1. Chemical Reactions
8.5E Investigate how evidence of chemical reactions indicates that new substances with different properties are formed.

2. 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

3. 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 ✓ ✓ ✓ ✓

4. 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

5. 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

6. 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

7. 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.

8. What is a precipitate?
A precipitate is a solid that forms during a chemical reaction between two liquids.

9. Most Importantly….
Always ask “Is it a new substance with different properties?”