Phase changes are reversible transitions between the three states of matter: solids, liquids, and gases. These transitions are represented in a phase diagram, which includes phase change curves that separate the different states. In a typical phase diagram, the blue line indicates the boundary between the solid and liquid phases, known as the melting curve or fusion curve. This curve represents the process of melting, where a solid transforms into a liquid.

The green line in the diagram separates the solid phase from the gaseous phase, representing sublimation, the process where a solid changes directly into a gas. Lastly, the red line indicates the boundary between the liquid and gas phases, known as the vaporization curve, which corresponds to the process of vaporization, where a liquid turns into a gas.

Understanding these phase changes also involves the concept of normal pressure, defined as 1 atmosphere (760 torr or 760 millimeters of mercury). At this standard pressure, we can identify the normal melting point and normal boiling point of a substance. The normal melting point is the temperature at which a solid transitions to a liquid under normal pressure, while the normal boiling point is the temperature at which a liquid transitions to a gas under the same conditions.

In the phase diagram, tracing the blue line down to the temperature axis reveals the normal melting point, which, for example, might be at 50 degrees Celsius. Similarly, tracing the red line down indicates the normal boiling point, which could be at 80 degrees Celsius. By understanding these curves and points, one can determine the phase transitions of any given substance at normal pressure.