Phase Changes: Heating Curve

Concepts

What happens to a solid substance when it is heated?

In the absence of reactions that change the molecular structure of a compound, two types of behavior are possible when a compound is heated: The compound can simply get hotter (that is, its temperature increases) or a phase change can occur.

The transition from the solid phase to the liquid phase is an example of a phase change. This phase change is called melting. Boiling or vaporization is an example of a phase change from the liquid to the gas phase.

This exercise explores the changes that occur to a substance during heating. At the outset of the experiment, a cylinder equipped with a movable barrier contains 0.200 mole of a pure solid. The barrier is exposed to the atmosphere and thus the material in the cylinder experiences a constant pressure at all times during the experiment.

The walls of the cylinder contain heating elements, the controls for which appear below the graph. When the button labeled "Heat" is pressed, current is passed through the heating elements and heat is released into the cylinder. Heat is only released when this button is selected. As soon as the button is released, heating stops. Thus it is possible to repeatedly stop and restart the heating during an experiment.

The amount of current that passes through the heating elements determines the heating rate. The heating rate is the amount of energy (with units of joules) delivered to the sample each second. The watt, 1 W = 1 J/sec, is a unit of power that applies to the heating rate.

Successive sets of experimental data are plotted on the graph. This feature allows heating curves for different heating rates to be compared.

The phase of the substance may be identified by its color: the solid phase is blue, the liquid phase is red, and the gas phase is yellow.

Experiment

To perform the experiment:

1. Click the Reset Experiment button. This returns the substance to 25.0 °C and allows the heating rate to be changed.
2. Enter the heating rate for the experiment. For the first experiment, try using 200 W.
3. Click on the Heat button to transfer heat to the substance in the cylinder. Heat is only transferred while this button is activated.
4. Carefully observe how the temperature varies with the heating time. The T vs t plot is shown in the graph.
5. Click the Reset Experiment button and repeat the experiment with a heating rate of 400 W.
6. The data from all experiments is cumulative on the graph. The Reset Graph button clears the graph and resets the experiment.

Be aware that this representation of the experiment is highly idealized. In particular it is assumed that the heat is instantaneously distributed in a uniform fashion throughout the system and that during a phase change the system is always at equilibrium. In practice one does not observe abrupt, sharp changes in slope for the temperature vs time plot, and overheating is common.

Questions:

1. Why are there regions where the temperature does not change with time, despite the fact that heat is being added to the system?
2. In most cases, the transfer of heat to an object increases its temperature. In the regions where the temperature does not change as heat is flowing into the substance, what change is the heat producing?
3. What is the melting point of the substance?
4. What is the boiling point of the substance?
5. What is the molar heat of fusion of the substance?
6. What is the molar heat of vaporization of the substance?
7. What are the molar heat capacities for the solid, liquid and gas?
8. How does the heating curve for a 400 W heating rate compare with that obtained using a 200 W heating rate? (Be quantitative in your answer.) Do the melting point and boiling point depend upon the heating rate? Do the molar heat of fusion and the molar heat of vaporization depend upon the heating rate?

Measurements can by made from the graph by positioning the cursor on the desired point and clicking the left mouse button. The time and temperature at that point will be displayed on the graph.