Under normal conditions, decreasing the temperature of a substance while simultaneously increasing the pressure tends to cause it to transition into a denser state, such as a liquid or a solid. However, there are specific combinations of temperature and pressure where a substance can exist as a gas even at high pressure and low temperature. This behavior is observed in a region known as the "supercritical fluid" state.
A supercritical fluid is a state of matter that occurs when a substance is heated and compressed beyond its critical point. The critical point is the specific temperature and pressure at which the liquid and gas phases of a substance become indistinguishable from each other. Above this critical point, the substance exists as a supercritical fluid that exhibits properties of both a gas and a liquid.
Supercritical fluids have distinct characteristics. They possess a density that is comparable to that of a liquid, allowing them to dissolve materials like a solvent. At the same time, they retain the ability to diffuse through materials like a gas. Supercritical fluids also exhibit unique properties related to temperature and pressure, such as increased solubility and enhanced chemical reactivity.
The critical point, and therefore the existence of supercritical fluids, varies depending on the substance. For example, carbon dioxide (CO2) has a critical point at around 31 degrees Celsius (88 degrees Fahrenheit) and 73 atmospheres of pressure. At temperatures below the critical point and lower pressures, CO2 would typically exist as a solid (dry ice) or a liquid. However, at temperatures above the critical point and higher pressures, it can be found in a supercritical fluid state, exhibiting gas-like and liquid-like properties simultaneously.
In summary, while most substances tend to transition into denser states like liquids or solids at high pressure and low temperature, there are cases where substances can exist as supercritical fluids under specific temperature and pressure conditions.