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Chemical Changes: Crash Course Kids #19.2

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Physical changes involve states of matter and energy. No new substance is created during a physical change, although the matter takes a different form. The size, shape, and color of matter may change. Physical changes occur when substances are mixed but don't chemically react.

How to Identify a Physical Change

One way to identify a physical change is that such a change may be reversible, especially a phase change. For example, if you freeze water into an ice cube, you can melt it into the water again. Ask yourself:

Is the change reversible? Not all physical changes are easy to reverse.

Was there a color change (with exceptions), bubble formation, or formation of a precipitate? These are all signs of a chemical change, not a physical change.

Is the chemical identity of the end product the same as it was before the change? If the answer is yes, it's a physical change. If the answer is no, it's a chemical change.

Examples of Physical Changes

Remember, the appearance of matter changes in a physical change, but its chemical identity remains the same.

Crushing a can

Melting an ice cube

Boiling water

Mixing sand and water

Breaking a glass

Dissolving sugar and water

Shredding paper

Chopping wood

Mixing red and green marbles

Sublimation of dry ice

Crumpling a paper bag

Melting solid sulfur into liquid sulfur. This is an interesting example since the state change does cause a color change, even though the chemical composition is the same before and after the change. Several nonmetals, such as oxygen and radon, change color as they change phase.

Chopping an apple

Mixing salt and sand

Filling a candy bowl with different candies

Vaporizing liquid nitrogen

Mixing flour, salt, and sugar

Mixing water and oil

Indications of a Chemical Change

Sometimes the easiest way to identify a physical change is to rule out the possibility of a chemical change. There may be several indications that a chemical reaction has occurred. Note: It's possible for a substance to change color or temperature during a physical change.

Evolving bubbles or releasing gas

Absorbing or releasing heat

Changing color

Releasing an odor

Inability to reverse the change

Precipitation of a solid from a liquid solution

Formation of a new chemical species. This is the best and surest indicator. A change in the chemical properties of the sample may indicate a chemical change (e.g., flammability, oxidation state).

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Examples of Physical Changes Compared With Chemical Changes

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Physical changes involve states of matter and energy. No new substance is created during a physical change, although the matter takes a different form. The size, shape, and color of matter may change. Physical changes occur when substances are mixed but don't chemically react.

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Definition of physical change and examples, and how physical changes can be reversed.

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Lesson Objectives

Define and give examples of physical changes in matter.

Define and give examples of chemical changes in matter.

State the law of conservation of mass.

Lesson Vocabulary

chemical change

law of conservation of mass

physical change

Introduction

You hit a baseball out of the park and head for first base. You're excited. The score is now tied. Your team has a chance of getting a winning home run. Then, you hear a crash. Oh no! The baseball hit a window in a neighboring house. The glass has a big hole in it, surrounded by a web of cracks (see Figure below). The glass has changed. It has been broken into jagged pieces. But the glass is still glass. Breaking the window is an example of a physical change in matter. It may also be bad news when the owner of the house finds out.

Physical Changes in Matter

In each of these changes, only the physical properties of matter change. The chemical properties remain the same. It didn't turn into a different type of matter. Because the type of matter remains the same with physical changes, the changes are often easy to undo. For example, braided hair can be unbraided again. Melted chocolate can be put in a fridge to re-harden. Dissolving salt in water is also a physical change. How do you think you could undo it?

When glass breaks, its physical properties change. Instead of one solid sheet of glass, it now has holes and cracks.

Physical Changes in Matter

A physical change in matter is a change in one or more of matter's physical properties. Glass breaking is just one example of a physical change. Some other examples are shown in Figure below. In each example, matter may look different after the change occurs, but it's still the same. It is still the same substance with the same chemical properties. For example, smaller pieces of wood have the ability to burn just as larger logs do.

In each of these changes, only the physical properties of matter change. The chemical properties remain the same.

Chemical Changes in Matter

Did you ever make a "volcano," like the one in Figure below. It's easy, just mix baking soda with vinegar. What happens when they combine? They produce an eruption of foamy bubbles. This happens because of a chemical change. A chemical change occurs when matter changes into an entirely different substance. The new substance has different properties. When vinegar and baking soda combine, they form carbon dioxide. Carbon dioxide is a gas. It is this gas that causes the bubbles. It is the same gas that gives soft drinks their fizz.

This girl is pouring vinegar on baking soda. This causes a bubbling "volcano."

Not all chemical changes are as dramatic as this "volcano." Some are slower and less obvious. Figure below and the video below show other examples of chemical changes.

These chemical changes all result in the formation of new substances with different chemical properties. Do you think any of these changes could be undone?

Signs of Chemical Change

How can you tell whether a chemical change has occurred? Often, there are clues. Several are demonstrated in Figures above and above and in the video below.

These chemical changes all result in the formation of new substances. These new substances have different chemical properties. Do you think any of these changes could be undone?

Signs of Chemical Change

How can you tell whether a chemical change has occurred? Often, there are clues. Several are demonstrated in Figures above.

To decide whether a chemical change has occurred, look for these signs:

Gas bubbles are released. (Example: Baking soda and vinegar mix and produce bubbles.)

