Chemical Reactions 8.5E Investigate how evidence of chemical reactions indicates that new substances...

Chemical Reactions8.5E Investigate how evidence of chemical reactions indicates that new substances with different properties are formed.

How is a chemical change different from a physical change?• Matter can be changed in many ways, but all of these changes

can be classified as either a chemical change or physical change.

Physical Changes Chemical Changes

A change that DOES NOT result in a new substance

A change that DOES result in a new substance

Can be reversed or undone Can’t be reversed or undone

Change in size or shape More than a change in size or shape

A change in the state of matter (solid, liquid, gas)

More than a change in the state of matter

Tear paper Burn paper

Chop wood Burn wood

What is a chemical reaction?• A change in matter that results in a new substance with

different chemical and physical properties than the original substance.

• Identify the chemical changes:• Glass breaking • Rusting metal• Burning toast• Cutting grass• Fireworks exploding• Melting butter• Digesting food

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What are the signs that a chemical reaction has occurred?The best way to identify that a chemical reaction has occurred is the formation of a new substance with different properties. Here are some clues:• Color Change• Gas Production • Change in Energy • Precipitate formation

When does color change indicate that a chemical reaction has occurred?• When a color change is unexpected. For example, you

would expect to get a green liquid when you mix a blue and yellow liquid. But getting a green liquid from combining two clear liquids is unexpected.• Examples of chemical reactions that result in color

change:• Bleaching clothes• Dying hair• Burning toast

What are some examples of gas production during a chemical reaction?

• Bubbles, fizzing, foaming, releasing a vapor (not boiling)• Burning wood – the smoke is a

new product made of carbon dioxide and water vapor.• Combining vinegar and baking

soda – bubbles and fizzing releasing carbon dioxide gas

What does “a change in energy mean?”• A change in energy in a chemical reaction means that energy is

either absorbed or released during a chemical reaction.• It is very common for energy in the form of heat (thermal

energy) to be absorbed or released during a chemical reaction. This results in either a temperature increase or decrease.

• Sometimes the change in energy is light energy being absorbed or released. Photosynthesis is an example of a reaction that absorbs light energy. A glow stick is an example of a chemical reaction releasing light energy.

What is a precipitate?• A precipitate is a solid that forms during a chemical reaction

between two liquids.

Most Importantly….

•Always ask “Is it a new substance with different properties?”