Physical and Chemical
Change
Notes #19
Molecule Reminder (not notes)
• A molecule is two or more atoms put together.
• We can represent a molecule either writing by its formula or drawing
its structure.
• The lines in the structure represent the bonds between atoms.
CO2
O=C=O
Formula for carbon dioxide
Structure of carbon dioxide
Molecular bonds (not notes)
• Bonds between atoms come in several different types
• The most common for this class are known as “covalent” bonds and
involve two atoms sharing a pair of electrons
Molecular New (notes)
• When you change the atoms a molecule has you create a new
substance
+O
Structure of water
Structure of hydrogen peroxide
Reminder (not notes)
• A physical property is how something
is right now
• A chemical property is how
something reacts with something
else
Physical Change (notes)
• A physical change is one that does not alter the molecules a
substance is made of
• A physical change can affect physical properties but not chemical
properties
• Examples of physical changes:
•
•
•
•
Changing shape
Changing size
Phase change
Dissolving*
*Dissolving in the sense of dissolving in water. Not
when something is dissolved by an acid.
Chemical Change (notes)
• A chemical change is one that alters the molecules a substance is
made of
• A chemical change can affect both physical properties and chemical
properties (you have a whole new substance)
• Examples of chemical changes:
•
•
•
•
Burning
Rusting
Rotting
Dissolving in an acid
Burning (not notes)
• For example, burning is the combination of different molecules with
the oxygen in the air to produce water and carbon dioxide.
• Many different substances can burn because oxygen is VERY reactive
+
O=O
O=O
+
Methane (CH4) combines with two oxygen molecules (O2) to make two water molecules
(H2O) and carbon dioxide (CO2)
Signs of Chemical Change (notes)
• Some of the common signs of chemical changes are:
•
•
•
•
•
Change in temperature
Change in color
Change in odor
Releases a gas (bubbles)
Gives off light
Remember: color, odor, etc. are
PHYSICAL PROPERTIES even though we
are talking about a chemical change.