Final answer:
The formula to calculate the required heat during a phase change is Q = mL, where Q is the heat, m is the mass, and L is the latent heat specific to the phase change (fusion or vaporization).
Step-by-step explanation:
To calculate the required heat during a phase change segment of a phase diagram, you use the formula Q = mL. Here, Q is the amount of heat, m is the mass of the substance, and L represents the latent heat, which is specific to the phase change under consideration. For melting (fusion) or freezing, L would be the latent heat of fusion, Lf, and for vaporization or condensation, L would be the latent heat of vaporization, Lv. This equation applies when the substance is undergoing a phase change at constant temperature, where the input energy is being used to break intermolecular bonds rather than increasing the temperature.
For example, if you want to calculate how much energy is needed to melt 1 kilogram of ice at 0 °C, you would use the value for the latent heat of fusion of water and multiply it by the mass of the ice.