critical temperature

Examples of critical temperature in the following topics:

• Temperature and Superconductivity

  • In superconducting materials, the characteristics of superconductivity appear when the temperature T is lowered below a critical temperature Tc.
  • The value of this critical temperature varies from material to material.
  • Solid mercury, for example, has a critical temperature of 4.2 K.
  • High-temperature superconductors can have much higher critical temperatures.
  • For example, YBa2Cu3O7, one of the first cuprate superconductors to be discovered, has a critical temperature of 92 K; mercury-based cuprates have been found with critical temperatures in excess of 130 K.

• Supercritical Fluids

  • However, close to the critical point, the density can drop sharply with a slight increase in temperature.
  • Therefore, close to the critical temperature, solubility often drops with increasing temperature, then rises again.
  • The critical point of a binary mixture can be estimated as the arithmetic mean of the critical temperatures and pressures of the two components,
  • Thus, above the critical temperature a gas cannot be liquified by pressure.
  • At slightly above the critical temperature (310 K), in the vicinity of the critical pressure, the line is almost vertical.

• Interpreting Phase Diagrams

  • Phase diagrams can also be used to explain the behavior of a pure sample of matter at the critical point.
  • General observations from the diagram reveal that certain conditions of temperature and pressure favor certain phases of matter.
  • The critical point, which occurs at critical pressure (Pcr) and critical temperature (Tcr), is a feature that indicates the point in thermodynamic parameter space at which the liquid and gaseous states of the substance being evaluated are indistinguishable.
  • At temperatures above the critical temperature, the kinetic energy of the molecules is high enough so that even at high pressures the sample cannot condense into the liquid phase.
  • A typical phase diagram illustrating the major components of a phase diagram as well as the critical point.

• Three States of Matter

  • Ice has fifteen known crystal structures, each of which exists at a different temperature and pressure.
  • The volume is definite (does not change) if the temperature and pressure are constant.
  • The highest temperature at which a particular liquid can exist is called its critical temperature.
  • A gas at a temperature below its critical temperature can also be called a vapor.
  • A supercritical fluid (SCF) is a gas whose temperature and pressure are greater than the critical temperature and critical pressure.

• Dependence of Resistance on Temperature

  • Resistivity and resistance depend on temperature with the dependence being linear for small temperature changes and nonlinear for large.
  • The resistivity of all materials depends on temperature.
  • The temperature coefficient is typically +3×10−3 K−1 to +6×10−3 K−1 for metals near room temperature.
  • Above that critical temperature, its resistance makes a sudden jump and then increases nearly linearly with temperature.
  • Compare temperature dependence of resistivity and resistance for large and small temperature changes

• Phase Changes and Energy Conservation

  • Temperature increases linearly with heat, until the melting point .
  • The curve ends at a point called the critical point, because at higher temperatures the liquid phase does not exist at any pressure.
  • The critical temperature for oxygen is -118ºC, so oxygen cannot be liquefied above this temperature.
  • Note that water changes states based on the pressure and temperature.
  • This graph shows the temperature of ice as heat is added.

• Radiative Shocks

  • Astrophysically the rate that gas cools can depend very sensitively on the temperature of the gas.
  • Imagine if the gas before the shock was just below the critical temperature at which cooling set in.
  • As it passes through the shock, it goes above this temperature and then rapidly begins to cool and rapidly returns to its initial temperature.
  • The additional condition that we seek is that final temperature equals the initial temperature.
  • From the diagram it is apparent that the entropy of the gas decreases through an isothermal shock; as a gas is compressed at constant temperature, its entropy decreases.

• Sex Determination

  • Sex determination in some crocodiles and turtles, for example, is often dependent on the temperature during critical periods of egg development.
  • This is referred to as environmental sex determination or, more specifically, as temperature-dependent sex determination.
  • In many turtles, cooler temperatures during egg incubation produce males, while warm temperatures produce females.
  • In some crocodiles, moderate temperatures produce males, while both warm and cool temperatures produce females.
  • In some species, sex is both genetic- and temperature-dependent.

• Real Gases

  • Equations other than the Ideal Gas Law model the non-ideal behavior of real gases at high pressures and low temperatures.
  • (Isotherms refer to the different curves on the graph, which represent a gas' state at different pressure and volume conditions but at constant temperature; "Iso-" means same and "-therm" means temperature—hence isotherm.)
  • Real-gas models must be used near the condensation point of gases (the temperature at which gases begin to form liquid droplets), near critical points, at very high pressures, and in other less common cases.
  • Note that the isotherms representing high temperatures deviate less from ideal behavior (Z remains close to 1 across the graph), while for isotherms representing low temperatures, Z deviates greatly from unity.
  • At low temperatures, the compressibility factor for a generalized gas greatly deviates from unity, indicating non-ideal gas behavior; at high temperatures, however, the compressibility factor is much less affected by the increased pressure.

• Temperature and Water

  • This is a reflection of evolutionary response to typical temperatures.
  • Enzymes are most efficient within a narrow and specific range of temperatures; enzyme degradation can occur at higher temperatures.
  • Therefore, organisms must either maintain an internal temperature or inhabit an environment that will keep the body within a temperature range that supports metabolism.
  • Temperature can limit the distribution of living things.
  • Water is required by all living things because it is critical for cellular processes.