melting point

Examples of melting point in the following topics:

• Crystalline Solids

  • Most organic compounds have melting points below 200 ºC.
  • If two crystalline compounds (A & B) are thoroughly mixed, the melting point of that mixture is normally depressed and broadened, relative to the characteristic sharp melting point of each pure component.
  • 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.
  • The compound was first prepared in England in 1946, and had a melting point of 58 ºC.

• Boiling & Melting Points

  • The melting points of crystalline solids cannot be categorized in as simple a fashion as boiling points.
  • Spherically shaped molecules generally have relatively high melting points, which in some cases approach the boiling point.
  • This structure or shape sensitivity is one of the reasons that melting points are widely used to identify specific compounds.
  • Notice that the boiling points of the unbranched alkanes (pentane through decane) increase rather smoothly with molecular weight, but the melting points of the even-carbon chains increase more than those of the odd-carbon chains.
  • The last compound, an isomer of octane, is nearly spherical and has an exceptionally high melting point (only 6º below the boiling point).

• Variation of Physical Properties Within a Group

  • The physical properties (notably, melting and boiling points) of the elements in a given group vary as you move down the table.
  • For example, pure carbon can exist as diamond, which has a very high melting point, or as graphite, whose melting point is still high but much lower than that of diamond.
  • Different groups exhibit different trends in boiling and melting points.
  • For Groups 1 and 2, the boiling and melting points decrease as you move down the group.
  • The noble gases (Group 18) decrease in their boiling and melting points down the group.

• Fats & Oils

  • Thus, the melting points of triglycerides reflect their composition, as shown by the following examples.
  • Natural mixed triglycerides have somewhat lower melting points, the melting point of lard being near 30 º C, whereas olive oil melts near -6 º C.

• Physical Properties of Carboxylic Acids

  • The table at the beginning of this page gave the melting and boiling points for a homologous group of carboxylic acids having from one to ten carbon atoms.
  • The boiling points increased with size in a regular manner, but the melting points did not.
  • Unbranched acids made up of an even number of carbon atoms have melting points higher than the odd numbered homologs having one more or one less carbon.
  • In the table of fatty acids we see that the presence of a cis-double bond significantly lowers the melting point of a compound.
  • Thus, palmitoleic acid melts over 60º lower than palmitic acid, and similar decreases occur for the C18 and C20 compounds.

• Variation of Physical Properties Across a Period

  • Another physical property that varies across a period is the melting point of the corresponding halide.
  • The melting point is correlated to the strength of intermolecular bonds within the element.
  • All of the alkali halides and alkaline earth halides are solids at room temperature and have melting points in the hundreds of degrees centigrade.
  • For example, the melting point of sodium chloride (NaCl) is 808 °C.
  • In contrast, the melting points of the non-metal halides from Periods 2 and 3, such as CCl4, PCl3, and SCl2, are below 0 °C, so these materials are liquids at room temperature.

• Fatty Acids

  • Natural fatty acids may be saturated or unsaturated, and as the following data indicate, the saturated acids have higher melting points than unsaturated acids of corresponding size.
  • The higher melting points of the saturated fatty acids reflect the uniform rod-like shape of their molecules.
  • The trans-double bond isomer of oleic acid, known as elaidic acid, has a linear shape and a melting point of 45 ºC (32 ºC higher than its cis isomer).

• Molecular Crystals

  • Molecular solids tend to be soft or deformable, have low melting points, and are often sufficiently volatile to evaporate directly into the gas phase.
  • Whereas the characteristic melting point of metals and ionic solids is ~1000 °C, most molecular solids melt well below ~300 °C.
  • Because dispersion forces and the other van der Waals forces increase with the number of atoms, large molecules are generally less volatile, and have higher melting points than smaller ones.
  • A molecular solid, white phosphorus has a relatively low density of 1.82 g/cm3 and melting point of 44.1 °C; it is a soft material which can be cut with a knife.
  • How does changing the Van der Waals attraction or charging the atoms affect the melting and boiling point of the substance?

• Liquid to Solid Phase Transition

  • For most substances, the melting and freezing points are the same temperature; however, certain substances possess different solid-liquid transition temperatures.
  • This is a first-order thermodynamic phase transition, which means that as long as solid and liquid coexist, the equilibrium temperature of the system remains constant and equal to the melting point.
  • 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.
  • How quickly do the more energetic atoms melt the solid?

• Thermosetting vs. Thermoplastic Polymers

  • This reflects the fact that above Tg they may be shaped or pressed into molds, spun or cast from melts or dissolved in suitable solvents for later fashioning.
  • Because of their high melting point and poor solubility in most solvents, Kevlar and Nomex proved to be a challenge, but this was eventually solved.