critical point

Examples of critical point in the following topics:

• Supercritical Fluids

  • A supercritical fluid is a substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist.
  • A supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist.
  • However, close to the critical point, the density can drop sharply with a slight increase in temperature.
  • The critical point of a binary mixture can be estimated as the arithmetic mean of the critical temperatures and pressures of the two components,
  • At the critical point, (304.1 K and 7.38 MPa) there is no difference in density, and the two phases become one fluid phase.

• Interpreting Phase Diagrams

  • Phase diagrams can also be used to explain the behavior of a pure sample of matter at the critical point.
  • 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 this point and beyond it, the substance being evaluated exists as a "supercritical fluid".
  • 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.

• Real Gases

  • The Ideal Gas Law assumes that a gas is composed of randomly moving, non-interacting point particles.
  • At a certain point of combined low temperature and high pressure, real gases undergo a phase transition from the gaseous state into the liquid or solid state.
  • 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.

• The Structure and Properties of Water

  • The electrical attraction between water molecules caused by this dipole pulls individual molecules closer together, making it more difficult to separate the molecules, and therefore raising the boiling point.
  • 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.
  • The triple point (TP), a well-defined coordinate where the curves intersect, at which the three states of matter (solid, liquid, gas) exist at equilibrium with each other.
  • The oxygen end is partially negative, and the hydrogen end is partially positive; because of this, the direction of the dipole moment points from the oxygen toward the center position between the two hydrogens.
  • One such property is its relatively high melting and boiling points; more energy is required to break the hydrogen bonds between molecules in order to change to a higher energy phase.

• 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.
  • Freezing, or solidification, is a phase transition in which a liquid turns into a solid when its temperature is lowered to or below its freezing point.
  • The crystal growth is the subsequent growth of the nuclei that succeed in achieving and surpassing the critical cluster size.
  • Crystallization of pure liquids usually begins at a lower temperature than the melting point, due to the high activation energy of homogeneous nucleation.
  • The melting point of water at one atmosphere of pressure is very close to 0 °C (32 °F, 273.15 K), and in the presence of nucleating substances the freezing point of water is close to the melting point.

• Three States of Matter

  • When a solid is heated above its melting point, it becomes liquid because the pressure is higher than the triple point of the substance.
  • The highest temperature at which a particular liquid can exist is called its critical temperature.
  • A liquid can be converted to a gas through heating at constant pressure to the substance's boiling point or through reduction of pressure at constant 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.

• Boiling & Melting Points

  • The melting points of crystalline solids cannot be categorized in as simple a fashion as boiling points.
  • 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.
  • Spherically shaped molecules generally have relatively high melting points, which in some cases approach the boiling point.
  • The data in the following table serves to illustrate these points.
  • The last compound, an isomer of octane, is nearly spherical and has an exceptionally high melting point (only 6º below the boiling point).

• Crystalline Solids

  • Most organic compounds have melting points below 200 ºC.
  • For a given compound, this temperature represents its melting point (or freezing point), and is a reproducible constant as long as the external pressure does not change.
  • The lowest mixture melting point, e, is called the eutectic point.
  • The A:B complex has a melting point of 54 ºC, and the phase diagram displays two eutectic points, the first at 50 ºC, the second at 30 ºC.
  • Polymorphism has proven to be a critical factor in pharmaceuticals, solid state pigments and polymer manufacture.

• An Introduction to Synthesis

  • At this point one is tempted to convert bromocyclohexane to cyclohexanol by an SN2 reaction with hydroxide ion.
  • Plausible solutions for the second and third problem will also appear above at this point.
  • Each simpler structure, so generated, becomes the starting point for further disconnections, leading to a branched set of interrelated intermediates.
  • Computer assisted analysis has proven helpful, but in the end the instincts and experience of the chemist play a critical role in arriving at a successful synthetic plan.

• Properties of Hydrogen

  • Hydrogen has a melting point of -259.14 °C and a boiling point of -252.87 °C.
  • When hydrogen bonds with fluorine, oxygen, or nitrogen, it can participate in a form of medium-strength noncovalent (intermolecular) bonding called hydrogen bonding, which is critical to the stability of many biological molecules.