Physical or Chemical 3

​Physical or Chemical

By JENNIFER MEDLOCK

These stunning rock arches in Utah were carved by wind-blown sand. Repeated scouring by the sand wore away the rock, bit by tiny bit, like sandpaper on wood. The bits of rock worn away by the sand still contain the same minerals as they did when they were part of the large rock. They have not changed chemically in any way. Only the size and shape of the rock have changed, from a single large rock to millions of tiny bits of rock. Changes in size and shape are physical changes in matter

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​Physical Changes

A physical change is a change in one or more physical properties of matter without any change in chemical properties. In other words, matter doesn’t change into a different substance in a physical change. Examples of physical change include changes in the size or shape of matter. Changes of state—for example, from solid to liquid or from liquid to gas—are also physical changes. Some of the processes that cause physical changes include cutting, bending, dissolving, freezing, boiling, and melting. Four examples of physical change are pictured.

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Open Ended

What physical changes are happening in the picture?

​Reversing a Chemical Change

When matter undergoes physical change, it doesn’t become a different substance. Therefore, physical changes are often easy to reverse. For example, when liquid water freezes to form ice, it can be changed back to liquid water by heating and melting the ice.

Physical Changes Summary

Go to the next available page in your notebook and title it "Physical Changes Summary." Copy these notes on the page.

• A physical change in matter is a change in one or more of matter’s physical properties. In a physical change, matter may change its size, shape, or state, but its chemical properties do not change.

• Because the chemical properties of matter remain the same in a physical change, a physical change is often easy to reverse.

Communities often use fireworks to celebrate important occasions. Fireworks certainly create awesome sights and sounds! Do you know what causes the brilliant lights and loud booms of a fireworks display? The answer is chemical changes.

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​Chemical Changes

What is a Chemical Change?

A chemical change occurs whenever matter changes into an entirely different substance with different chemical properties. A chemical change is also called a chemical reaction. Many complex chemical changes occur to produce the explosions of fireworks. An example of a simpler chemical change is the burning of methane. Methane is the main component of natural gas, which is burned in many home furnaces. During burning, methane combines with oxygen in

Most chemical changes are not as dramatic as exploding fireworks, so how can you tell whether a chemical change has occurred? There are usually clues. You just need to know what to look for. A chemical change has probably occurred if bubbles are released, there is a change of color, or an odor is produced. Other clues include the release of heat, light, or loud sounds. Examples of chemical changes that produce these clues are shown in the​ picture below.

Identifying Chemical Changes

Open Ended

In addition to iron rusting, what is another example of matter changing color? Do you think this color change is a sign that a new chemical substance has been produced?

​Can chemical changes be reversed?

Because chemical changes produce new substances, they often cannot be undone. For example, you can’t change ashes from burning logs back into wood. Some chemical changes can be reversed, but only by other chemical changes. For example, to undo tarnish on copper pennies, you can place them in vinegar. The acid in the vinegar combines with the copper oxide of the tarnish. This changes the copper oxide back to copper and oxygen, making the pennies reddish brown again. You can try this at home to see how well it works.  

• A chemical change occurs whenever matter changes into an entirely different substance with different chemical properties. Burning is an example of a chemical change.

• Signs of chemical change include the release of bubbles, a change of color, production of an odor, release of heat and light, and production of loud sounds.

• Because chemical changes result in different substances, they often cannot be undone. Some chemical changes can be reversed, but only by other chemical changes.

Go to the next available page in your notebook and title it "Chemical Changes Summary." Copy these notes onto the page.

Chemical Changes Summary