THE
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 doesn’t have
to stay at that temperature. Ice can be chilled to any temperature below the
freezing point. Our ice will begin at -50oC.
150oC
Temp oC
100oC
0oC
-50oC
Time (minutes)
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
Temp oC
100oC
B
0oC
-50oC
A
Time (minutes)
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 it’s not warming the ice?
150oC
Temp oC
100oC
B
0oC
C
-50oC
A
Time (minutes)
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!
Temp oC
100oC
B
0oC
C
-50oC
A
Time (minutes)
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
Temp oC
100oC
B
0oC
C
-50oC
A
Time (minutes)
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/g•oC.
But the S.H. of water is 4.18 J/g•oC
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
Temp oC
100oC
B
0oC
C
-50oC
A
Time (minutes)
Since we are always adding the same amount of
heat energy (Joules) per minute, it’s 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
Temp oC
100oC
B
0oC
C
-50oC
A
Time (minutes)
More strangeness!
Once again the line goes flat. We are still adding energy but there is
no temperature increase. Can you guess why?
RIGHT! It’s 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
D
Temp oC
100oC
B
0oC
C
-50oC
A
Time (minutes)
E
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
D
Temp oC
100oC
B
0oC
C
-50oC
A
Time (minutes)
E
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
Temp oC
100oC
B
0oC
C
-50oC
A
Time (minutes)
E
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. It’s stored there as potential energy.
The H2O has the least potential energy at A and the most at F.
F
150oC
D
Temp oC
100oC
B
0oC
C
-50oC
A
Energy Being Added
Time (minutes)
E
150oC
But if the energy is stored doesn’t 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
D
Temp oC
100oC
B
0oC
C
-50oC
A
Energy Being Released
Time (minutes)
E
Try this. For each description, you give the letter where it’s found.
1) Liquid water at 100oC. That’s letter D.
2) Solid ice at 0oC. Did you say letter B?
3) Water vapor at 100oC. That’s letter E.
4) How about liquid water at 0oc. Definitely letter C.
OK, now let’s try something else.
F
150oC
D
Temp oC
100oC
B
0oC
C
-50oC
A
Time (minutes)
E
You provide the name of the process that occurs between the two letters
given. Pay attention to the order of the letters!
What’s 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.
What’s happening between C and B? Water is freezing!
And lastly, between D and E. Liquid water is evaporating (vaporizing).
F
150oC
D
Temp oC
100oC
B
0oC
C
-50oC
A
Time (minutes)
E
A few more questions that get asked over and over again.
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
Temp oC
100oC
B
0oC
C
-50oC
A
Time (minutes)
E