critical temperature

Examples of critical temperature in the following topics:

• 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.

• The Structure and Properties of Water

  • At room temperature (approximately 25 degrees Celsius), it is a tasteless, odorless, and colorless liquid.
  • When water achieves a specific critical temperature and a specific critical pressure (647 K and 22.064 MPa), the liquid and gas phases merge into one homogeneous fluid phase that shares properties of both gas and liquid.
  • Phase diagrams help describe how water changes states depending on the pressure and temperature.
  • The density of ice and water as a function of temperature
  • The three phases of water – liquid, solid, and vapor – are shown in temperature-pressure space.

• 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.

• Liquid to Solid Phase Transition

  • Freezing is a phase transition in which a liquid turns into a solid when its temperature is lowered to its freezing point.
  • All known liquids, except helium, freeze when the temperature is low enough.
  • For most substances, the melting and freezing points are the same temperature; however, certain substances possess different solid-liquid transition temperatures.
  • The crystal growth is the subsequent growth of the nuclei that succeed in achieving and surpassing the critical cluster size.
  • Freezing does not start until the temperature is low enough to provide enough energy to form stable nuclei.

• Boiling & Melting Points

  • For general purposes it is useful to consider temperature to be a measure of the kinetic energy of all the atoms and molecules in a given system.
  • As temperature is increased, there is a corresponding increase in the vigor of translational and rotation motions of all molecules, as well as the vibrations of atoms and groups of atoms within molecules.
  • Experience shows that many compounds exist normally as liquids and solids; and that even low-density gases, such as hydrogen and helium, can be liquified at sufficiently low temperature and high pressure.
  • Thus, in order to break the intermolecular attractions that hold the molecules of a compound in the condensed liquid state, it is necessary to increase their kinetic energy by raising the sample temperature to the characteristic boiling point of the compound.
  • Molecular size is important, but shape is also critical, since individual molecules need to fit together cooperatively for the attractive lattice forces to be large.

• Solid Solubility and Temperature

  • Solubility often depends on temperature; the solubility of many substances increases with increasing temperature.
  • The solubility of a given solute in a given solvent typically depends on temperature.
  • Many salts show a large increase in solubility with temperature.
  • Some solutes exhibit solubility that is fairly independent of temperature.
  • A few, such as cerium(III) sulfate, become less soluble in water as temperature increases.

• Gas Solubility and Temperature

  • Solubility of a gas in water tends to decrease with increasing temperature, and solubility of a gas in an organic solvent tends to increase with increasing temperature.
  • Several factors affect the solubility of gases: one of these factors is temperature.
  • In severe cases, temperature changes can result in large-scale fish kills.
  • The trend that gas solubility decreases with increasing temperature does not hold in all cases.
  • There are several molecular reasons for the change in solubility of gases with increasing temperature, which is why there is no one trend independent of gas and solvent for whether gases will become more or less soluble with increasing temperature.

• Crystalline Solids

  • When a pure crystalline compound is heated, or a liquid cooled, the change in sample temperature with time is roughly uniform.
  • Below the temperature of the isothermal line ced, the mixture is entirely solid, consisting of a conglomerate of solid A and solid B.
  • Above this temperature the mixture is either a liquid or a liquid solid mixture, the composition of which varies.
  • Melting or freezing takes place over a broad temperature range and there is no true eutectic point.
  • Polymorphism has proven to be a critical factor in pharmaceuticals, solid state pigments and polymer manufacture.