Final answer:
The formula for latent heat does not change when cooling a substance down compared to heating it up. The energy change is the same but with an opposite sign, involving the release of energy during freezing or condensation and the absorption of energy during melting or vaporization. The equations Q = mLf for freezing and Q = mLy for condensation can be used to calculate the heat released.
Step-by-step explanation:
The formula for latent heat of vaporization or fusion does not change whether we are heating or cooling a substance. The latent heat refers to the amount of energy per unit mass required for a phase change without a temperature change. This energy is represented by the heat of fusion (δLf) when a solid changes to a liquid, or the heat of vaporization (δLy) when a liquid changes to a gas, and the process is reversible.
Therefore, whether a substance is melting or freezing, or vaporizing or condensing, the amount of energy involved is the same but with opposite sign. The energy is released in the process of freezing (solidifying) or condensing and is absorbed during melting or vaporization.
Quantitatively, the energy, Q, involved in these phase changes is given by the equation Q = mL, where m is the mass of the substance and L is the latent heat (either Lf or Ly). For cooling down, when a gas turns into a liquid (condensation) or a liquid turns into a solid (freezing), the heat released can be calculated using Q = mLy for condensation and Q = mLf for freezing.
In the context of cooling curves, the heat released can be represented in a similar way as the heat absorbed is represented in heating curves. It is important to note that the processes of fusion, vaporization, and sublimation are exothermic when occurring in reverse, meaning they release heat, which can be calculated with the same respective latent heat values used for the endothermic processes.