supercritical
Summary
A supercritical fluid is a substance above both its critical temperature and critical pressure, where it is no longer distinctly liquid or gas. The testable point: above the critical temperature, compression alone cannot liquefy a gas, because the liquid–gas line on a phase diagram ends at the critical point.
Detail
Meaning: A substance is supercritical when its temperature and pressure both exceed those of its critical point. On a phase diagram the liquid–gas boundary stops at the critical point, so beyond it there is no boundary and no meniscus. At that high temperature, molecular kinetic energy is large enough that intermolecular attractions cannot hold a liquid together, however much the pressure rises. The fluid is dense like a liquid, so it can dissolve solutes, but it diffuses and flows more like a gas.
Equation: supercritical if T > Tc and P > Pc, where T is temperature (K), Tc is the critical temperature (K), P is pressure (Pa), and Pc is the critical pressure (Pa).
On the MCAT: A passage gives a phase diagram and asks you to label the supercritical region, or asks what happens when a gas above Tc is compressed at constant temperature. Answer: density rises smoothly with no phase change. Supercritical CO₂ appears as a nonpolar solvent, for example in decaffeinating coffee, and it is easy to remove by lowering the pressure.
Common confusion: Students call it just a very hot gas. Both conditions are required: a gas above Tc at low pressure is an ordinary gas. Moving from gas to supercritical fluid also involves no sharp phase transition, only gradual change in properties.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
- Chemistry: A Molecular Approach
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.