ZingPath: Heating Curves

All matter is composed of atoms or molecules that are in constant motion. The average kinetic energy of the particles in an object is the measure we call temperature. Because of the constant motion, all particles have heat, or thermal energy. When heat is being transferred away from an object, the particles move slower and slower. When a substance gains heat, the particles move faster and faster. An object that gains heat may undergo a phase change. When a substance is in a phase change-melting or boiling-it is gaining heat energy. All of the energy in the substance at that temperature is used to overcome the attractive forces of the particles, and therefore, the temperature remains constant during the phase change. When the attractive forces are overcome, the energy is used to increase the temperature of the particles.

In this Activity Object, students investigate heating curves. During two experiments, students heat 100-gram samples of sulfur and paraffin at three different levels of heat. They observe that when there is a phase change of a substance, such as melting or boiling, the temperature remains constant, and when the substance is not changing phase, it is increasing in temperature. As heating curves are graphed, it is easy to see that the rate of heating a substance does not change the temperatures at which those substances start to melt and boil, but the time required for a phase change is affected. Additionally, every substance has its own boiling point and melting point because they are characteristic properties of matter.