Physical and
chemical
changes
Handout 170
Objectives
›  List
and identify known physical and
chemical properties of an object
›  Describe, contrast and identify chemical
changes
Physical Changes affect the physical
properties of an object
›  How
earth looks today is not how earth
looked when it first formed. Even if we go
back 100,000 years, it will not look the
same. Earth is constantly undergoing
change.
›  We will examine how this change occurs
in this unit.
›  One way the Earth changes is through
physical change.
Phys props and changes
›  A
physical change is a change to an
object’s physical properties.
›  Physical properties are properties of an
object that can be observed without
changing the chemical composition of
the object.
›  Think about minerals: what did we
observe that didn’t change the chemical
composition of the mineral?
Phys props continued
›  Think
of a basketball. How can you
describe it? If you were to change those
properties, would the basketball still be
chemically the same?
›  Examples: shape, size, texture, density,
mass, color, volume, etc.
Phys prop continued
›  Thinking
of the same basketball, what
could we do to it to change its physical
properties, but still have it be the same
chemical basketball?
›  There are three common physical
changes.
Physical Changes
›  Changes
in color. Dying a substance can
change its color but keep it chemically
the same.
›  Breaking a substance. If you break, cut, or
snap it apart, it’s still the same thing.
›  Change in state of matter. If you melt,
freeze, or gasify something, it’s chemical
composition is the same!
Chemical Changes
›  Chemical
changes reveal the chemical
properties of an object and change the
physical properties of an object.
›  If we start something on fire (yea!), it’s not
the same chemically anymore (boo!).
›  Flammability (similarly, combustability) is
an example of a chemical property.
›  Why?
Chem Props
›  The
only way we know if something is
flammable is to start it on fire.
›  Chemical properties of an object can
only be observed when the substance’s
chemical identity changes.
›  Another example of a chemical property
is rusting.
Chemical Changes
›  A
chemical change is the change of one
substance into another substance.
›  A chemical change affects the
substances involved in the change.
›  During a chemical change, combinations
of atoms in the original substance are
rearranged to make a new substance.
›  This is known as a chemical reaction.
Chemical change
›  Typically,
two or more substances are
combined to cause a reaction. Examples
include:
›  Rust forming on a piece of iron. (O + Fe)
›  Metal tarnishing is the result of metals
oxidizing.
›  Substances burning to produce a new
substance.
Chemical Changes
›  Not
all chemical changes are destructive.
›  For example, adding heat to some
changes causes “happy” chemical
changes:
›  Cooking
an egg
›  Making a cake.
›  Yea! Cake! Oh, and cookies.
›  Cookies!
ID’ing a chemical change
›  There
is only one true indication of a
chemical change: a new substance has
been formed.
›  However, there are other signs that may
(but DO NOT GUARANTEE) a chemical
change has occurred.
Signs of a chemical change
›  Odor,
as a result of chemical reaction.
›  Temperature change (positive or
negative). This means that energy is being
“used” which is done when atoms
rearrange.
›  Endothermic:
object feels cooler.
›  Exothermic: object will warmer.
More ways to ID a chemical
change
›  Color.
Isn’t color a physical change?
How to ID a chem change
›  Yes,
color can indicate a physical
change. However…
›  Look for an unexpected chemical
change. Ex: you mix two clear liquids
together and it turns chartreuse.
›  Formation of bubbles can indicate the
production of a gas.
›  Formation of a new solid
Bubbles
Be careful
›  Many
of these changes can indicate a
physical change, not a chemical change.
›  The only true way to tell if a chemical
change has occurred is to find out if the
chemical makeup has changed OR if a
new substance is formed as a result.