Chemical vs. Physical Changes
• Physical Change
–No new compounds are formed
–Ex. Cutting, ripping, dissolving,
phase changes…
Chemical vs. Physical Changes
• Chemical Change
–Bonds are broken and the atoms in
compounds are rearranged
–New Compounds are created
–Are the result of chemical reactions
Chemical Reactions
• Evidence
– Bubbling, Cloudy solution, Temperature
Change, Color Change, Smoke, Light,
Heat
• Reactants: what you start with
• Products: what you end with
• Can be written as a sentence or an
equation:
Methane reacts with oxygen to form
carbon dioxide and water.
or
CH4 + O2  CO2 + H2O
(REACTANTS)  (PRODUCTS)
Types of Reactions
• Addition/Synthesis: two or more elements
combine to form a new compound
A + B  AB
Shortcut –
only one product
Types of Reactions
• Decomposition: one compound breaks
apart to form two or more products
AB  A + B
Shortcut –
only one reactant
Types of Reactions
• Single-Replacement (Displacement): an
element replaces another element in a
compound
A +BX  AX + B
Shortcut one element &
one compound
on both sides
Types of Reactions
• Double-Replacement (Displacement): two
elements in different compounds switch
places
AX + BY  BX + AY
Shortcut Two
compounds
with elements
switching
places in
each
Combustion: a compound containing
carbon and hydrogen combines with
oxygen to form carbon dioxide and water
AB + O2  CO2 + H2O
Types of Reactions
• Combustion: a compound containing
carbon and hydrogen combines with
oxygen to form carbon dioxide and water
AB + O2  CO2 + H2O
Types of Reactions
• Exothermic: reactions that give
energy off to their surroundings.
• See a flame or feel heat (hand
warmers, MREs)
• Endothermic: reactions that
absorb energy from their
surroundings.
• Feel cold (instant ice packs)
Law of Conservation of Mass
• During a chemical reaction, atoms CANNOT be
created nor destroyed, only rearranged!
• In other words, the number of atoms of each
element in the reactants must equal the number
of atoms in the products.
• CHEMICAL EQUATIONS MUST BE
BALANCED!!!
How to Balance Equations
1. Count the number of atoms on each side
of the reaction.
2. Put a coefficient in front of one molecule
that has too few atoms of an element.
3. Count the number of atoms again.
4. If all are equal—YOU ARE DONE! If
not—REPEAT steps 2-3 until they are!
CH4 + O2  CO2 + H2O
Count the number of atoms:
• Reactants:
C–1
H–4
O–2
NOT BALANCED!!!!!!!
• Products:
C–1
H–2
O–2+1=3
CH4 + 2O2  CO2 + 2H2O
Put in coefficients:
• Reactants:
C–1
H–4
O–2*2=4
BALANCED!!!!!!!!!!
• Products:
C–1
H–2*2=4
O–2+2=4