This document discusses how to identify chemical reactions and changes through various signs and properties. It defines chemical and physical changes, noting that a chemical change results in a new substance. It then outlines several signs that a chemical reaction has occurred, including color changes, gas production, changes in energy, and formation of precipitates. Specific examples are provided for each, such as burning toast resulting in a color change and vinegar and baking soda producing gas. The document emphasizes that a chemical reaction can be identified by the formation of a new substance with different chemical and physical properties compared to the original substances.

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 – it changes its properties
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
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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?”