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?”