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A substance goes through phase changes when transitioning between different states of matter: solid, liquid, and gas. Fusion (melting) occurs when a substance transitions from a solid to a liquid state, requiring energy to overcome the attractive forces between the particles. Vaporization (boiling) happens when a substance transforms from a liquid to a gas state, requiring even more energy to overcome the stronger attractive forces between particles in the liquid state. During phase changes, the temperature remains constant. However, the energy added is used to overcome the attractive forces between particles, allowing them to move more freely. The amount of energy required to change the phase of a unit mass of a substance is known as its latent heat.

The heat of fusion (or latent heat of melting) is the energy required to change a substance from a solid to a liquid at its melting point. In this phase change, the substance absorbs energy, while the attractive forces between its particles loosen up. A certain amount of energy is needed to overcome these forces, which are weaker in the solid state, allowing the particles to move past one another in the liquid state.

The heat of vaporization (or latent heat of boiling) is the energy required to change a substance from a liquid to a gas state at its boiling point. In this phase change, particles must separate even further than in the transition from solid to liquid. The attractive forces between the particles are stronger in the liquid state, requiring more energy to overcome them, allowing the particles to move independently as a gas.

Comparing the energy requirements for the heat of fusion and heat of vaporization, it becomes evident that the heat of vaporization is generally higher than the heat of fusion. This is because the stronger attractive forces between particles in the liquid state need to be overcome during vaporization. In contrast, the forces between particles in the solid state are weaker, requiring less energy to overcome in the fusion process. In conclusion, any substance's heat of fusion is generally lower than its heat of vaporization because the energy required to overcome the attractive forces between particles in the solid state is less than the energy required to overcome the stronger attractive forces in the liquid state.