chemical change

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 the air to produce entirely different chemical substances, including the gases carbon dioxide and water vapor. Click image to the left or use the URL below. URL:

can chemical changes be reversed

Because chemical changes produce new substances, they often cannot be undone. For example, you cant 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.

identifying chemical changes

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 Figure 1.1. Q: 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? A: Another example of matter changing color is a penny changing from reddish brown to greenish brown as it becomes tarnished. The color change indicates that a new chemical substance has been produced. Copper on the surface of the penny has combined with oxygen in the air to produce a different substance called copper oxide. Q: Besides food spoiling, what is another change that produces an odor? Is this a chemical change? A: When wood burns, it produces a smoky odor. Burning is a chemical change. Q: Which signs of chemical change do fireworks produce? A: Fireworks produce heat, light, and loud sounds. These are all signs of chemical change.

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instructional diagrams

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questions

a chemical change always involves one or more

a) chemical reactions.

b) chemical properties.

c) states of matter.

-->  d) two of the above

an example of a property that could change only if a chemical change occurs is

a) size.

b) shape.

c) volume.

-->  d) flammability.

ice melting is an example of a chemical change in matter.

a. true

-->  b. false

signs that a chemical change has occurred include

a) a change in color.

b) the release of bubbles.

c) the production of an odor.

-->  d) all of the above

all of the following are chemical changes except

a) iron rusting.

-->  b) ice cubes melting.

c) pennies tarnishing.

d) food spoiling.

the production of a loud sound may be a sign that a chemical change has occurred.

-->  a. true

b. false

diagram questions

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