# what is the supercritical state?

is the supercritical state of matter a separate state of matter or is it a combination of a liquid and a gas? could someone give me an overview of this state and its properties. i have seen some videos in which the meniscus of a liquid vanishes and reappears as the liquid turns to its supercritical state and back. i find this hard to comprehend as i am used to seeing the meniscus of a liquid go down the container as it vaporises. could someone explain this spontaneous change in state?

In the supercritical state the difference between liquid and gas vanishes.

The sharp distinction between liquid and gas only exists up to a critical pressure and temperature at which the energy needed to vaporize the liquid vanishes and the densities of the liquid and the gas get equal, above this points no different liquid and gas phases exist.

In other words, it is possible to transform a liquid into a gas without encountering a change of state (but rather some a cross-over). But when you are below the critical temperature and the critical pressure, there is a sharp distinction due to the first order phase transition line. The Van der Waals gas gives a good qualitative explanation for this.

In a way the properties of the supercritical state are a combination of the ones of the liquid and the gas, its viscosity is low (as for a gas) but usually it is a good solvent (supercritical $\text{CO}_2$ is used to extract caffeine from coffee beans in decaffeination).

Around the critical point (when the pressure is the critical pressure and the temperature is the critical temperature) additional phenomena can be observed, such as long range fluctuations leading to clouding in the fluid (as small temperature fluctuations drive the system between liquid and gaseous state, thus causing the light to be scattered), this phenomenon is called critical opalescence. For more on this read more about the critical point.

At pressures and temperatures above the critical point, the boundary between fluid and gas disappears. A supercritical fluid has no surface tension, as there is no liquid-gas boundary. That's why the meniscus of the fluid state disappears.

In liquid water, for example, the polar molecules of water tend to stick to one another because the positive and negative poles of the molecules attract. Glass also is composed of polar molecules, and when water is placed in a glass tube, positive and negative ends of the glass molecules attract opposite polarities of the water molecules, resulting in a meniscus because polar attraction causes the water molecules to creep up the sides of the glass slightly.

Kinetic energy of the gaseous phase breaks the bonds between molecules of water, and likewise breaks the bonds between water and glass molecules, so there is no meniscus. Likewise, the molecules of a supercritical fluid are too energetic for a meniscus to form.

Above the critical point, the attributes of a supercritical fluid can be "fine-tuned" to be more gas-like or more fluid-like by varying the pressure and temperature. A supercritical fluid may be considered a mixture of two phases, rather than a separate phase.

Here is a temperature/pressure phase diagram of carbon dioxide that shows where the supercritical fluid exists: http://en.wikipedia.org/wiki/Supercritical_fluid#/media/File:Carbon_dioxide_pressure-temperature_phase_diagram.svg