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, which is often 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 appears 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 (that is, the mouse 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 unit for which is watts (1 W = 1 J/sec). The heating rate may be set to any value between zero and 1000.0 W.
Successive sets of experimental data are plotted on the graph, which may be reset using the "Reset Graph" button. 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.
To perform the experiment:
Note: When the cursor is over the graph, pressing the left mouse button will display the position of the cursor. This information can be used to determine numerical values from the plot.
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 phase-change process 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.