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