Mr. Shields
Regents Chemistry
U03 L03
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Phase changes
Consider the following:
Steam (G )  Water (L)  Ice cube (S)
>100 deg C
0-100 deg C <0 deg C
This transition represents a change from one state of
matter to another
A Change of state is also known as a PHASE CHANGE
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Phase Changes
 Consider Ice (s) going through a phase change to
water (l).
 H20 (s)  H20 (l)
 In both solid and liquid phases, the molecules are
H2O. The identity of a substance during a phase
change is not altered.
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Phase Changes
Let’s consider the phase changes in our water example
Steam (G )  Water (L)  Ice cube (S)
>100 deg C
0-100 deg C <0 deg C
What’s happening to the molecules as we make these
transitions?
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Phase Changes
Molecules are like magnets
- they feel a force of attraction between one another
As temperature decreases K.E. decreases
- The force of attraction between particles begins to
play a bigger and bigger role
So what exactly is going on?
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Phase Changes
1. As T decreases Molecules begin to attract one
another more strongly
As they cool down further…
2. The Molecules start arranging themselves in a more
orderly fashion
- they grab on to each other and stay in place
Liquid phase  solid phase
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Phase Changes
3. Motion of the molecule becomes more restricted
- The molecule is trapped in place by it’s neighbor
unorganized
High KE (T)
Little organization
Highly organized
Lower KE
Even Lower KE
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Phase Change & Energy
There are 2 specific energy terms associated
With Phase changes
Which energy term applies depends upon which
way the temperature is changing
1. Temperature (T) is either increasing
2. Temperature (T) is either decreasing
Increasing T represents an Increase in Energy
Decreasing T represents a Decrease in Energy
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Phase Change Diagram
We can show this relationship between Energy, T and
Phase change in a Phase Change Energy diagram
T increases
Energy Absorbed
Gas
Liquid
Endothermic
Exothermic
Energy Released
T decreases
Solid
In an endothermic phase change the particles absorb energy
Does the T of the phase increase or decrease?
In an exothermic phase change the particles release energy
Does the T of the phase increase or decrease?
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Phase Change – Liquid / Gas
Let’s look at the Liquid to Gas phase changes first
When a liquid absorbs energy it’s temperature increase.
Eventually the liquid becomes a gas (steam).
Is this an endothermic or exothermic process?
exothermic
endothermic
What about the reverse process (Gas to Liquid)?
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Phase Change – Liquid / Gas
If enough energy is absorbed a liquid becomes a gas.
This is called VAPORIZATION.
Gas
Vaporization
“endo”
Liquid
Condensation
“exo”
Solid
What happens if the gas is cooled (i.e. it gives up energy)?
The gas phase undergoes a phase change to the liquid phase.
This is called CONDENSATION.
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Phase Change – Solid / Liquid
Let’s next look at Solid to Liquid phase changes
When a solid absorbs energy it’s temperature increase.
The solid becomes a liquid.
Is this an endothermic or exothermic process?
exothermic
endothermic
What about the reverse process (liquid to solid)?
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Phase Change – Solid / Liquid
If enough energy is absorbed a solid becomes a liquid
This is called FUSION (commonly called melting).
Gas
Fusion
“endo”
Liquid
Solidification
“exo”
Solid
What if the liquid is cooled (i.e. gives up energy).
What eventually happens?
The liquid becomes a solid. This is called SOLIDIFICATION,
(commonly called freezing)
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Phase Change – Solid to Gas
Let’s next look at Solid to Gas phase changes
When certain solids absorb energy they go directly from the
Solid to Gas phase, i.e. they skip the Liquid phase. There are
only a few materials that do This.
Iodine (I2) and dry Ice (CO2)
endothermic
This process is known as SUBLIMATION. Is it an ENDO
Or EXO phase change?
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Sublimation of Iodine
Iodine solid
Iodine Gas
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Phase Change – Solid to Gas
Sublimation is a unique Process. Certain criteria must
Be met for it to occur.
So what kind of compounds SUBLIME?
Compounds with Weak intermolecular forces of
attraction between molecules and High Vapor Pressures
endothermic
CO2 or I2
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Phase Change – Solid to Gas
Let’s next look at the opposite of sublimation. In other
Words the process of going from Gas to Solid.
Since this is the reverse process the same criteria of weak
Molecular forces and high VP still apply.
exothermic
This process is known as Deposition. It’s an exo process.
In other words, heat is released.
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Phase Change Quiz
Gas
e
Energy Absorbed
Endothermic
a
Exothermic
c
Energy Released
Liquid
f
d
b
Solid
ENDOTHERMIC
T ?
a.
Sublimation
EXOTHERMIC
T?
d.
Deposition
b. Fusion
e.
Condensation
c.
f.
Solidification
Vaporization
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