Final answer:
The pressure at which a pure substance changes phase at a given temperature is known as its melting point for solid to liquid and boiling point for liquid to gas.
Step-by-step explanation:
At a given temperature, the pressure at which a pure substance changes phase is referred to as the substance's melting point when changing from solid to liquid, or boiling point when changing from liquid to gas. The conditions under which these phase changes occur are typically represented on a phase diagram, which depicts the state of a substance as a function of its temperature and pressure. These points are indicated on a phase diagram, which shows the varying states of a substance based on pressure and temperature.
As an example, water at standard pressure of 1 atm has a boiling point of 100 °C. Increasing the pressure above 1 atm raises the boiling point, while reducing the pressure lowers it. Similarly, the melting point of ice at 1 atm is 0 °C, but this can also vary with pressure. The crossover from one phase to another under these conditions is characterized by a flat line in a phase diagram indicating equilibrium between the phases. Furthermore, elements like the triple point mark the unique set of conditions where solid, liquid, and gas phases all coexist in balance.