Sensible Heat and Latent Heat -

A temperature change can be measured or felt. Heat transfer that causes a temperature change is called sensible heat transfer. Heat transfer that does not cause a temperature change is called latent heat transfer. During latent heat transfer, there are changes in molecular energy, but these changes do not result in a temperature change. Instead, they result in a phase change. The three phases, or states, of matter are solids, liquids, and gases. A heated beaker of ice can be used to illustrate phase changes and latent heat transfer. When the heat is applied, the ice begins to melt. Initially, the temperature stays at 32 degrees Fahrenheit, because latent heat is being absorbed by the ice. The latent heat causes the molecules of ice to overcome the molecular forces holding them together. As a result, the molecules move freely as a liquid. After the ice has melted, the addition of heat does produce a temperature change. At 212 degrees, the water boils into steam, but the temperature stays the same. The water undergoes a phase change, but not a temperature change. At 212 degrees, the latent heat causes the molecules to move fast enough to overcome the molecular forces holding them together. When the molecules absorb enough latent heat, they leave the water as steam. A graph is used in the video presentation to illustrate the relationship between temperature and heat when heat is applied to ice. One section of the graph represents sensible heat transfer. From the beginning to the end of this section, the temperature of the water changes as a result of heat transfer. Other sections of the graph represent latent heat transfer, because there are no temperature changes in these sections, only phase changes. A horizontal line at 32 degrees represents the latent heat of fusion, or melting. The latent heat of fusion is the amount of heat required to melt a substance without a change in temperature or pressure. The beginning of this line represents ice at 32 degrees, and the end of the line represents water at 32 degrees. Between these two points, the line represents an ice/ water mixture. A horizontal line at 212 degrees represents the latent heat of vaporization. The latent heat of vaporization is the amount of heat required to vaporize a substance without a change in temperature or pressure. The beginning of this line represents water at 212 degrees, and the end of the line represents steam at 212 degrees. Between these two points, the line represents a water/steam mixture. In the ice example, heat was added. If heat is removed, the heat transfer line on the graph moves in the opposite direction. When heat is removed from steam, the steam starts to condense into water. When heat is removed, the horizontal line at 212 degrees represents the latent heat of condensation. The latent heat of condensation is the amount of heat that has to be removed from a substance to condense it without producing a change in temperature or pressure. When heat is removed from water, there is a sensible heat transfer, and the water temperature drops until it reaches 32 degrees. At 32 degrees, any heat removed is latent heat, so the temperature remains at 32 degrees. A horizontal line at 32 degrees represents the latent heat of freezing. The latent heat of freezing is the amount of heat that has to be removed from a substance to freeze it without producing a change in temperature or pressure.