not change the original identity of the substance. A, wood burning. Well, when I burn a piece of wood, what's going to be left at the end will no longer be wood. It'll be soot; it'll be ash; it'll be something completely new and different. So its 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 them. Well, that's when the metal surface of the car has interacted with oxygen from the air. The oxygen 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 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 the identity is different and this would not be a physical change. The answer here is D because dissolving sugar in water counts as a physical change. So the sugar acts as the solute. 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, heat it up enough, the water will evaporate, and I'll be left at the bottom of my pot with the original sugar. Its identity hasn't been changed. It's just been dissolved in water. I can always go back and recollect that sugar if I really wanted. So just remember, physical changes do not change the identity of the original substance.

Physical & Chemical Changes

So chemical changes. Chemical changes are changes in the chemical composition that create a new chemical bond or bonds and product or products. Basically, a chemical change changes the identity of your original substance. Some common types of chemical changes involve the rusting of metals. The example that we had 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, baking, or cooking, 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. When you're eating food, it goes through your digestive tract, your body absorbs nutrients, and 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, the last two, we can 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 undergoes a chemical change and starts smelling okay or loses its smell altogether. Remember, a chemical change is basically changing the identity of a substance by changing its chemical bonds and connections.

Physical & Chemical Changes Example 2

So if we take a look here at this example question, it says, which of 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 to its 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, if we're 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 c, we have the condensing of water vapor. So, condensation is a term you might have heard of 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 a liquid, it's still water. Its 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 wood. I haven't burned the wood, so it's not ash or soot, it's still wood. So that would just be a physical change. So out of all the options here, only 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.

Physical & Chemical Changes

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 can be reversed to restore the original substance of a given compound. Now, what is classified as a reversible change, we have phase changes, we have the dissolving of compounds in liquids, and, of course, physical changes. If we take a look here at this example, we have carbon dioxide, which is CO₂, s here means it's in its 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 gaseous form. So we're going from the solid form to the gaseous form. Phase changes, we said, are a common type of 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 condensation. Remember when we talked about condensation of water vapor on windows? We could go from liquid to solid. Liquid to solid, we put water in an ice 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 to a solid, you're depositing a solid, so this is called deposition. 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 just the fancy way of talking about it. Now, fusion is an interesting word because it can mean different things depending on what area in 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, when 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 phase changes, dissolving, and, of course, our physical changes. Now that we've looked at reversible changes, let's move on to irreversible changes.

Physical & Chemical 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 our irreversible change, we can't go backwards. We're stuck with whatever we make. Now, the most common example of irreversible changes deals with chemical changes. Remember, chemical changes alter the identity of our original material.

A great example here is we have nitrogen gas, hydrogen gas, and together they combine to give us NH3 gas. They have chemically bonded to each other, N2 and H2 to form our compound of NH3. And because they're chemically bonded this way, it's going to be an irreversible process. Okay? So we're going forward in one direction. Once we've made that NH3 group, we can't go backwards and remake our N2 and H2 by normal conditions. So just remember, an irreversible process creates something brand new and it forever alters the way our original material was. We can't go back to it, at least not by normal means. 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.

Physical & Chemical Changes Example 3

So if we take a look at this example question, it says, are the following processes reversible or irreversible? Remember, we tend to connect to the term reversible physical changes. And with 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 in water. Remember, we said earlier when you're dissolving your solute, 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 powder. Now this one may not be as clear. 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 said, 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 and 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.

Here’s what students ask on this topic:

What is the difference between physical and chemical changes?

Physical changes involve alterations in the physical state or appearance of a substance without changing its chemical composition. Examples include dissolving, mixing, chopping, and crushing. The substance retains its original identity. Chemical changes, on the other hand, involve changes in the chemical composition, resulting in the formation of new substances with different properties. Examples include rusting, burning, and metabolism of food. These changes create new chemical bonds and products, altering the original substance's identity.

Can you provide examples of reversible and irreversible changes?

Reversible changes are those that can be undone to restore the original substance. Common examples include phase changes like melting (fusion), freezing, condensation, and sublimation. Dissolving a solute in a solvent is also reversible. Irreversible changes, however, cannot be undone by normal means. These typically involve chemical changes, such as the formation of rust (iron oxide) from iron and oxygen, or the synthesis of ammonia (NH₃) from nitrogen (N₂) and hydrogen (H₂).

What are some common indicators of a chemical change?

Common indicators of a chemical change include the formation of a new substance, color change, temperature change, gas production, and odor change. For example, rusting of iron results in a color change from metallic gray to reddish-brown. Burning wood produces heat, light, and new substances like ash and gases. The metabolism of food in the body results in new compounds and energy release.

How do phase changes relate to physical changes?

Phase changes are a type of physical change where a substance transitions between different states of matter: solid, liquid, and gas. These changes do not alter the chemical composition of the substance. Examples include melting (solid to liquid), freezing (liquid to solid), vaporization (liquid to gas), condensation (gas to liquid), sublimation (solid to gas), and deposition (gas to solid). The identity of the substance remains the same throughout these changes.

Why are chemical changes considered irreversible under normal conditions?

Chemical changes are considered irreversible under normal conditions because they involve the formation of new chemical bonds and products, fundamentally altering the original substance's identity. For instance, when nitrogen (N₂) and hydrogen (H₂) react to form ammonia (NH₃), the original gases cannot be easily separated back into their original forms. The new chemical bonds created during the reaction make it difficult to reverse the process without special conditions or additional chemical reactions.