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