Final answer:
The transition temperature is the temperature at which two phases of a substance are in equilibrium, such as the boiling point where liquid and gas are in equilibrium.
Step-by-step explanation:
The transition temperature is the temperature at which two phases are in equilibrium. This concept is especially important in understanding the nature of phase changes, such as during the boiling or melting of substances. For instance, the boiling point of a liquid is the temperature at which its liquid and gas phases are in equilibrium. This means that at the boiling temperature, the rate of vaporization (liquid turning into gas) and the rate of condensation (gas turning into liquid) are the same. The phase diagram illustrates the temperatures and pressures at which different phases are in equilibrium, and at a specific combination of temperature and pressure known as the triple point, all three phases can coexist in equilibrium.