# What happenes to the temperature during a phase change?

• Freezing (heat of fusion, $\Delta {H}_{\text{fus}}$) and condensation are exothermic (heat of vaporization, $\Delta {H}_{\text{vap}}$), since they involve slowing down molecular motion, and thus energy is released and $\Delta H < 0$.
• Melting (heat of fusion, $\Delta {H}_{\text{fus}}$) and vaporization (heat of vaporization, $\Delta {H}_{\text{vap}}$) are endothermic, since they involve speeding up molecular motion, and thus energy is absorbed and $\Delta H > 0$.