| How Does One Substance Change
into Another?
Chemical Changes
burn, rot, rust, decompose, ferment, explode, and corrode
Chemical Property
The ability of a substance to undergo a specific chemical
change
During a chemical change, the composition of matter
always changes,
and a new substance is formed.
Chemical Change or Chemical Reaction
One or more substances change in to one or more new
substances
Reactants
Substances present at the start of a chemical reaction
Products
New substances formed by a chemical reaction
Example:
Iron plus sulfur yields iron sulfide, an exothermic
reaction (it gives off energy):
|
Fe (s) + S (s)
|
—>
|
FeS (s) + energy |
| reactants |
|
products
|
Recognizing Chemical Reactions
-
Possible clues include transfer of energy (including heat),
change
in color, production of a gas, or formation of a precipitate.
Precipitate
A solid that forms and settles out of a liquid mixture
Conservation of Mass
-
During any chemical reaction, the mass of the products is
always equal to the mass of the reactants.
The Law of Conservation of Mass
In any physical change or chemical reaction, mass
is conserved.
Mass is never created nor destroyed.
Example:
The balanced chemical equation for photosynthesis demonstrates
the law:
|
(sunlight)
|
|
|
6 CO2
+ 6 H2O
|
—>
|
C6H12O6
+ 6 O2 |
C 6
H 12
O 18 |
|
C 6
H 12
O 18
|
Note how there are the same number of atoms in the products
as there are in the reactants.
How does one substance change
into another?
Practice Problems
Answer the following questions: |
|
| 1. |
Identify each of the following as a physical
change or a chemical reaction.
If it is a chemical reaction, state is the visible
evidence. |
|
.
| a) Propane fuel burning in a barbecue. |
| . |
| b) Chopping a carrot |
| . |
| c) Using peroxide to bleach hair |
|
|
|
| 2. |
What evidence would you see that indicates
that lighting a match is a chemical reaction? |
|
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