LSS_Heating and Cooling Curves Lesson

Heating and Cooling Curves

What do you think happens to the temperature of a chemical as it is heated?

This question might sound very simple, but let's look at it in depth

Visualize heating a beaker of ice on a hot plate while measuring the temperature. You begin with solid ice. As you heat it, the temperature rises. But, at a certain point during the heating process the temperature stops increasing. What happened? Did you accidentally turn off the heat source? You check your heat source and see that it is still operating. Why isn't the temperature increasing? Then you notice that although the temperature is no longer increasing, something else is happening. The ice is melting, changing from solid ice to liquid water. Then, unexpectedly, the temperature starts to rise again in the water. You anticipate that it will continue to rise. But, again the temperature stops rising. This time you see that the liquid is boiling.  

Let's think back to our original question. What do you think happens to the temperature of a chemical as it is heated? If your answer was simply that the temperature would increase, you missed something.  

In order to interpret what is happening in this experiment you need to consider the Law of Conservation of Energy.   As the name implies, energy, like matter, cannot be created or destroyed. Most often the Law of Conservation of Energy is discussed in what is called a closed system. This refers to a system that is isolated from its surroundings so that the total energy of that entire system is a constant amount. In this example, this is not a closed system. In a closed system you expect the energy of one component to decrease while the other increases. In this system we are supplying a continual source of energy from the heat source. So, while the heat source is supplying energy, that energy must be accounted for at all times during the experiment. It does not just disappear. You also need to know that temperature is a measure of average kinetic energy. (Energy will be discussed more in depth in a later module.)

So, since the heat source continually supplied energy to the ice/water, what happened to that energy?  

• Most of the time, the energy from the hot plate caused the kinetic energy to increase (as was evident by the increase in temperature).  
• But, during the phase changes the kinetic energy did not increase (as was evident by the stable temperature). In this experiment (an open system) this means that the potential energy must have increased during those times.

That is exactly what happens during phase changes. Instead of increasing the kinetic and potential energy, the heat source increases only the potential energy of the molecules. This causes the molecules to spread apart, increasing the PE and causing the phase to change. It is important to note that the PE of the molecules also increases in between phase changes, but the KE does not (think temperature). This phenomenon is often graphed on what is called a heating curve.

Below is a heating curve for water. Based on what you learned above and what you already know about phase changes and the melting and boiling points for water, drag the labels to the correct spots. You will label each phase and each phase change. Make note of the models shown above the graph to guide you.

As the process is reversed, the graph is called a cooling curve. It looks the same as a heating curve except at the freezing point. As the liquid cools to its freezing point often the liquid continues cooling instead of turning to solid. This occurs because of the disorganization of the liquid particles. While the molecules are arranging themselves into the solid structure, the temperature continues to decline below the freezing point. Finally a crystal forms and the other molecules join. Then the temperature returns to the freezing point. This is known as supercooling and is shown in the cooling curve to the right.

Watch this video that demonstrates the composition of a heating curve. Note that you are not responsible for energy (enthalpy) until a later module. 

Remember to work on the module practice problems as you complete each section of content.  

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