Something changes color. (Example: Leaves turn from green to other colors.)

An odor is produced. (Example: Logs burn and smell smoky.)

A solid comes out of a solution. (Example: Eggs cook and a white solid comes out of the clear liquid part of the egg.)

Reversing Chemical Changes

Chemical changes produce new substances. For this reason, they often cannot be undone. Some chemical changes can be reversed, sort of. To do so requires another chemical change to take place. For example, you can undo the tarnish on copper pennies by placing them in vinegar. The acid in the vinegar reacts with the tarnish. This is a chemical change that makes the pennies bright and shiny again. You can try this yourself at home to see how well it works. Other chemical changes cannot be reversed at all or may be difficult to do. Rusting is a chemical change. If metal rusts, the best you can do is to sand off the rust to get down to the shiny metal. Although the metal may now be shiny, the rust was removed. The rust was not changed back into the original metal.

Some chemical reactions occur in only one direction. These reactions are called irreversible reactions. For example, you cannot change a fried egg back into a raw egg. Can you think of some other irreversible reactions related to cooking? Would you like a piece of cake? To make a cake you mix items such as such as eggs and flour. They are mixed together and baked to form the cake. The cake can’t be “unbaked” and “unmixed.”  So making a cake is irreversible change. Therefore, baking is a form of chemical change.

How Temperature Affects Changes in Matter

Have you are burned something on the stove or in the oven? Do you know why things burn? It all has to do with temperature. Changes in matter are affected by temperature. For example, what if you place a cake in the oven and you leave it in too long? What do you think will happen? That's right, it will probably burn. The same thing will happen if the oven setting is too high and you apply too much heat. In either case, heat has affected the rate of change in matter. It's probably obvious that when the temperature is increased a chemical change, like cooking, will occur faster. What about a physical change? Are physical changes affected by temperature?

Most of us have seen steam rising off a wet road after a summer rainstorm. This happens because the road surface is very warm. The warm road and warm air temperature causes the water to evaporate quickly. The liquid water is turning into water vapor, but it is still water. The evaporation of water is a physical change.  So yes, temperature affects the rate that physical change occurs.

Are there other examples you can think of? What about how fast something melts? Have you ever left a candy bar outside in the sun? Just like how temperature increases the rate a substance dissolves, temperature affects the rate of change in many other ways. As temperature increases, changes in matter happen more quickly. For example, do you think an ice cube will melt faster in warm temperatures? Of course they will, which is why we keep ice in the freezer. If not, they would melt and eventually evaporate. When temperatures are lower, change happens more slowly. Think about your refrigerator. What happens if you leave the door open? Items in the refrigerator may thaw, but they may also spoil more quickly.

We use refrigerators to slow down the natural rate of change in the foods we eat. Food left out can spoil quickly. That is why it is really important to keep foods cool. The next time your mother tells you to close the refrigerator door, don't just do it because she told you so. Do it because you know a lot about physics and rates of change of matter.

Conservation of Mass

If you build a campfire, you start with a large stack of sticks and logs. As the fire burns, the stack slowly shrinks. By the end of the evening, all that is left is a small pile of ashes. What happened to the matter that you started with? Was it destroyed by the flames? It may seem that way. What do you think happened? The truth is that the same amount of matter still exists. The wood changed not only to ashes, but also to carbon dioxide, water vapor, and other gases. The gases floated off into the air, leaving behind just the ashes. Although matter was changed, it was not created or destroyed. Because the same amount of matter still exists, we can say that matter is conserved. You may wonder how it can be conserved if something is now missing?

Burning is a chemical process. Is mass destroyed when wood burns?

Assume you had measured the mass of the wood before you burned it. Assume you had also trapped the gases released by the burning wood and measured their mass and the mass of the ashes. What would you find? The ashes and gases combined have the same mass as the wood you started with.

This example illustrates the law of conservation of mass. The law states that matter cannot be created or destroyed. Even when matter goes through physical or chemical changes, the total mass of matter always remains the same.

Lesson Summary

Physical changes are changes in the physical properties of matter. They do not affect the makeup of matter. An example of a physical change is glass breaking.

Chemical changes are changes in the makeup and chemical properties of matter. An example of a chemical change is wood burning.

Matter cannot be created or destroyed even when it changes. This is the law of conservation of mass.

Lesson Review Questions

Recall

1. What is a physical change in matter?

2. What happens during a chemical change in matter?

3. State the law of conservation of mass.

Apply Concepts

4. When a plant grows, its mass increases over time. Does this mean that new matter is created? Why or why not?

5. Butter melts when you heat it in a pan on the stove. Is this a chemical change or a physical change? How can you tell?

6. Is popping popcorn a physical or chemical change? How can you tell?

Think Critically

6. Compare and contrast physical and chemical changes in matter. Give an example of each type of change.

Points to Consider

Some physical changes in matter are changes of state.

What are the states of matter?

What might cause matter to change state?

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Welcome to CK-12 Foundation | CK-12 Foundation

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Choose three writing prompts to complete.

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It can change back to normal state, change qualitative properties.

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Add extreme heat; create new substance.