Final answer:
Phase transitions such as melting and evaporation are endothermic, requiring heat input, while transitions like freezing and condensation are exothermic, releasing heat. The energy involved is measured as heat of fusion or vaporization.
Step-by-step explanation:
Energy Changes in Phase Transitions
Some phase transitions require an input of heat, such as melting and evaporation. Other phase transitions require heat to be released, such as freezing and condensation. During a phase change, the temperature of the substance does not change; instead, the energy added or released is used to modify the intermolecular forces between particles.
In endothermic processes like melting (solid to liquid) and evaporation (liquid to gas), heat is absorbed by the substance. This is necessary to overcome the attractive forces between molecules and allow a transition to a less ordered state. In contrast, exothermic processes like freezing (liquid to solid) and condensation (gas to liquid) release energy as the substance transitions to a more ordered state, reforming intermolecular bonds.
The amount of energy involved in these transitions is quantified by the heat of fusion (for melting and freezing) and the heat of vaporization (for evaporation and condensation). These energies are expressed in terms of the amount of heat per unit mass necessary for the phase change to occur.