Understanding

1
Understanding
THE
Water Phase Change Graph
From RegentsEarth.com
2
The graph below shows what happens when heat
energy (Joules) is added to a block of ice at
-50oC. Keep in mind that because energy is
added at a constant rate (J/min) more and more
energy is being pumped into the system as time
goes by.
You should also realize that although water
freezes at 0oC, it doesnt have to stay at that
temperature. Ice can be chilled to any
temperature below the freezing point. Our ice
will begin at -50oC.
150oC
100oC
Temp oC
0oC
-50oC
Time (minutes)
3
As we add heat energy the first thing that
happens is that the ice gets warmer and warmer.
Not very surprising.
Look carefully at what just happened.
We started at A with solid ice at -50oC and we
got to B where we still have solid ice which is
now at 0oC.
From point A to point B the only thing that
happened was that ice got warmer.
150oC
100oC
Temp oC
B
0oC
-50oC
A
Time (minutes)
4
Now something very strange is about to happen!
We are still adding heat energy at the same rate
as before yet the temperature has stopped
increasing!
How could that be? What is the energy doing if
its not warming the ice?
150oC
100oC
Temp oC
B
C
0oC
-50oC
A
Time (minutes)
5
According to Properties of Water (ESRT page 1)
it takes the addition of 334 joules of heat
energy per gram to melt ice.
All of the energy added from B to C is going to
melt the ice. The temperature will not increase
again until all the ice has melted!
150oC
Whenever the slope of the line is flat it means a
phase change is occurring!
100oC
Temp oC
B
C
0oC
-50oC
A
Time (minutes)
6
After all the ice has melted and we have nothing
but liquid water at 0oC the temperature will
start to increase once again.
But something is different! The slope of the line
has changed!
Why is the slope of the line from A to B so much
steeper than the slope from C to D?
Slope rate! If the slope is steeper from A to B
it means that the ice was warming faster from A
to B than the water was from C to D. But why?
150oC
D
100oC
Temp oC
B
C
0oC
-50oC
A
Time (minutes)
7
It all has to do with Specific Heat, the energy
needed to change the temperature of 1g of a
substance by 1oC. (See ESRT page 1)
The S.H. of ice is only 2.11 J/goC.
But the S.H. of water is 4.18 J/goC
Since the S.H. of water is so much higher,
more energy is needed to warm the water than to
warm the ice.
150oC
D
100oC
Temp oC
B
C
0oC
-50oC
A
Time (minutes)
8
Since we are always adding the same amount
of heat energy (Joules) per minute, its going to
take much longer to heat the water than to heat
the ice. That why the slope of the line changes.
Because water has a high Specific Heat it heats
up VERY SLOWLY. More slowly than almost any other
common substance!
150oC
D
100oC
Temp oC
B
C
0oC
-50oC
A
Time (minutes)
9
More strangeness!
Once again the line goes flat. We are still
adding energy but there is no temperature
increase. Can you guess why?
RIGHT! Its another phase change. This time the
change is from liquid water to water vapor, a gas.
This process is called vaporization or
evaporation. Either term is acceptable.
150oC
E
D
100oC
Temp oC
B
C
0oC
-50oC
A
Time (minutes)
10
How much energy does it take to evaporate water?
Once again look at the Properties of Water from
ESRT page 1.
It takes a whopping 2260 Joules of heat energy
for each gram of water you want to evaporate!
150oC
E
D
100oC
Temp oC
B
C
0oC
-50oC
A
Time (minutes)
11
Finally, after all the water has evaporated and
turned into vapor, the temperature will once
again begin to rise.
Can you guess why the slope of the line is as
steep as it was when ice was warming?
Right! The specific heat of water vapor is small
so it heats rapidly.
F
150oC
D
E
100oC
Temp oC
B
C
0oC
-50oC
A
Time (minutes)
12
Just a few more important things you need to know
As you go UP the line from A to B to C to D to E
and finally to F, energy is being added.
Where is that energy?
The energy is in the H2O. Its stored there as
potential energy.
The H2O has the least potential energy at A and
the most at F.
F
150oC
D
E
100oC
Temp oC
B
C
0oC
-50oC
A
Energy Being Added
Time (minutes)
13
But if the energy is stored doesnt that mean we
can get it back?
Sure! As we go down the line from F towards A the
same energy we put into the system is released
into the environment as the H2O........
Cools from F to E.
Condenses back into a liquid from E to D.
Cools from 100oC back down to 0oC.
Freezes, becoming a solid between C and B.
And finally, chilling from 0oC ice at point B to
-50oC ice at point A.
The energy we put in is the same amount we get
out.
F
150oC
D
E
100oC
Temp oC
B
C
0oC
-50oC
A
Energy Being Released
Time (minutes)
14
Try this. For each description, you give the
letter where its found.
Thats letter D.
1) Liquid water at 100oC.
2) Solid ice at 0oC.
Did you say letter B?
3) Water vapor at 100oC.
Thats letter E.
4) How about liquid water at 0oc.
Definitely letter C.
OK, now lets try something else.
F
150oC
D
E
100oC
Temp oC
B
C
0oC
-50oC
A
Time (minutes)
15
You provide the name of the process that occurs
between the two letters given. Pay attention to
the order of the letters!
Whats going on between letters B and C?
Ice is melting!
How about between E and D?
Water vapor is condensing!
Between A and B?
Ice is warming.
Whats happening between C and B?
Water is freezing!
And lastly, between D and E.
Liquid water is evaporating (vaporizing).
F
150oC
D
E
100oC
Temp oC
B
C
0oC
-50oC
A
Time (minutes)
16
A few more questions that get asked over and over
again.
Bye!
Between which 2 letters is the most energy being
added?
Between D and E
How can you tell where a phase change is
occurring?
The line is flat!
When energy is released, where does it go?
It enters the environment and warms the air
immediately surrounding the H2O.
F
150oC
D
E
100oC
Temp oC
B
C
0oC
-50oC
A
Time (minutes)