Learning Objectives Track the heat associated with a phase change. Use the terms endothermic and exothermic to describe the transfer of energy during a phase change. You recall from prior reading that matter can exist in one of several different states, including a gas, liquid, or solid state and that the amount of energy in molecules of matter determines the state of matter . You will also recall that If heat is added to a substance, such as in melting, vaporization, and sublimation, the process is endothermic . In this instance, heat is increasing the speed of the molecules causing them move faster (examples: solid to liquid; liquid to gas; solid to gas). If heat is removed from a substance, such as in freezing and condensation, then the process is exothermic . In this instance, heat is decreasing the speed of the molecules causing them move slower (examples: liquid to solid; gas to liquid). These changes release heat to the surroundings. The amount of heat needed to change a sample from solid to liquid would be the same to reverse from liquid to solid. The only difference is the direction of heat transfer. Phase Change and Heat Transfer For each phase change of a substance, there is a characteristic quantity of heat needed to perform the phase change per gram (or per mole) of material. The heat of fusion (Δ H fus ) is the amount of heat per gram (or per mole) required for a phase change that occurs at the melting point. The heat of vaporization (Δ H vap ) is the amount of heat per gram (or per mole) required for a phase change that occurs at the boiling point. If you know the total number of grams or moles of material, you can use the Δ H fus or the Δ H vap to determine the total heat being transferred for melting or solidification using these expressions: \[\text{heat} = n \times ΔH_{fus} \label{Eq1a} \] You will not be asked to complete calculations of the energies involved in the heats of vaporization or heats of fusion. But the process is very similar to the demonstration calculations that were shown in the previous section. Table \(\PageIndex{1}\) lists the heats of fusion and vaporization for some common substances. Note that the amount of heat required to melt a solid can vary significantly from substance to substance. Also note that the heat of vaporization is always larger than the heat of fusion, which is sensible considering the amount of energy required to add enough kinetic energy to a liquid to separate its particles from each other to become a gas. Substance ΔHfus (cal/g) ΔHvap (cal/g) aluminum (Al) 94.0 2602.0 gold (Au) 15.3 409.0 iron (Fe) 63.2 1504.0 water (H2O) 79.9 540.0 sodium chloride (NaCl) 123.5 691.0 ethanol (C2H5OH) 45.2 200.3 benzene (C6H6) 30.4 94.1 Key Takeaway There is an energy change associated with any phase change.