Open the IR Imaging of Phase Changes Learning Tool. to watch the accelerated IR imaging video of ice which was melted on the hot plate and then heated to boiling. A standard visible light video is played beside the IR imaging video. As you watch the video, play close attention to the color of the ice as it melts and the liquid water as it boils.
Worked Example
In the IR Imaging of Phase Changes Learning Tool., does the color of the ice change as it melts? Does the color of the water change as it boils? What does this indicate about the temperature of the ice and water, respectively, as a phase change occurs?
While the ice melts, its color does not change; it remains a dark blue. Similarly, as the water boils at the end of the video, the water remains a bright red with patches of white. Therefore, we can conclude that the temperature of the ice does not change substantially while it is melting and the temperature of the water does not change substantially while it is vaporizing.
With the IR imaging camera, one can see that the temperature of ice or liquid water remains constant during a phase change. In addition, scientists can show quantitatively that the temperature of a substance remains constant during a phase change by constructing a heating curve. Scientists obtain these curves by adding energy to their sample slowly enough that the phase change remains close to equilibrium with both phases at the same temperature. A heating curve displays the temperature of a substance as energy is added to the sample as heat.
Watch the video in the IR Imaging of Phase Changes Learning Tool. again, but, this time, click “Show Heating Curve” to display a simulated heating curve which is drawn as the video plays.
Your Turn
The phase changes of water, which play an important role in Earth’s energy balance, can be graphically displayed with a heating curve. Refer to the simulated heating curve in the IR Imaging of Phase Changes Learning Tool. to answer the following questions.
- In a heating curve, what do you think the regions with an upwards, positive slope indicate? What do the horizontal portions of the heating curve indicate?
- Based on the heating curve of water, when is the temperature of water constant and when does the temperature of the water change?
- The parts of the graph with a positive slope show that the temperature of the water is increasing. The flat regions of the graph show that the water sample absorbs energy without experiencing a change in temperature.
- The horizontal portions of the heating curve correspond to the phase changes of water; the temperature of the water does not change during melting or boiling, even though heat is being absorbed. However, when the water is not experiencing a phase change, absorption of heat causes the temperature of the ice or water to increase.
The heating curve of water can be constructed by taking precise temperature measurements as ice is heated to boiling under highly controlled conditions, providing clear evidence that temperature does not change during a phase change.