According to the Newton "Ask a Scientist" page, exothermic-phase changes are those that give off energy or heat, such as liquid water freezing into ice or water vapor condensing to liquid water. Scientific American explains that water freezing into ice is recognized as an exothermic process; it is the opposite process to melting ice, which requires energy to occur and is therefore endothermic.Continue Reading
Chemistry at About.com explains the endothermic and exothermic processes. In order to determine if a phase change is endothermic or exothermic, consider the energy levels of the atoms in both the initial and final states. For an exothermic reaction, the atoms in the initial state have more energy, which is typically indicated by greater kinetic energy. For instance, water moves more than ice because the molecules in water have more kinetic energy. Likewise, water molecules in water vapor move even more than those in liquid water. In order to condense water vapor to water, the molecules must lose energy.
Chemistry at About.com states that most exothermic reactions, unlike phase changes, result in higher entropy in the products. An example of an exothermic reaction that is not a phase change is combustion. After wood burns, the result is much less ordered than the reactants that went into the reaction, according to Scientific American. However, ice is more ordered than liquid water, so the best way to determine if a reaction is exothermic is to determine if energy or heat is released during the reaction or phase change.Learn more about Chemistry