THE Time (minutes) Temp o C 0oC0oC -50 o C 100 o C 150 o C The graph below shows what happens when heat energy (Joules) is added to a block of ice at.

THE

Time (minutes)

Tem

p o C

0oC

-50oC

100oC

150oC

The graph below shows what happens when heat energy (Joules) isadded to a block of ice at -50oC. Keep in mind that because energy is addedat a constant rate (J/min) more and more energy is being pumped into thesystem as time goes by.

You should also realize that although water freezes at 0oC, it doesn’t haveto stay at that temperature. Ice can be chilled to any temperature below thefreezing point. Our ice will begin at -50oC.

Time (minutes)

Tem

p o C

0oC

-50oC

100oC

150oC

As we add heat energy the first thing that happens is that the ice gets warmerand warmer. Not very surprising.Look carefully at what just happened.

A

B

We started at A with solid ice at -50oC and we got to B where we still havesolid ice which is now at 0oC.From point A to point B the only thing that happened was that ice got warmer.

Time (minutes)

Tem

p o C

0oC

-50oC

100oC

150oC

A

B

Now something very strange is about to happen!

C

We are still adding heat energy at the same rate as before yet the temperaturehas stopped increasing!

How could that be? What is the energy doing if it’s not warming the ice?

Time (minutes)

Tem

p o C

0oC

-50oC

100oC

150oC

A

B C

According to “Properties of Water” (ESRTpage 1) it takes the addition of 334 joulesof heat energy per gram to melt ice.All of the energy added from B to C is goingto melt the ice. The temperature will notincrease again until all the ice has melted!

Whenever the slope of the line is flat it means a phase change is occurring!

Time (minutes)

Tem

p o C

0oC

-50oC

100oC

150oC

A

B C

After all the ice has melted and we have nothing but liquid water at 0oCthe temperature will start to increase once again.But something is different! The slope of the line has changed!

D

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 waswarming faster from A to B than the water was from C to D. But why?

Time (minutes)

Tem

p o C

0oC

-50oC

100oC

150oC

A

B C

D

It all has to do with Specific Heat, the energy neededto change the temperature of 1g of a substance by1oC. (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•oCSince the S.H. of water is so much higher, moreenergy is needed to warm the water than to warmthe ice.

Time (minutes)

Tem

p o C

0oC

-50oC

100oC

150oC

A

B C

D

Since we are always adding the same amount ofheat energy (Joules) per minute, it’s going to takemuch 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 itheats up VERY SLOWLY. More slowly thanalmost any other common substance!

Time (minutes)

Tem

p o C

0oC

-50oC

100oC

150oC

A

B C

D

More strangeness! Once again the line goes flat. We are still adding energy but there isno temperature increase. Can you guess why?

E

RIGHT! It’s another phase change. This time the change is from liquid waterto water vapor, a gas.

This process is called vaporization or evaporation. Either term isacceptable.

Time (minutes)

Tem

p o C

0oC

-50oC

100oC

150oC

A

B C

D E

How much energy does it take toevaporate water?Once again look at the “Properties ofWater” from ESRT page 1.It takes a whopping 2260 Joules of heat energy for each gram of wateryou want to evaporate!

Time (minutes)

Tem

p o C

0oC

-50oC

100oC

150oC

A

B C

D E

Finally, after all the water has evaporatedand turned into vapor, the temperature willonce again begin to rise.

F

Can you guess why the slope of the lineis as steep as it was when ice was warming?Right! The specific heat of water vapor issmall so it heats rapidly.

Time (minutes)

Tem

p o C

0oC

-50oC

100oC

150oC

A

B C

D E

F

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 isbeing added.

Energy Being Added

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.

Where is that energy?

Time (minutes)

Tem

p o C

0oC

-50oC

100oC

150oC

A

B C

D E

F

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 weput 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.

Energy Being Released

Time (minutes)

Tem

p o C

0oC

-50oC

100oC

150oC

A

B C

D 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.

F

That’s letter E.4) How about liquid water at 0oc. Definitely letter C.

OK, now let’s try something else.

Time (minutes)

Tem

p o C

0oC

-50oC

100oC

150oC

A

B C

D E

F

You provide the name of the process that occurs between the two lettersgiven. 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).

Time (minutes)

Tem

p o C

0oC

-50oC

100oC

150oC

A

B C

D E

F

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.