A typical phase diagram. The dotted green line gives the anomalous behaviour of water

The critical temperature, T_c, of a material is the temperature above which distinct liquid and gas phases do not exist. Water is a common Chemical substance that is essential for the survival of all known forms of Life. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Liquid is one of the principal States of matter. A liquid is a Fluid that has the particles loose and can freely form a distinct surface at the boundaries of This page is about the physical properties of gas as a state of matter In the Physical sciences a phase is a Set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties 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. A supercritical fluid is any substance at a Temperature and Pressure above its Thermodynamic critical point. Above the critical temperature a liquid cannot be formed by an increase in pressure, but with enough pressure a solid may be formed. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface The critical pressure is the vapor pressure at the critical temperature. Vapor pressure (also known as equilibrium vapor pressure or saturation vapor pressure) is the Pressure of a Vapor in equilibrium On the diagram showing the thermodynamic properties for a given substance, the point at critical temperature and critical pressure is called the critical point of the substance. In Physics, thermodynamics (from the Greek θερμη therme meaning " Heat " and δυναμις dynamis meaning " In Physical chemistry, Thermodynamics, Chemistry and Condensed matter physics, a critical point, also called a critical state The critical molar volume is the volume of one mole of material at the critical temperature and pressure. The mole (symbol mol) is a unit of Amount of substance: it is an SI base unit, and almost the only unit to be used to measure this The volume of any solid plasma vacuum or theoretical object is how much three- Dimensional space it occupies often quantified numerically

Critical properties vary from material to material, just as is the case for the melting point and boiling point. The melting point of a solid is the temperature range at which it changes state from solid to Liquid. The boiling point of a liquid is the temperature at which the Vapor pressure of the liquid equals the environmental pressure surrounding the liquid 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. Miscibility is a term commonly used in Chemistry that refers to the property of Liquids to mix in all proportions forming a Homogeneous Solution

Table of critical temperature and pressure for selected substances

Other uses

Superconductivity

In superconductivity applications, critical temperature refers to the temperature below which a given material becomes superconductive. Superconductivity is a phenomenon occurring in certain Materials generally at very low Temperatures characterized by exactly zero electrical resistance

Construction

In construction, critical temperature refers to the temperature above which structural steel loses its strength and is no longer fully capable of loadbearing support. In the fields of Architecture and Civil engineering, construction is a process that consists of the Building or assembling of Infrastructure Steel is an Alloy consisting mostly of Iron, with a Carbon content between 0 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. Passive fire protection (PFP is an integral component of the three components of structural Fire protection and fire safety in a Building.

Mathematical definition

For pure substances, there is an inflection point in the critical isotherm on a pV diagram. In Differential calculus, an inflection point, or point of inflection (or inflexion) is a point on a Curve at which the Curvature 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. :

T_r = T / T_c

p_r = p / p_c

V_r = V / V_c

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 p_r

See also

• Critical phenomena
• Critical exponent
• Critical point
• Aerogel

References

1. ^ Emsley, John (1991). In Physics, critical phenomena is the collective name associated with thephysics of critical points Most of them stem from the divergence of the Correlation length Critical exponents describe the behaviour of physical quantities near continuous Phase transitions. In Physical chemistry, Thermodynamics, Chemistry and Condensed matter physics, a critical point, also called a critical state Aerogel is a low-density solid-state material derived from Gel in which the liquid component of the gel has been replaced with gas The Elements, (Second Edition), Oxford University Press. ISBN 0-19-855818-X.  
2. ^ (2002) Thermodynamics: An Engineering Approach, (Fourth Edition), McGraw-Hill, page 824. The McGraw-Hill Companies Inc, ( is a Publicly traded corporation headquartered in Rockefeller Center in New York City. ISBN 0-07-238332-1.  
3. ^ Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam (PDF). Erlangen, Germany. International Association for the Properties of Water and Steam (September 1997). Retrieved on 2006-12-03. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 1800 - War of the Second Coalition: Battle of Hohenlinden, French
4. ^ Critical Temperature and Pressure. Purdue University. Retrieved on 2006-12-19. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 324 - Licinius abdicates his position as Roman Emperor.

Further reading

• Critical Temperature and Pressure (HTML). HTML, an initialism of HyperText Markup Language, is the predominant Markup language for Web pages It provides a means to describe the structure Department of Chemistry. Purdue University. Retrieved on 2006-12-03. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 1800 - War of the Second Coalition: Battle of Hohenlinden, French
• Hagen Kleinert and Verena Schulte-Frohlinde, Critical Properties of φ⁴-Theories, World Scientific (Singapur, 2001); Paperback ISBN 981-02-4658-7 (readable online here)