Physical and Chemical
Changes
Chapter 2 Section 4
Energy and Change
Two kinds of changes in matter:
 Physical Change
 Chemical Change
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Physical Change
A physical change alters the form of a
substance, but does not change it to
another substance.
 Examples are: tearing paper, when a nail
bends, spinning wool into yarn.
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Chemical Change
When a substance undergoes a chemical
change, it is changed into a different
substance with different properties.
 Examples burning wood, turning sugar into
caramel.
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Thermal and Chemical Energy
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Every substance has energy from the movement
of its particles, called thermal energy.
The higher the temperature of a substance, the
greater its thermal energy.
Another form of energy comes from the chemical
bonds within matter.
This form of energy is called chemical energy.
Changes in Matter
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Any substance can either gain energy or lose
energy.
In either case, the substance changes in some
way.
Matter changes whenever energy is added or
taken away.
When something is heated, it gains thermal
energy
When something is cooled, it loses thermal
energy.
Law of Conservation of Energy
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In every physical change and chemical change,
the total amount of energy stays the same.
The word conservation comes from conserve
which means “to protect from loss.”
Energy can change from one form to another but
can never be lost.
For example, an unlit match contains chemical
energy. When it is lit, the chemical energy
changes to light and thermal energy.
The total amount of chemical energy that was in
the match before it was lit is equal to the light
and thermal energy after it is lit.
Changes Between Liquids and
Solids
Under certain conditions, a substance can
change from any one state of matter to
any other.
 Each substance changes state at
temperatures typical for that substance.
 But the overall pattern for the way
substances change state is the same.
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Melting
The change in state from a solid to a liquid
is melting.
 In most pure substances, melting occurs
at a temperature that is specific to the
substance, called the melting point.
 The melting point of a substance depends
on how strongly its particles attract one
another.
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Freezing
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Freezing is the change of state from liquid to
solid-just the reverse of melting.
When you put liquid water into a freezer, the
water loses energy to the cold air.
The water molecules move more slowly.
This means that the temperature of the water is
dropping.
When the temperature reaches 0 degrees
Celsius, the molecules are moving so slowly that
they form regular patterns. These patterns are
the ice crystals that form ice.
Changes Between Liquid and Gas
The change from liquid water to water
vapor is an example of vaporization.
 Vaporization occurs when a liquid gains
enough energy to become a gas.
 There are two main types of vaporization
 Evaporation – takes place at the surface of
a liquid
 Boiling – takes place inside a liquid as well
as on the surface.
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Evaporation
An example of evaporation is when a
puddle dries up after it rains.
 The water in the puddle gains enough
energy from the ground, air, and the sun to
change the particles on the surface of the
liquid from a liquid to a gas.
 The energy enables the molecules on the
surface of the puddle to escape into the
air.
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Boiling
Each liquid boils at a specific temperature,
called its boiling point.
 Like the melting point of a solid, the boiling
point of a liquid depends on how strongly
the particles of the substance are attracted
to one another.
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Boiling Point and Air Pressure
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Boiling point also depends on the pressure of
the air above a liquid.
The lower the air pressure above the liquid, the
less energy the liquid molecule need to escape
into the air.
As you go up in elevation, air pressure
decreases.
At the air pressure near sea level, the boiling
point is 100 degrees Celsius.
In the mountains, air pressure is lower and so is
the boiling point of water.
Condensation
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The opposite of vaporization is called
condensation.
Condensation occurs when a gas loses enough
thermal energy to become a liquid.
Examples are clouds form when water vapor in
the atmosphere condenses into liquid droplets.
When the droplets get heavy enough, they fall to
the ground as rain.
Condensation cont.
When you observe condensation by breathing onto
a mirror, you can see the warm water vapor in
your breath condense on the cooler surface of
the mirror.
The droplets then evaporate in water vapor again.
You cannot see water vapor in the air. When you
see steam from water boiling it is tiny droplets of
liquid water suspended in the air.
Changes Between Solid and Gas
Sublimation occurs when the surface
particles of a solid gain enough energy to
become a gas.
 Particles do not pass through the liquid
state at all.
 An example of sublimation is the change
that dry ice undergoes.
 Dry ice is the common name for solid
carbon dioxide.
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Why do we use Dry Ice?
As dry ice changes from a solid to a liquid,
it absorbs energy, so materials near it stay
cold and dry.
 For this reason using dry ice is an
excellent way to keep temperatures low
when a refrigerator is unavailable.
 When dry ice becomes a gas, it cools
water vapor in the air, which form fog.
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Chemical Changes
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Chemical Reaction is another name for chemical
change.
In some chemical reactions, one substance
breaks down into two or more other substances.
In other reactions, two or more substances
combine, forming one or more new substances.
All chemical reactions have one thing in
common. They all produce new substances.
Energy and Chemical Reactions
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Like all changes in matter, chemical reactions
occur when substances gain or lose energy.
All chemical reactions either absorb energy or
release energy.
What is the source of energy that a plant uses to
grow?
A plant changes the sun’s light energy into the
chemical energy that it needs to make it grow.
Stored Chemical Energy
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Chemical energy that has been absorbed and
stored can be later released.
An example is burning wood in a woodstove.
Wood combines with oxygen in the air,
producing substances such as carbon dioxide,
water, and soot.
Chemical energy is transformed into thermal and
electromagnetic energy during this process.
Your body releases chemical energy that you
obtain from foods that you eat.
Controlling Chemical Energy
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How can you make a chemical reaction occur
faster or slower?
You just have to add energy or take energy
away.
If you are baking a cake, and the kitchen is
warm, reactions in the cake batter happen faster.
The hotter the oven the faster the cake bakes.
You discontinue the chemical reaction by taking
the cake out of the oven.
Putting the cake in the refrigerator, slows down
the chemical reactions that make the cake spoil.