Final answer:
The phase-transition temperatures of a substance are the points at which it changes from solid to liquid (freezing/melting point), liquid to gas (boiling point), and solid to gas (sublimation point), as seen in phase diagrams and calculated by the formula Q = mL.
Step-by-step explanation:
If the phase-transition temperatures of a substance are the points at which the substance changes its state, these would refer to the specific temperatures where a substance transitions between solid, liquid, and gas. These phase-transition temperatures are notable as the boiling, freezing (or melting), and sublimation points. Each substance has fixed temperatures at which these phase changes occur, assuming constant pressure.
For example, the phase-transition temperatures for water at standard atmospheric pressure are 0°C for freezing/melting and 100°C for boiling. Similarly, substances like carbon dioxide have a phase-transition temperature for sublimation rather than melting at standard pressure. During these phase changes, the heat that is absorbed or released can be calculated with the formula Q = mL, where Q is the heat transferred and L is the latent heat coefficient.
Phase diagrams are very useful in visualizing these transitions, as they graphically represent the physical states of a substance under varying conditions of pressure and temperature. The triple point on a phase diagram indicates the unique temperature and pressure at which all three phases of a substance coexist in thermal equilibrium.