Download Phase Changes - Vincent Sapone

Phase Changes
Surface Tension and Hydrogen Bonding
Phase Change of Water
110
100
Water Boiling
T = Constant
90
80
Temperature (C)
70
Water Heating
Temp = Rising
60
50
Temperature is constant during a phase
change. Normally adding heat energy to
something will increase its temperature as
the molecules move faster. During a phase
change the energy is used not to make the
molecules move faster but to break the
chemical bonds holding them together.
40
30
20
Ice Melting
T = Constant
10
0
-10
0
5
10
15
20
25
Time (min)
30
35
40
45
50
Energy vs Temperature
110
Gas
100
Liquid Boiling
90
80
Temperature(C)
70
Liquid Warming
60
50
Why are steam burns worse than water burns?
40
30
20
10
Solid Melting
0
-10
Solid Warming
In order for liquid water to turn into a gas (steam) you have
to add a lot of extra energy to it. When hot steam hits your
skin all the energy from the steam is released onto your skin
as the steam cools to the same temperature of your skin
(thermal equilibrium). Remember heat always transfers from
hot to cold. All this extra heat is called latent heat.
Turning a liquid into steam takes a LOT of energy and the
opposite is true. Going from steam to a liquid (condensation)
releases all this energy.
Heat Energy Added (J)
Energy vs Temperature
110
100
90
80
Temperature(C)
70
60
Energy Released by water!
50
40
30
Energy Absorbed by water
20
10
0
-10
Heat Energy Added (J)
Draw a Phase Change graph of Heat (yaxis) vs Temperature (x-axis).
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•
•
•
During Condensation energy is_______ by water?
During evaporation energy is ________ by water?
During melting energy is __________ by water?
During freezing energy is __________ by water?
• Why is evaporation called a cooling process?
• What is latent heat of vaporization?
• What is latent heat of fusion?
Why does water have such a high
latent heat? Why is so much
energy released or absorbed?
Water is A Polar
Molecule!
Water has a + side and a – side!
N
S
S
N
Opposites Poles Attract
Electrical Charges are like tiny magnets.
•
•
Opposite Charges Attract ( + and - )
Like Charges Repel ( + and + or - and - )
+ -
Like Poles Repulse
N
N
S
S
7
Water Molecules
are attracted to
one another!
+
O
H
During a phase change the energy
is used not to move the molecules
but first to break the chemical
(hydrogen) bonds holding the
water molecules together!
H
-
O
H
HYDROGEN BONDS
Require energy to break!
H
Water as a Polar Molecule
Phet Water Simulation
Watch Closely.
Perform Polarity mini-Lab
Surface Tension Demo and Paper towels.
9
SURFACE TENSION:
Hydrogen bonding is
stronger at the surface.
A water strider uses the high surface
tension of water to walk on its surface.
• “A molecule within the bulk of
a liquid experiences
attractions to neighboring
molecules in all directions, but
since these average out to
zero, there is no net force on
the molecule. For a molecule
that finds itself at the surface,
the situation is quite different;
it experiences forces only
sideways and downward, and
this is what creates the
stretched-membrane effect.”
Via Chemwiki
Violating Kinetic Theory: Polarity of water
• What happens if you
put a can of soda in the
freezer and leave it
there?
• Shouldn’t it contract
(get smaller) as it cools?
• Why does it expand and
explode?
11
•
Density Decreases as Temperature Increases
•
Density Increases as Temperature Decreases
•
Substances Expand when Heated
•
Substances Contract when Cooled
T↑ V↑ D↓
T↓ V↓ D↑
Did Mr. Sapone Lie?
• Ice is colder than water so
shouldn’t it be more dense?
• Does Ice Sink in water?
ICE CUBE
12
100°C
This is important for life as we know it on earth
and explains why soda explodes in the freezer!
PHET SIM
Water expands when heated
Water contracts when cooled
4°C
Water behaves different but
only in this range! (32F-40F)
0°C
-10°C
Water expands when heated
Water contracts when cooled
Naked Science Video
• Water at 4°C will
EXPAND when
cooled until 0°C
at which point it
becomes ice.
• Likewise, as Ice at
0°C melts it
CONTRACTS as it
turns into a
liquid.
13
Water as a Polar Molecule
Phet Water Simulation
Watch Closely.
Perform Polarity mini-Lab
Surface Tension Demo and Paper towels.
14
Penny Drop Data
Student
# Drops
J. Medina
73?
Student
# Drops Student
# Drops
Student
# Drops
15
Defenitions
•
•
•
•
•
•
•
Cohesion
Adhesion
Surface tension
Capillary action (link)
Hydrogen Bond
Intermolecular forces
Why does a meniscus form in a graduated cylinder?