A phase diagram is a scientific chart that maps the stable physical states, or phases, of a substance under varying conditions. It illustrates the specific combinations of temperature and pressure at which a pure substance exists as a solid, liquid, or gas in thermodynamic equilibrium. This visual guide allows a reader to determine the phase of matter at any given condition. Interpreting the boundaries that represent phase changes, such as boiling, provides a deeper understanding of a substance’s physical properties.
Interpreting the Pressure-Temperature Axes
The layout of a typical phase diagram for a single component substance is defined by two axes. The horizontal axis (x-axis) represents temperature, increasing from left to right. The vertical axis (y-axis) represents pressure, increasing as you move upward. These two variables are the primary determinants of a substance’s physical state.
The diagram is divided into three distinct areas, each corresponding to a stable state of matter. The region on the left, at lower temperatures, represents the solid phase. The central area represents the liquid phase, found at intermediate pressures and temperatures. The region on the right and bottom, corresponding to lower pressures and higher temperatures, indicates the gaseous phase.
The vast majority of points within these three areas correspond to conditions where only a single phase is stable. Moving from one region to another signifies a phase transition, such as melting or vaporization. The boundaries separating these areas are where two phases can coexist in equilibrium.
Identifying the Vaporization Curve
The boiling point is not a fixed point on the diagram but a range of temperatures that depend directly on external pressure. It is represented by the curved line separating the liquid region from the gas (or vapor) region. This boundary is known as the vaporization curve, and every point along it shows a temperature and pressure combination where the liquid and gas phases are in dynamic equilibrium.
Boiling occurs when the vapor pressure of the liquid equals the surrounding atmospheric pressure. Since vapor pressure increases with temperature, the temperature required for boiling also increases as the external pressure is raised. This relationship explains why the vaporization curve slopes upward and to the right.
To find the normal boiling point for a substance, locate the point on the vaporization curve that corresponds to a pressure of exactly one atmosphere (atm). The temperature value associated with this specific intersection point on the x-axis is the standard reference for the substance’s boiling temperature.
Related Phase Change Lines
In addition to the vaporization curve, a complete phase diagram contains two other lines that represent different phase transitions. The first is the fusion curve, which is the boundary separating the solid and liquid regions. This line indicates the melting or freezing point of the substance at different pressures, where the solid and liquid phases coexist in equilibrium.
For most substances, the fusion curve slopes slightly upward and to the right, meaning that higher pressure slightly increases the melting temperature. The third boundary is the sublimation curve, which separates the solid region directly from the gas region. Points on this line represent conditions where a solid can transition directly into a gas, or vice versa, without passing through the liquid state.
These three lines—the vaporization, fusion, and sublimation curves—are the only locations on the diagram where two distinct phases can exist simultaneously.
The Triple and Critical Points
Two unique points define the limits of the phase diagram and possess distinct physical significance. The first is the triple point, the precise location where the three phase transition lines intersect. This single combination of pressure and temperature is the only condition under which the solid, liquid, and gas phases of the substance can all coexist in stable equilibrium.
The second defining feature is the critical point, which marks the endpoint of the vaporization curve. This point is characterized by the critical temperature and critical pressure. Beyond this point, the distinction between the liquid and gas phases disappears entirely, and the substance exists as a single, homogeneous supercritical fluid. Above the critical temperature, it is impossible to condense the gas into a liquid simply by increasing the pressure.