For Physics
HG-1
THE HEAT OF FUSION OF ICE
Introduction When heat flows into a system there are a number of things that can happen. One result could be a temperature rise. Or, the system might even catch fire. A third possibility is a change of state. As you know, matter exists in three states (or maybe four, the fourth being a plasma), solid, liquid and gaseous. At different temperatures, the same substance may be in different states. Each state is characterized by the way the interatomic forces act. In solids, the atoms (or molecules) are held firmly together in a closely packed crystal structure. In liquids, the crystal has fallen apart rendering the substance shapeless. Nevertheless a liquidhas certain coherence, being held together in a blob by surface tension forces. In the gaseous state, the atoms (or molecules) are far apart and act individually. There is no coherence. To disintegrate a crystal structure, work must be done against atomic forces in order to pull the crystal apart. As this goes on, the solid crystalline substance is said to melt. One way to supply the energy necessary to do the work of melting is to heat the substance. When heat is used to melt the substance, it does work against the crystalline forces rather than becoming kinetic energy. Therefore the temperature (which is a measure of atomic kinetic energy) does not change while melting goes on, but remains constant at a value called the melting point. If the temperature of a substance is below the melting point, then heating it simply raises its temperature until the melting point is reached. (Formula: Q = crnA T). However, once the melting point is reached, further heating leaves the temperature constant and starts breaking down the crystal structure. This continues until all the substancCis melted. If still more heat is added, the temperature once again starts to ~ i s e . To melt a given amount of different substances requires differing amounts of heat because of differences of crystal structure.
THE HEAT OF FUSION OF ICE
Introduction When heat flows into a system there are a number of things that can happen. One result could be a temperature rise. Or, the system might even catch fire. A third possibility is a change of state. As you know, matter exists in three states (or maybe four, the fourth being a plasma), solid, liquid and gaseous. At different temperatures, the same substance may be in different states. Each state is characterized by the way the interatomic forces act. In solids, the atoms (or molecules) are held firmly together in a closely packed crystal structure. In liquids, the crystal has fallen apart rendering the substance shapeless. Nevertheless a liquidhas certain coherence, being held together in a blob by surface tension forces. In the gaseous state, the atoms (or molecules) are far apart and act individually. There is no coherence. To disintegrate a crystal structure, work must be done against atomic forces in order to pull the crystal apart. As this goes on, the solid crystalline substance is said to melt. One way to supply the energy necessary to do the work of melting is to heat the substance. When heat is used to melt the substance, it does work against the crystalline forces rather than becoming kinetic energy. Therefore the temperature (which is a measure of atomic kinetic energy) does not change while melting goes on, but remains constant at a value called the melting point. If the temperature of a substance is below the melting point, then heating it simply raises its temperature until the melting point is reached. (Formula: Q = crnA T). However, once the melting point is reached, further heating leaves the temperature constant and starts breaking down the crystal structure. This continues until all the substancCis melted. If still more heat is added, the temperature once again starts to ~ i s e . To melt a given amount of different substances requires differing amounts of heat because of differences of crystal structure.