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The heat of fusion of a substance is the energy measured during a 1. phase change 2. temperature change 3. chemical change 4. pressure change
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option 1, phase change.
The phase change is a physical change that involves the passage of a substance from one state to another.
The key stage changes are:
1. Fusion or melting: change from solid to liquid
2. Freezing: the reverse to fusion or melting, ie the change from liquid to solid
3. Boiling: switch from liquid to gas
4. Condensation: the reversal to boiling, ie the transition from gas to liquid
5. Sublimation: the pass from solid to gas, without passing through a liquid state.
6. Deposition: reversal to sublimation, ie the transition from gas to solid, without passing through a liquid.
During all phase changes, the energy supplied to or released by the system is used to modulate the space between the particles that make up the substance, so the temperature of the substance does not change.
Fusion heat, also known as latent fusion heat, is the energy measured during the fusion or melting process, which is the energy absorbed by the substance when it is transformed from a solid state to a liquid, which occurs at constant temperature, and constant pressure.
The absorbed energy is used to overcome the large intermolecular forces that keep the particles tightly packed in the solid state, increasing the energy of the particles and forcing them to separate (occupy more space) in the liquid state. Thus, fusion is an endothermic process.