During a phase change, such as when ice melts to become water or water boils to become steam, the temperature of the substance does not change even though heat energy is being added or removed. This phenomenon can be explained by understanding how energy is used during the phase transition process.
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Energy and Molecular Structure: When a substance changes from one phase to another (e.g., solid to liquid or liquid to gas), the energy applied (in the form of heat) does not go into increasing the temperature of the material. Instead, this energy is used to overcome the intermolecular forces that hold the molecules together in a specific phase.
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Latent Heat: The energy required for a phase change at a constant temperature is known as latent heat. For example, when ice melts, the heat energy provided is used to break the hydrogen bonds between water molecules in the solid state, transforming it into liquid water. Similarly, when water boils, energy is used to overcome the attractions between water molecules so that they can escape into the gas phase.
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Thermal Equilibrium: During the phase change, all the added heat energy is directed towards changing the phase, rather than increasing the kinetic energy of the molecules, which would raise the temperature. The system remains in a state of thermal equilibrium, where the heat energy added balances the energy needed to break the intermolecular forces.
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Constant Temperature: Because the temperature reflects the average kinetic energy of the molecules, and during a phase transition that energy is not increasing (it is being used for structural changes rather than kinetic changes), the temperature remains constant throughout the phase change.
In summary, during a phase change, heat energy is absorbed or released, but it is utilized for changing the molecular structure rather than increasing temperature; therefore, the temperature of a substance remains unchanged until the phase change is complete.