- Critical temperature
The critical temperature, Tc, of a material is the
temperatureabove which distinct liquidand gasphases do not exist. As the critical temperature is approached, the properties of the gas and liquid phases become the same resulting in only one phase: the supercritical fluid. Above the critical temperature a liquid cannot be formed by an increase in pressure, but with enough pressure a solid may be formed. The critical pressure is the vapor pressureat the critical temperature. On the diagram showing the thermodynamicproperties for a given substance, the point at critical temperature and critical pressure is called the critical point of the substance. The critical molar volumeis the volume of one mole of material at the critical temperature and pressure.
Critical properties vary from material to material, just as is the case for the
melting pointand boiling point. Critical properties for many pure substances are readily available in the literature. Obtaining critical properties for mixtures is somewhat more problematic.
Two immiscible liquids, such as oil and water, will also have a critical temperature and pressure at which the two phases will become consolute.
Table of critical temperature and pressure for selected substances
superconductivityapplications, critical temperature refers to the temperature below which a given material becomes superconductive.
construction, critical temperature refers to the temperature above which structural steelloses its strength and is no longer fully capable of loadbearing support. Maintaining structural and important process steel building components below this critical temperature, which varies from country to country but is generally between 500 and 560°C, is an important function of passive fire protection.
For pure substances, there is an
inflection pointin the critical isothermon a "pV" diagram. This means that at the critical point:
This relation can be used to evaluate two parameters for an equation of state in terms of the critical properties.
Sometimes a set of reduced properties are defined in terms of the critical properties, ie.:
The "principle of corresponding states" indicates that substances at equal reduced pressures and temperatures have equal reduced volumes. This relationship is approximately true for many substances, but becomes increasingly inaccurate for large values of pr
*cite web | title = Critical Temperature and Pressure | work = Department of Chemistry
publisher = Purdue University | url = http://www.chem.purdue.edu/gchelp/liquids/critical.html | format =
HTML| accessdate = 2006-12-03
Hagen Kleinertand Verena Schulte-Frohlinde, "Critical Properties of φ4-Theories", [http://www.worldscibooks.com/physics/4733.html World Scientific (Singapur, 2001)] ; Paperback ISBN 981-02-4658-7" (readable online [http://www.physik.fu-berlin.de/~kleinert/b8 here] )"
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