A phase diagram is a graphical tool used across chemistry, physics, and materials science to map the physical state of a substance under varying external conditions. It visually summarizes the relationship between pressure, temperature, and the resulting phase (state of matter) for a specific material. For a simple, single-component substance, the standard diagram plots pressure on the vertical axis and temperature on the horizontal axis. This representation allows scientists and engineers to predict the physical behavior of a material at any given combination of these two variables. The diagram is divided into distinct sections, each revealing information about a substance’s stability and transformation points.
Mapping Stable States of Matter
The expansive, labeled areas within the boundaries of a phase diagram represent the conditions where a single, distinct phase is thermodynamically stable. These regions typically correspond to the solid, liquid, or gas phases of the substance. To interpret the diagram, one locates the point where a specific pressure and temperature intersect on the graph. The area in which that intersection falls identifies the stable physical state of the material under those conditions.
The region at low temperatures and high pressures generally corresponds to the solid state, where molecules are closely packed. Conversely, the region at high temperatures and low pressures indicates the gas phase, characterized by widely separated particles. The liquid phase occupies the space between these two extremes, existing under intermediate conditions of pressure and temperature.
Conditions for Phase Transitions
The curved lines, or boundaries, that separate the stable regions are the most informative features of the phase diagram. These lines are known as coexistence curves, representing the specific combinations of pressure and temperature where two different phases can exist simultaneously in equilibrium. Every point along the line separating the solid and liquid regions is a melting point where the solid and liquid forms coexist. Similarly, the curve separating the liquid and gas regions represents the boiling or vaporization point, showing the conditions for liquid-gas equilibrium.
The slope of these curves reveals how a change in pressure affects the temperature required for a phase transition. For the liquid-gas boundary, an increase in pressure consistently raises the boiling temperature. The solid-liquid boundary typically slopes slightly to the right, showing that higher pressure often increases the melting point. The exception is water, where the line slopes to the left because solid ice is less dense than liquid water. The third boundary, separating the solid and gas phases, is the sublimation curve, which dictates the conditions under which a solid transitions directly into a gas.
Unique Reference Points
Beyond the broad regions and phase boundaries, a phase diagram features specific, unique intersection points that serve as fixed reference markers. The most notable is the Triple Point, which is the single, precise combination of pressure and temperature where the solid, liquid, and gas phases all coexist simultaneously in thermodynamic equilibrium. This point is unique for every pure substance and represents a physical constant sometimes used in calibrating temperature scales.
The other important marker is the Critical Point, which represents the end of the liquid-gas coexistence curve. At this point, a substance reaches its critical temperature and critical pressure, and the distinct physical properties differentiating a liquid from a gas cease to exist. Above the critical point, the material exists as a Supercritical Fluid, a state that has properties of both a gas and a liquid. Increasing the pressure in this state will no longer cause the fluid to condense into a separate liquid phase.