Coexistence Curve
Definition and meaning of Coexistence Curve in chemistry.
A coexistence curve is the line on a phase diagram that marks the combinations of temperature, pressure, or composition at which two phases of a substance remain in equilibrium with each other.
In more detail
Along the curve, the chemical potential of each component is equal in both phases, so any relative amount of the two phases can coexist stably at that fixed temperature and pressure. Moving off the curve favors one phase completely. For a liquid-vapor system, the coexistence curve rises from the triple point and terminates at the critical point, where the densities (and all other properties) of the liquid and vapor become identical and the two-phase distinction vanishes. Near the critical point, the curve's shape follows a universal power law governed by a critical exponent, a key result in the theory of critical phenomena.
Key facts
| Field | Physical Chemistry |
|---|---|
| Also called | binodal curve, saturation curve |
| Terminates at | critical point |
| Equilibrium condition | chemical potential equal in both phases |
On the pressure-temperature diagram for water, the liquid-vapor coexistence curve runs from the triple point (0.01 degrees C, 0.006 atm) to the critical point (374 degrees C, 218 atm), passing through 100 degrees C at 1 atm, the normal boiling point.
Frequently asked questions
Is a coexistence curve the same as a phase boundary?
Yes, in a two-phase region the coexistence curve is the phase boundary; the term is used especially for liquid-vapor and liquid-liquid equilibria and in the study of critical phenomena.
What happens to the coexistence curve at the critical point?
The curve ends there because the two coexisting phases become indistinguishable in density and other properties, leaving a single supercritical fluid phase beyond that point.