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Article: Melting point

Melting point

Definition and Importance of Melting Point

  • The melting point is the temperature at which a substance changes from a solid to a liquid state.
  • It is an important characteristic property of a substance.
  • The melting point is determined by the equilibrium between the solid and liquid phases.
  • The melting point of a substance depends on pressure and is usually specified at a standard pressure.
  • The freezing point is the temperature at which a substance changes from a liquid to a solid state.

Examples and Measurements of Melting Points

  • Most substances have melting and freezing points that are approximately equal.
  • Agar is an example of a substance with different solid-liquid transition temperatures, known as hysteresis.
  • The melting point of ice at 1 atmosphere of pressure is very close to 0°C, also known as the ice point.
  • The freezing point of water can be different from the melting point in the presence of nucleating substances.
  • Tungsten has the highest melting point among metals, making it suitable for electrical filaments in incandescent lamps.
  • Melting point measurements can be done using a Kofler bench, differential scanning calorimetry, or a basic melting point apparatus.
  • Online measurement of freeze points is used in oil refineries for more frequent and convenient measurements.
  • Refractory materials with extremely high melting points can be measured using an optical pyrometer.

Thermodynamics and Pressure Dependence

  • Heat is required to raise the temperature of a solid to its melting point.
  • The heat of fusion is the additional heat needed for the melting process.
  • At the melting point, the change in Gibbs free energy is zero.
  • The enthalpy and entropy of the material increase at the melting point.
  • The melting point is determined by the temperature at which the Gibbs free energy of the liquid becomes lower than that of the solid.
  • The melting point is sensitive to changes in pressure.
  • Most substances are more dense in the solid state, causing the melting point to increase with pressure.
  • Water is an exception and its melting point decreases with pressure.
  • Silicon's melting point decreases at pressures above 10 GPa.
  • Changes in pressure have a smaller effect on the melting point compared to the boiling point.

Melting Point Characterization and Freezing-Point Depression

  • Melting points are used to characterize organic and inorganic compounds.
  • A pure substance has a higher melting point and a narrower range compared to impure substances or mixtures.
  • The solidus temperature marks the beginning of melting in a mixture.
  • The liquidus temperature indicates complete melting.
  • Eutectics are mixtures that melt sharply at a constant temperature.
  • Adding another compound to a solvent lowers its freezing point.
  • This phenomenon is used to prevent freezing, such as adding salt or ethylene glycol to water.

Melting Point Prediction and Notable Melting Points

  • Frederick Lindemann proposed a theory to predict the bulk melting point of crystalline materials based on thermal vibrations.
  • Alfa Aesar released over 10,000 melting points of compounds as open data.
  • The dataset has been used to create a random forest model for melting point prediction.
  • Open melting point data are also available from Nature Precedings.
  • High-quality data can be mined from patents.
  • The melting points of various elements vary across the periodic table.
  • Notable melting points include helium (0.95K), carbon (3823K), iron (1811K), and gold (1337.33K).
  • Sources of melting point data include Alfa Aesar, Nature Precedings, patents, and the CRC Handbook of Chemistry and Physics.

Melting point Data Sources

Reference URL
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