Changes of State
Section 18.3
Demo
• Tape 4 students together with
masking tape. (Wrap around
several times)
• What state do they represent?
• Are they able to move without
breaking the tape?
Demo - continued
• When the students move more
and the tape breaks, what
state do they represent now?
• What do we call it when this
change occurs in a solid?
Changes of State
• Remember that the 3 most
common states of matter are
solid, liquid and gas.
• A change of state occurs when a
substance changes from one
physical state to another.
Changes of State
• We will be looking
at changes between
the 3 common
states of matter.
• There are
actually six
changes of state that can happen as a
substance switches from one physical form to
another.
Realplayer\18.3\Changes of states of matter - Bing Videos.flv
Solids and Liquids
• Melting is the process of
changing any solid into its liquid
form.
• In order for this to happen,
energy must be added to the
solid (an endothermic process).
• Why must energy be added?
• To answer this question, we
must consider the solid at the
atomic/ molecular level.
Solids and Liquids
• For a solid substance to melt, it
must gain sufficient energy to overcome
intermolecular attraction.
• As a solid (such as ice) absorbs energy, the
individual molecules of the solid vibrate faster
and faster, weakening the forces that hold the
molecules in their fixed positions.
Solids and Liquids
• Eventually the particles will
be vibrating so much that
they are able to break free of
the attractions to
surrounding particles, so
they are no longer held
rigidly in place.
Solids and Liquids
• Freezing (solidification) is the
process of changing any liquid
into its solid form.
• In order for this to happen,
energy must be removed
from the particles in the
liquid phase (an exothermic
process).
• Why must energy be
removed?
Solids and Liquids
• To form a solid, the particles
must slow down enough that
intermolecular attractions can
hold them firmly in place
relative to the neighboring
molecules. This requires
the liquid to be placed in a
cooler environment (so it can
lose heat to its surroundings).
Solids and Liquids
• Most solids, besides having a more orderly
structure than their liquid form, have a slightly
higher density than their liquid forms.
• Water is unusual in this respect because as the
solid forms, its crystalline structure actually has
more space in it
than the liquid form
does.
Solids and Liquids
• The temperature at which a substance changes
from the liquid to the solid phase is known as
the freezing point.
• If the change is in the other directions, solid to
liquid, then it is called the melting point.
• These two temperatures are the same for any
particular substance (the temp. at which it
melts is the same temp. at which it freezes).
Realplayer\18.3\Melting and freezing - Bing Videos.flv
Liquids and Gases
• Vaporization is the general
term for any liquid changing
to the gaseous state.
• Evaporation is the term used
for the process of a liquid
changing into a gas at the
surface of a liquid.
• Evaporation is what cools
your skin when you sweat.
sciencecity.oupchina.com.hk
Liquids and Gases
• Recall that temperature
is a measure of the
motion of the particles
in a substance. It is
actually a measure of
the average motion,
since not all particle will
be moving with the
same speed.
images.usatoday.com
Liquids and Gases
• Some of the particles are moving fast
enough to break free of the attractive
forces at the surface of the liquid – this is
evaporation.
www.school-for-champions.com
Interesting Fact
• Most people wear an antiperspirant
to avoid sweating. These contain
compounds that clog the pores of the skin, so
sweat cannot be excreted by the body.
• However, sweat is the body’s air conditioner.
So when you wear antiperspirant, you cause
your body to cool less efficiently.
Liquids and Gases
• Boiling is the term used for the process of a
liquid changing into a gas that occurs
throughout the liquid.
• Bubbles of the gaseous
liquid form anywhere
within the liquid that the
particles have enough
energy to form a gas. This is why
you see bubbles forming and rising from the
bottom of a pan being heated on a stove.
Liquids and Gases
• Boiling occurs when the pressure inside the
bubbles that form (called vapor pressure)
equals the pressure outside the bubbles (which
is atmospheric pressure).
• The boiling point is the
temperature at which
boiling will occur for a
particular liquid.
Realplayer\18.3\Boiling and
evaporation - Bing Videos.flv
Interesting Fact
• Atmospheric pressure changes
depending upon the altitude of a particular
location. Because boiling depends upon
atmospheric pressure, being at a lower or
higher altitude will affect the temperature at
which a liquid boils.
• This is why some recipes have extra instructions
for “high altitudes.” When boiling something in
water, the water boils at a lower temperature,
requiring a longer time to cook the food!
Liquids and Gases
• Condensation is the process of changing a
gas back into the liquid phase.
• The condensation point is the temperature
at which a gas will become a liquid. It is
the same as the boiling point for the liquid.
Realplayer\18.3\condensation time lapse.flv
Liquids and Gases
• Condensation is an exothermic process because
it requires the particles to lose energy to their
surroundings so they slow down enough that
they can become attracted to other particles
and begin clumping together,
forming a liquid.
Liquids and Gases
• Dew and fog form from condensation
when the atmosphere holds more water
than it should at a particular temperature.
Liquids and Gases
• Fog is just tiny water droplets suspended in
air; it is like a cloud, just at a lower altitude.
www.sci.uidaho.edu
Solids and Gases
• Sublimation is the process in which a solid
turns directly into a gas, without going
through a liquid phase (like dry-ice).
Solids and Gases
• Dry ice is just another name for frozen
carbon dioxide. It is what they frequently
use on stage if there is need of a hazy
effect.
Solids and Gases
• For sublimation to occur, the particles must
gain enough energy to change from a solid
to a gas (an endothermic process).
Solids and Gases
Realplayer\18.3\Sublimation - Bing Videos.flv
Realplayer\18.3\Sublimation of Iodine.flv
Solids and Gases
• Deposition (or desublimation) is the
process in which a gas turns directly into a
solid, without going through a liquid phase.
• Some books and videos call any direct
change between solid and gas sublimation.
This can be confusing and should be
avoided!
Solids and Gases
• This heavy frost formed on all exposed
surfaces due to deposition of water out of
the air onto cooler surfaces.
Solids and Gases
• Frost that forms on windshields and other
surfaces overnight during the cooler
months is also an example of deposition.
Application – Changes of State
• The water
cycle is an
excellent
everyday
example
involving
changes of
state.
cd7.e2bn.net
Interesting Fact
• When a sample undergoes a phase change, it
doesn’t change temperature until the phase
change is complete, even though heat is being
added to or taken away from the sample.
• Where does the energy go?
• When changing from solidliquidgas, the
energy is used up breaking the attractions
between particles.
Interesting Fact
• Example: If you stir ice water so that the
liquid and ice stay well mixed, a
thermometer in the mixture will read 32OF
(0OC) until either the ice is all melted or the
water is all frozen.
Phase Change Diagram
• A phase change diagram is a plot of the
change in temperature over time of a
substance as heat is added at a constant
rate.
• The flat areas
correspond to
phase changes
occurring.
Phase Change Diagram
• The diagonal lines in the diagram indicate
times when the substance is all in one state
and is warming up (temperature is
increasing) as heat is added.
Realplayer\18.3\C
hange in State.flv
Phase Diagram
• A phase diagram indicates what phase a
substance would be in at a particular
combination of temperature and pressure.
• A phase diagram for
water is shown to
the right.
Realplayer\18.3\Phase
Diagrams and the
States of Matter.flv
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http://www.valdosta.edu/~dnshaner/015.JPG
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S0xmPA&01RI=7AB4A5FA1863BBE&01NA=
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