Chemical Reactions
Chemical Reactions:
takes place when one or more substances
change into new substances.
Starting substances:
Formed substances:
Reactants
Products
Chemical Reactions
• Chemical Property: the ability of a
substance to undergo a chemical reaction
and form new substances. A chemical
change always results in a change in
chemical composition
• Examples: burn, rust, decompose,
ferment, explode, corrode
Reactants
Iron and Sulfur
change into, produce
Products
Iron sulfide
Reactants
Iron and Sulfur
change into, produce
Chemical
Reaction
Products
Iron sulfide
Chemical Reactions
• Energy is always absorbed or given off in
a chemical reaction
• Evidence of a chemical reaction: change
in color or odor, gas or solid formed from a
liquid, irreversibility of a reaction
• Conservation of Mass: in any physical or
chemical reaction, mass is neither created
nor destroyed; it is conserved.
• Mass of Product = Mass of Reactant
Examples of Chemical Reactions
• A silver spoon tarnishes. The silver reacts with sulfur in the
air to make silver sulfide, the black material we call tarnish.
2 Ag + S
Ag2S
• An iron bar rusts. The iron reacts with oxygen in the air to
make rust.
4 Fe + 3 O2
2 Fe2O3
• Methane burns. Methane combines with oxygen in the air
to make carbon dioxide and water vapor.
CH4 + 2 O2
CO2 + 2 H2O
• An antacid (calcium hydroxide) neutralizes stomach acid
(hydrochloric acid).
Ca(OH)2 + 2 HCl
CaCl2 + 2 H2O
• Glucose (simple sugar) ferments to ethyl alcohol and
carbon dioxide. The sugar in grapes or from grain ferments
with yeast to make the alcohol and carbon dioxide. The
carbon dioxide is the gas that bubbles out of beer or
champaign.
C6H12O6 (glucose)
2 C2H5OH (ethyl alcohol) + 2 CO2
Review
Physical or Chemical Change
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Cookies are baked
Water boils
Salt dissolves in water
A firefly emits light
Milk spoils
A metal chair rusts
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Chemical
Physical
Physical
Chemical
Chemical
Chemical
Review
State the difference between a physical and
chemical change
• In a chemical change, the chemical composition of
the reactants changes as product(s) are formed. In
a physical change, the chemical composition of the
substance remains the same even if its physical
appearance changes
• In a physical change, the chemical composition of
the reactants changes as product(s) are formed. In
a chemical change, the chemical composition of the
substance remains the same even if its physical
appearance changes
Review
• State several physical or chemical properties
that could be used to distinguish between each
of the following pairs of substances and mixtures
• Gasoline and Water
• color, odor, reaction upon heating, boiling point
• Copper and Silver
• color, melting point, reactions with other
substances, hardness, brittleness, strength
• Water and Salt Water Solution
• boiling point, freezing point, density
Review
Which properties are
physical and which are
chemical?
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Reaction upon heating
Color
Odor
Boiling point
Melting point
Hardness
Reaction with other
substances
Brittleness
Strength
Freezing point
Density
Physical
• Color
• Odor
• Boiling point
• Melting point
• Hardness
• Brittleness
• Strength
• Freezing point
• Density
Chemical
• Reaction upon heating
• Reaction with other
substances