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

Chemistry focuses on the study of matter and its transformations, which can be classified into physical and chemical changes. Physical changes involve alterations in the physical state without changing the substance's composition, preserving the identity of the material. Examples include dissolving a solute, mixing substances, and altering the shape or form of an object, such as crushing a can. Conversely, chemical changes result in the formation of new chemical bonds, creating new substances with different identities. This includes processes like rusting, burning, and metabolism, which involve breaking and forming bonds, leading to a change in chemical composition.

Matter can undergo reversible changes, where the original substance can be restored, typically through phase changes, dissolving, or other physical changes. These include transitions between solid, liquid, and gas states, such as melting (fusion), freezing, vaporization, condensation, sublimation, and deposition. Irreversible changes, on the other hand, are permanent and cannot be undone under normal conditions. These are often chemical changes where new substances are formed, such as the reaction of nitrogen and hydrogen gases to form ammonia: N₂+3H₂→2NH₃ which cannot be easily reversed to yield the original reactants.

Understanding the distinction between reversible and irreversible changes is crucial in chemistry, as it informs the potential

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. 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 of solute. Now we'll talk about 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 we can have the mixing of substances. These are keywords 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 braking, we can have the crushing of a substance. For example, you have a can of soda. You take the can of soda and you crush it. Although it looks different, at the end of the day it's still a can. It's 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 a can, 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 my substance, a wood burning. When I burn a piece of wood, what's going to be left at the end will no longer be wood. It'll be sudd, 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 it. 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 leftover 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 deemed 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 get left at the bottom of my pot. 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 property. Physical changes do not change the identity of my original substance.

Physical & Chemical Changes

So chemical changes. Chemical changes are changes in the chemical composition that creates a new chemical bond or bonds and product or products. So basically a 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. 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 tract, 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 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 it 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 OK or lose its smell altogether.

Now remember, a chemical change is basically us 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, 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 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 South, much that it liquefies here. In both instances, whether it's a gas or 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. It went through this process, but it 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, as 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 we talked about condensation of water vapor on windows? We could go from 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 that 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. We can say melting or fusion. Fusion is 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.

When we're talking about fusion in nuclear chemistry, 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, 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 deal with chemical changes. Now remember, chemical changes change 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 an end to an H2 to form our compound of NH3. And because it chemically bonded this way, it's going to be irreversible process.

OK. 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.

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.

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 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 solute then that represents a physical change. Because all I'd 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 homec 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 in 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 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 are 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 and chemical changes are two types of processes that materials can undergo. A physical change involves a change in the physical properties of a substance, but its chemical identity remains the same. Examples of physical changes include changes in state (like ice melting into water), tearing a piece of paper, or stretching a rubber band. These changes are generally reversible.

On the other hand, a chemical change results in the formation of one or more new substances with different chemical properties. This type of change involves making or breaking chemical bonds, and it often results in a change in color, the production of a gas, a change in temperature, or the formation of a precipitate. Examples include rusting iron, burning wood, or baking a cake. Chemical changes are typically not easily reversible.

Understanding the difference between these changes is important in chemistry because it helps in predicting the behavior of materials and the conservation of matter in different reactions and processes.

What are physical changes in chemistry?

Physical changes in chemistry refer to alterations in the form or appearance of a substance without changing its chemical composition. These changes are generally reversible. Examples include changes in state (like ice melting into water), size (such as cutting a piece of paper), shape (bending a metal rod), or texture (crushing a can). During a physical change, the molecules of the substance rearrange, but the actual structure of the molecules—the atoms they're made of and the way those atoms are bonded together—remains the same. This means that a physical change affects only physical properties, such as density, color, ductility, hardness, or conductivity. Since the identity of the substance doesn't change, physical changes are often used in separation techniques like filtration, distillation, or chromatography to isolate or purify substances.

What is a chemical change in chemistry?

A chemical change in chemistry refers to a process where substances combine or break apart to form new substances with different chemical properties. This transformation involves the making or breaking of chemical bonds, leading to a change in the molecular composition of the substances involved. Unlike a physical change, which only affects the form of a material without altering its underlying structure, a chemical change results in the production of one or more new substances.

Examples of chemical changes include the rusting of iron, the combustion of gasoline, the baking of bread, and the digestion of food. These changes are often irreversible and can be accompanied by observable signs such as the release of gas, a change in color, the formation of a precipitate, or the emission or absorption of heat. Identifying a chemical change is crucial in understanding chemical reactions, which are fundamental to the study of chemistry and the processes that occur in living organisms and the environment.